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26 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
27
28 <html>
29 <head>
30 <title>The AWT Focus Subsystem</title>
31 </head>
32
33 <body bgcolor="white">
34 <h1 align=center>The AWT Focus Subsystem</h1>
35
36 <p>
37 Prior to Java 2 Standard Edition, JDK 1.4, the AWT focus subsystem
38 was inadequate. It suffered from major design and API problems,
39 as well as over a hundred open bugs. Many of these bugs were caused by
40 platform inconsistencies, or incompatibilities between the native
41 focus system for heavyweights and the Java focus system for
42 lightweights.
43 <p>
44 The single worst problem with the AWT focus implementation was the
45 inability to query for the currently focused Component. Not only was
46 there no API for such a query, but also, because of an insufficient
47 architecture, such information was not even maintained by the code.
48 <p>
49 Almost as bad was the inability of lightweight children of a Window
50 (not a Frame or a Dialog) to receive keyboard input. This problem
51 existed because Windows never received <code>WINDOW_ACTIVATED</code>
52 events and thus could never be activated, and only active Windows
53 could contain focused Components.
54 <p>
55 In addition, many developers noted that the APIs for FocusEvent and
56 WindowEvent were insufficient because they did not provide a way for
57 determining the "opposite" Component involved in the focus or
58 activation change. For example, when a Component received a FOCUS_LOST
59 event, it had no way of knowing which Component was gaining
60 focus. Since Microsoft Windows provides this functionality for free,
61 developers migrating from Microsoft Windows C/C++ or Visual Basic to
62 Java had been frustrated by the omission.
63 <p>
64 To address these and other deficiencies, we have designed a new focus
65 model for the AWT in JDK 1.4. The primary design changes were the
66 construction of a new centralized KeyboardFocusManager class, and a
67 lightweight focus architecture. The amount of focus-related,
68 platform-dependent code has been minimized and replaced by fully
69 pluggable and extensible public APIs in the AWT. While we have
70 attempted to remain backward compatible with the existing
71 implementation, we were forced to make minor incompatible changes in
72 order to reach an elegant and workable conclusion. We anticipate that
73 these incompatibilities will have only a trivial impact on existing
74 applications.
75 <p>
76 This document is a formal specification both of the new APIs and of
77 existing APIs which remain relevant in the new model. Combined with
78 the javadoc for focus-related classes and methods, this document
79 should enable developers to create substantial AWT and Swing
80 applications with a focus behavior that is customized yet consistent
81 across platforms. This document has the following sections:
82 <ul>
83 <li><a href=#Overview>Overview of KeyboardFocusManager</a>
84 <li><a href=#BrowserContexts>KeyboardFocusManager and Browser Contexts</a>
85 <li><a href=#KeyEventDispatcher>KeyEventDispatcher</a>
86 <li><a href=#FocusEventAndWindowEvent>FocusEvent and WindowEvent</a>
87 <li><a href=#EventDelivery>Event Delivery</a>
88 <li><a href=#OppositeComponents>Opposite Components and Windows</a>
89 <li><a href=#TemporaryFocusEvents>Temporary FocusEvents</a>
90 <li><a href=#FocusTraversal>Focus Traversal</a>
91 <li><a href=#FocusTraversalPolicy>Focus Traversal Policy</a>
92 <li><a href=#FocusTraversalPolicyProviders>Focus Traversal Policy Providers</a>
93 <li><a href=#ProgrammaticTraversal>Programmatic Traversal</a>
94 <li><a href=#Focusability>Focusability</a>
95 <li><a href=#FocusableWindows>Focusable Windows</a>
96 <li><a href=#RequestingFocus>Requesting Focus</a>
97 <li><a href=#FocusAndPropertyChangeListener>Focus and PropertyChangeListener</a>
98 <li><a href=#FocusAndVetoableChangeListener>Focus and VetoableChangeListener</a>
99 <li><a href=#ZOrder>Z-Order</a>
100 <li><a href=#ReplacingDefaultKeyboardFocusManager>Replacing DefaultKeyboardFocusManager</a>
101 <li><a href=#Incompatibilities>Incompatibilities with Previous Releases</a>
102 </ul>
103
104 <a name="Overview"></a>
105 <h3>Overview of KeyboardFocusManager</h3>
106 <p>
107 The focus model is centralized around a single class,
108 KeyboardFocusManager, that provides a set of APIs for client code to
109 inquire about the current focus state, initiate focus changes, and
110 replace default focus event dispatching with a custom dispatcher.
111 Clients can inquire about the focus state directly, or can register a
112 PropertyChangeListener that will receive PropertyChangeEvents when a
113 change to the focus state occurs.
114 <p>
115 KeyboardFocusManager introduces the following main concepts and their
116 terminology:
117 <ol>
118 <li>The "focus owner" -- the Component which typically receives
119 keyboard input.
120 <li>The "permanent focus owner" -- the last Component to receive
121 focus permanently. The "focus owner" and the "permanent focus
122 owner" are equivalent unless a temporary focus change is
123 currently in effect. In such a situation, the "permanent focus
124 owner" will again be the "focus owner" when the temporary focus
125 change ends.
126 <li>The "focused Window" -- the Window which contains the "focus
127 owner".
128 <li>The "active Window" -- the Frame or Dialog that is either the
129 "focused Window", or the first Frame or Dialog that is an owner
130 of the "focused Window".
131 <li>"Focus traversal" -- the user's ability to change the "focus
132 owner" without moving the cursor. Typically, this is done using
133 the keyboard (for example, by using the TAB key), or an
134 equivalent device in an accessible environment. Client code can
135 also initiate traversal programmatically. Normal focus traversal
136 can be either "forward" to the "next" Component, or "backward" to
137 the "previous" Component.
138 <li>"Focus traversal cycle" -- a portion of the Component hierarchy,
139 such that normal focus traversal "forward" (or "backward") will
140 traverse through all of the Components in the focus cycle, but no
141 other Components. This cycle provides a mapping from an arbitrary
142 Component in the cycle to its "next" (forward traversal) and
143 "previous" (backward traversal) Components.
144 <li>"Traversable Component" -- Component that is in the focus traversal
145 cycle.
146 <li>"Non-traversable Component" -- Component that is not in the focus
147 traversal cycle. Note that a non-traversable Component can nevertheless
148 be focused in other way (e.g. by direct focus request).
149 <li>"Focus cycle root" -- Container that is the root of the Component
150 hierarchy for a particular "focus traversal cycle". When the
151 "focus owner" is a Component inside a particular cycle, normal
152 forward and backward focus traversal cannot move the "focus
153 owner" above the focus cycle root in the Component hierarchy.
154 Instead, two additional traversal operations, "up cycle" and
155 "down cycle", are defined to allow keyboard and programmatic
156 navigation up and down the focus traversal cycle hierarchy. </li>
157 <li>"Focus traversal policy provider" - Container which has
158 "FocusTraversalPolicyProvider" property as true. This Container will
159 be used to acquire focus traversal policy. This container doesn't
160 define new focus cycle but only modifies the order by which its
161 children are traversed "forward" and "backward". Focus traversal
162 policy provider can be set using
163 <code>setFocusTraversalPolicyProvider</code> on the Container.
164 </ol>
165
166 <p>
167 Every Window and JInternalFrame is, by default, a "focus cycle
168 root". If it's the only focus cycle root, then all of its
169 focusable descendants should be in its focus cycle, and its focus
170 traversal policy should enforce that they are by making sure that
171 all will be reached during normal forward (or backward)
172 traversal. If, on the other hand, the Window or JInternalFrame
173 has descendants that are also focus cycle roots, then each such
174 descendant is a member of two focus cycles: the one that it is
175 the root of, and the one of its nearest focus-cycle-root
176 ancestor. In order to traverse the focusable components belonging
177 to the focus cycle of such a "descendant" focus cycle root, one
178 first traverses (forward or backward) to reach the descendant,
179 and then uses the "down cycle" operation to reach, in turn, its
180 descendants.
181
182 <p>
183 Here is an example:<br> <img src="FocusCycle.gif" align=middle
184 alt="Three groups as described below: ABCF BDE and DGH. "><br>
185
186 <p>Assume the following:
187 <ul>
188 <li><b>A</b> is a <code>Window</code>, which means that it
189 must be a focus cycle root.
190 <li><b>B</b> and <b>D</b> are <code>Container</code>s that
191 are focus cycle roots.
192 <li><b>C</b> is a <code>Container</code> that is not a focus cycle root.
193 <li><b>G</b>, <b>H</b>, <b>E</b>, and <b>F</b> are all
194 <code>Component</code>s.
195 </ul>
196
197 There are a total of three focus cycle roots in this example:
198
199 <ol>
200 <li><b>A</b> is a root, and <b>A</b>, <b>B</b>, <b>C</b>,
201 and <b>F</b> are members of <b>A</b>'s cycle.
202 <li><b>B</b> is a root, and <b>B</b>, <b>D</b>, and
203 <b>E</b> are members of <b>B</b>'s cycle.
204 <li><b>D</b> is a root, and <b>D</b>, <b>G</b>,
205 and <b>H</b> are members of <b>D</b>'s cycle.
206 </ol>
207
208 Windows are the only Containers which, by default, are focus cycle
209 roots.
210
211
212 <code>KeyboardFocusManager</code> is an abstract class. AWT provides a default
213 implementation in the <code>DefaultKeyboardFocusManager</code> class.
214
215
216 <a name="BrowserContexts"></a>
217 <h3>KeyboardFocusManager and Browser Contexts</h3>
218 <p>
219 Some browsers partition applets in different code bases into separate
220 contexts, and establish walls between these contexts. Each thread and
221 each Component is associated with a particular context and cannot
222 interfere with threads or access Components in other contexts. In such
223 a scenario, there will be one KeyboardFocusManager per context. Other
224 browsers place all applets into the same context, implying that there
225 will be only a single, global KeyboardFocusManager for all
226 applets. This behavior is implementation-dependent. Consult your
227 browser's documentation for more information. No matter how many
228 contexts there may be, however, there can never be more than one focus
229 owner, focused Window, or active Window, per ClassLoader.
230
231
232 <a name="KeyEventDispatcher"></a>
233 <h3>KeyEventDispatcher and KeyEventPostProcessor</h3>
234 <p>
235 While the user's KeyEvents should generally be delivered to the focus
236 owner, there are rare cases where this is not desirable. An input
237 method is an example of a specialized Component that should receive
238 KeyEvents even though its associated text Component is and should
239 remain the focus owner.
240 <p>
241 A KeyEventDispatcher is a lightweight interface that allows client
242 code to pre-listen to all KeyEvents in a particular context. Instances
243 of classes that implement the interface and are registered with the
244 current KeyboardFocusManager will receive KeyEvents before they are
245 dispatched to the focus owner, allowing the KeyEventDispatcher to
246 retarget the event, consume it, dispatch it itself, or make other
247 changes.
248 <p>
249 For consistency, KeyboardFocusManager itself is a
250 KeyEventDispatcher. By default, the current KeyboardFocusManager will
251 be the sink for all KeyEvents not dispatched by the registered
252 KeyEventDispatchers. The current KeyboardFocusManager cannot be
253 completely deregistered as a KeyEventDispatcher. However, if a
254 KeyEventDispatcher reports that it dispatched the KeyEvent, regardless
255 of whether it actually did so, the KeyboardFocusManager will take no
256 further action with regard to the KeyEvent. (While it is possible for
257 client code to register the current KeyboardFocusManager as a
258 KeyEventDispatcher one or more times, there is no obvious reason why
259 this would be necessary, and therefore it is not recommended.)
260 <p>
261 Client-code may also post-listen to KeyEvents in a particular context
262 using the KeyEventPostProcessor interface. KeyEventPostProcessors
263 registered with the current KeyboardFocusManager will receive
264 KeyEvents after the KeyEvents have been dispatched to and handled by
265 the focus owner. The KeyEventPostProcessors will also receive
266 KeyEvents that would have been otherwise discarded because no
267 Component in the application currently owns the focus. This will allow
268 applications to implement features that require global KeyEvent post-
269 handling, such as menu shortcuts.
270 <p>
271 Like KeyEventDispatcher, KeyboardFocusManager also implements
272 KeyEventPostProcessor, and similar restrictions apply to its use in
273 that capacity.
274
275 <a name="FocusEventAndWindowEvent"></a>
276 <h3>FocusEvent and WindowEvent</h3>
277 <p>
278 The AWT defines the following six event types central to the focus
279 model in two different <code>java.awt.event</code> classes:
280 <ol>
281 <li><code>WindowEvent.WINDOW_ACTIVATED</code>: This event is
282 dispatched to a Frame or Dialog (but never a Window which
283 is not a Frame or Dialog) when it becomes the active Window.
284 <li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>: This event is
285 dispatched to a Window when it becomes the focused Window.
286 Only focusable Windows can receive this event.
287 <li><code>FocusEvent.FOCUS_GAINED</code>: This event is dispatched
288 to a Component when it becomes the focus owner. Only focusable
289 Components can receive this event.
290 <li><code>FocusEvent.FOCUS_LOST</code>: This event is dispatched
291 to a Component when it is no longer the focus owner.
292 <li><code>WindowEvent.WINDOW_LOST_FOCUS</code>: This event is
293 dispatched to a Window when it is no longer the focused Window.
294 <li><code>WindowEvent.WINDOW_DEACTIVATED</code>: This event is
295 dispatched to a Frame or Dialog (but never a Window which is
296 not a Frame or Dialog) when it is no longer the active Window.
297 </ol>
298
299 <a name="EventDelivery"></a>
300 <h3>Event Delivery</h3>
301 <p>
302 If the focus is not in java application and the user clicks on a focusable
303 child Component<b>a</b> of an inactive Frame <b>b</b>, the following events
304 will be dispatched and handled in order:
305
306 <ol>
307 <li><b>b</b> will receive a <code>WINDOW_ACTIVATED</code> event.
308 <li>Next, <b>b</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
309 <li>Finally, <b>a</b> will receive a <code>FOCUS_GAINED</code> event.
310 </ol>
311
312 If the user later clicks on a focusable child Component <b>c</b> of another
313 Frame <b>d</b>, the following events will be dispatched and handled in
314 order:
315 <ol>
316 <li><b>a</b> will receive a <code>FOCUS_LOST</code> event.
317 <li><b>b</b> will receive a <code>WINDOW_LOST_FOCUS</code> event.
318 <li><b>b</b> will receive a <code>WINDOW_DEACTIVATED</code> event.
319 <li><b>d</b> will receive a <code>WINDOW_ACTIVATED</code> event.
320 <li><b>d</b> will receive a <code>WINDOW_GAINED_FOCUS</code> event.
321 <li><b>c</b> will receive a <code>FOCUS_GAINED</code> event.
322 </ol>
323
324 Note that each event will be fully handled before the next event is
325 dispatched. This restriction will be enforced even if the Components
326 are in different contexts and are handled on different event
327 dispatching threads.
328 <p>
329 In addition, each event type will be dispatched in 1-to-1
330 correspondence with its opposite event type. For example, if a
331 Component receives a <code>FOCUS_GAINED</code> event, under no
332 circumstances can it ever receive another <code>FOCUS_GAINED</code>
333 event without an intervening <code>FOCUS_LOST</code> event.
334 <p>
335 Finally, it is important to note that these events are delivered for
336 informational purposes only. It is impossible, for example, to prevent
337 the delivery of a pending <code>FOCUS_GAINED</code> event by requesting
338 focus back to the Component losing focus while handling the preceding
339 <code>FOCUS_LOST</code> event. While client code may make such a request,
340 the pending <code>FOCUS_GAINED</code> will still be delivered,
341 followed later by the events transferring focus back to the original
342 focus owner.
343 <p>
344 If it is absolutely necessary to suppress the <code>FOCUS_GAINED</code> event,
345 client code can install a <code>VetoableChangeListener</code> which
346 rejects the focus change. See <a href="#FocusAndVetoableChangeListener">Focus
347 and VetoableChangeListener</a>.
348
349
350 <a name="OppositeComponents"></a>
351 <h3>Opposite Components and Windows</h3>
352 <p>
353 Each event includes information about the "opposite" Component or
354 Window involved in the focus or activation change. For example, for a
355 <code>FOCUS_GAINED</code> event, the opposite Component is the Component
356 that lost focus. If the focus or activation change occurs with a native
357 application, with a Java application in a different VM or context, or
358 with no other Component, then the opposite Component or Window is
359 null. This information is accessible using
360 <code>FocusEvent.getOppositeComponent</code> or
361 <code>WindowEvent.getOppositeWindow</code>.
362 <p>
363 On some platforms, it is not possible to discern the opposite
364 Component or Window when the focus or activation change occurs between
365 two different heavyweight Components. In these cases, the opposite
366 Component or Window may be set to null on some platforms, and to a
367 valid non-null value on other platforms. However, for a focus change
368 between two lightweight Components which share the same heavyweight
369 Container, the opposite Component will always be set correctly. Thus,
370 a pure Swing application can ignore this platform restriction when
371 using the opposite Component of a focus change that occurred within a
372 top-level Window.
373
374 <a name="TemporaryFocusEvents"></a>
375 <h3>Temporary FocusEvents</h3>
376 <p>
377 <code>FOCUS_GAINED</code> and <code>FOCUS_LOST</code> events are
378 marked as either temporary or permanent.
379 <p>
380 Temporary <code>FOCUS_LOST</code> events are sent when a Component is
381 losing the focus, but will regain the focus shortly. These events
382 can be useful when focus changes are used as triggers for validation
383 of data. For instance, a text Component may want to commit its
384 contents when the user begins interacting with another Component,
385 and can accomplish this by responding to <code>FOCUS_LOST</code> events.
386 However, if the <code>FocusEvent</code> received is temporary,
387 the commit should not be done, since the text field will be receiving
388 the focus again shortly.
389 <p>
390 A permanent focus transfer typically occurs as the result of a user
391 clicking on a selectable, heavyweight Component, focus traversal with
392 the keyboard or an equivalent input device, or from a call to
393 <code>requestFocus()</code> or <code>requestFocusInWindow()</code>.
394 <p>
395 A temporary focus transfer typically occurs as the result of showing a
396 Menu or PopupMenu, clicking or dragging a Scrollbar, moving a Window
397 by dragging the title bar, or making another Window the focused
398 Window. Note that on some platforms, these actions may not generate
399 any FocusEvents at all. On others, temporary focus transfers will
400 occur.
401 <p>
402 When a Component receives a temporary <code>FOCUS_LOST</code> event,
403 the event's opposite Component (if any) may receive a temporary
404 <code>FOCUS_GAINED</code> event, but could also receive a permanent
405 <code>FOCUS_GAINED</code> event. Showing a Menu or PopupMenu, or
406 clicking or dragging a Scrollbar, should generate a temporary
407 <code>FOCUS_GAINED</code> event. Changing the focused Window,
408 however, will yield a permanent <code>FOCUS_GAINED</code> event
409 for the new focus owner.
410 <p>
411 The Component class includes variants of <code>requestFocus</code> and
412 <code>requestFocusInWindow</code> which take a desired temporary state as a
413 parameter. However, because specifying an arbitrary temporary state
414 may not be implementable on all native windowing systems, correct
415 behavior for this method can be guaranteed only for lightweight
416 Components. This method is not intended for general use, but exists
417 instead as a hook for lightweight Component libraries, such as Swing.
418
419 <a name="FocusTraversal"></a>
420 <h3>Focus Traversal</h3>
421 <p>
422 Each Component defines its own Set of focus traversal keys for a given
423 focus traversal operation. Components support separate Sets of keys
424 for forward and backward traversal, and also for traversal up one
425 focus traversal cycle. Containers which are focus cycle roots also
426 support a Set of keys for traversal down one focus traversal cycle. If
427 a Set is not explicitly defined for a Component, that Component
428 recursively inherits a Set from its parent, and ultimately from a
429 context-wide default set on the current <code>KeyboardFocusManager</code>.
430 <p>
431 Using the <code>AWTKeyStroke</code> API, client code can specify
432 on which of two specific KeyEvents, <code>KEY_PRESSED</code> or
433 <code>KEY_RELEASED</code>, the focus traversal operation will occur.
434 Regardless of which KeyEvent is specified, however, all KeyEvents
435 related to the focus traversal key, including the associated
436 <code>KEY_TYPED</code> event, will be consumed, and will not be
437 dispatched to any Component. It is a runtime error to specify a
438 <code>KEY_TYPED</code> event as mapping to a focus traversal operation,
439 or to map the same event to multiple focus traversal operations for any
440 particular Component or for a <code>KeyboardFocusManager</code>'s defaults.
441 <p>
442 The default focus traversal keys are implementation-dependent. Sun
443 recommends that the all implementations for a particular native
444 platform use the same keys. For Windows and Unix, the recommendations
445 are:
446
447 <ul>
448 <li>traverse forward to the next Component:
449 <br><i>TextAreas</i>: <code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
450 <br><i>All others</i>: <code>TAB</code> on <code>KEY_PRESSED</code> and
451 <code>CTRL-TAB</code> on <code>KEY_PRESSED</code>
452 <li>traverse backward to the previous Component:
453 <br><i>TextAreas</i>: <code>CTRL-SHIFT-TAB</code> on
454 <code>KEY_PRESSED</code>
455 <br><i>All others</i>: <code>SHIFT-TAB</code> on <code>KEY_PRESSED</code>
456 and <code>CTRL-SHIFT-TAB</code> on
457 <code>KEY_PRESSED</code>
458 <li>traverse up one focus traversal cycle : <none>
459 <li>traverse down one focus traversal cycle : <none>
460 </ul>
461 <p>
462 Components can enable and disable all of their focus traversal keys en
463 masse using <code>Component.setFocusTraversalKeysEnabled</code>. When focus
464 traversal keys are disabled, the Component receives all KeyEvents for
465 those keys. When focus traversal keys are enabled, the Component never
466 receives KeyEvents for traversal keys; instead, the KeyEvents are
467 automatically mapped to focus traversal operations.
468 <p>
469 For normal forward and backward traversal, the AWT focus
470 implementation determines which Component to focus next based on the
471 <a href=#FocusTraversalPolicy><code>FocusTraversalPolicy</code></a> of
472 the focus owner's focus cycle root or focus traversal policy provider. If the
473 focus owner is a focus cycle root, then it may be ambiguous as to which
474 Components represent the next and previous Components to focus during
475 normal focus traversal. Thus, the current
476 <code>KeyboardFocusManager</code> maintains a reference to the
477 "current" focus cycle root, which is global across all contexts. The
478 current focus cycle root is used to resolve the ambiguity.
479 <p>
480 For up-cycle traversal, the focus owner is set to the current focus
481 owner's focus cycle root, and the current focus cycle root is set to
482 the new focus owner's focus cycle root. If, however, the current focus
483 owner's focus cycle root is a top-level window, then the focus owner
484 is set to the focus cycle root's default component to focus, and the
485 current focus cycle root is unchanged.
486 <p>
487 For down-cycle traversal, if the current focus owner is a focus cycle
488 root, then the focus owner is set to the current focus owner's default
489 component to focus, and the current focus cycle root is set to the
490 current focus owner. If the current focus owner is not a focus cycle
491 root, then no focus traversal operation occurs.
492
493
494 <a name="FocusTraversalPolicy"></a>
495 <h3>FocusTraversalPolicy</h3>
496 <p>
497
498 A <code>FocusTraversalPolicy</code> defines the order in which Components within
499 a particular focus cycle root or focus traversal policy provider are
500 traversed. Instances of <code>FocusTraversalPolicy</code> can be shared across
501 Containers, allowing those Containers to implement the same traversal policy.
502 FocusTraversalPolicies do not need to be reinitialized when the
503 focus-traversal-cycle hierarchy changes.
504
505 <p>
506 Each <code>FocusTraversalPolicy</code> must define the following
507 five algorithms:
508
509 <ol>
510 <li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
511 next Component after <b>a</b>.
512 <li>Given a focus cycle root and a Component <b>a</b> in that cycle, the
513 previous Component before <b>a</b>.
514 <li>Given a focus cycle root, the "first" Component in that cycle.
515 The "first" Component is the Component to focus when traversal
516 wraps in the forward direction.
517 <li>Given a focus cycle root, the "last" Component in that cycle.
518 The "last" Component is the Component to focus when traversal
519 wraps in the reverse direction.
520 <li>Given a focus cycle root, the "default" Component in that cycle.
521 The "default" Component will be the first to receive focus when
522 traversing down into a new focus traversal cycle. This may be the
523 same as the "first" Component, but need not be.
524 </ol>
525
526 <p>
527 A <code>FocusTraversalPolicy</code> may optionally provide an
528 algorithm for the following:
529 <blockquote>
530 Given a Window, the "initial" Component in that Window. The initial
531 Component will be the first to receive focus when the Window is
532 first made visible. By default, this is the same as the "default"
533 Component.
534 </blockquote>
535
536 In addition, Swing provides a subclass of <code>FocusTraversalPolicy</code>,
537 <code>InternalFrameFocusTraversalPolicy</code>, which allows developers
538 to provide an algorithm for the following:
539
540 <blockquote>
541 Given a <code>JInternalFrame</code>, the "initial" Component in that
542 <code>JInternalFrame</code>. The initial Component is the first to
543 receive focus when the <code>JInternalFrame</code> is first selected.
544 By default, this is the same as the <code>JInternalFrame</code>'s
545 default Component to focus.
546 </blockquote>
547
548 A <code>FocusTraversalPolicy</code> is installed on a Container using
549 Container.<code>setFocusTraversalPolicy</code>. If a policy is not explicitly
550 set, then a Container inherits its policy from its nearest focus-cycle-root
551 ancestor. Top-levels initialize their focus traversal policies using the context
552 default policy. The context default policy is established by using
553 KeyboardFocusManager. <code>setDefaultFocusTraversalPolicy</code>.
554
555 <p>
556 AWT provides two standard <code>FocusTraversalPolicy</code>
557 implementations for use by client code.
558
559 <ol>
560 <li><code>ContainerOrderFocusTraversalPolicy</code>: Iterates across the
561 Components in a focus traversal cycle in the order they were added
562 to their Containers. Each Component is tested for fitness using the
563 accept(Component) method. By default, a Component is fit only if it
564 is visible, displayable, enabled, and focusable.
565 <li>By default, ContainerOrderFocusTraversalPolicy implicitly transfers
566 focus down-cycle. That is, during normal forward focus traversal,
567 the Component traversed after a focus cycle root will be the
568 focus-cycle-root's default Component to focus, regardless of whether
569 the focus cycle root is a traversable or non-traversable Container
570 (see the pic.1,2 below). Such behavior provides backward compatibility
571 with applications designed without the concepts of up- and down-cycle
572 traversal.
573 <li><code>DefaultFocusTraversalPolicy</code>: A subclass of
574 <code>ContainerOrderFocusTraversalPolicy</code> which redefines
575 the fitness test. If client code has explicitly set the
576 focusability of a Component by either overriding
577 <code>Component.isFocusTraversable()</code> or
578 <code>Component.isFocusable()</code>, or by calling
579 <code>Component.setFocusable(boolean)</code>, then a
580 <code>DefaultFocusTraversalPolicy</code> behaves exactly
581 like a <code>ContainerOrderFocusTraversalPolicy</code>. If,
582 however, the Component is relying on default focusability, then a
583 <code>DefaultFocusTraversalPolicy</code> will reject all
584 Components with non-focusable peers.
585 <br>
586 The focusability of a peer is implementation-dependent. Sun
587 recommends that all implementations for a particular native platform
588 construct peers with the same focusability. The recommendations for
589 Windows and Unix are that Canvases, Labels, Panels, Scrollbars,
590 ScrollPanes, Windows, and lightweight Components have non-focusable
591 peers, and all other Components have focusable peers. These
592 recommendations are used in the Sun AWT implementations. Note that
593 the focusability of a Component's peer is different from, and does
594 not impact, the focusability of the Component itself.
595 </ol>
596 <p>
597 Swing provides two additional, standard FocusTraversalPolicy
598 implementations for use by client code. Each implementation is an
599 InternalFrameFocusTraversalPolicy.
600
601 <ol>
602 <li>SortingFocusTraversalPolicy: Determines traversal order by
603 sorting the Components of a focus traversal cycle based on a given
604 Comparator. Each Component is tested for fitness using the
605 accept(Component) method. By default, a Component is fit only if it
606 is visible, displayable, enabled, and focusable.
607 <li>By default, SortingFocusTraversalPolicy implicitly transfers focus
608 down-cycle. That is, during normal forward focus traversal, the
609 Component traversed after a focus cycle root will be the
610 focus-cycle-root's default Component to focus, regardless of
611 whether the focus cycle root is a traversable or non-traversable
612 Container (see the pic.1,2 below). Such behavior provides backward
613 compatibility with applications designed without the concepts of
614 up- and down-cycle traversal.
615 <li>LayoutFocusTraversalPolicy: A subclass of
616 SortingFocusTraversalPolicy which sorts Components based on their
617 size, position, and orientation. Based on their size and position,
618 Components are roughly categorized into rows and columns. For a
619 Container with horizontal orientation, columns run left-to-right or
620 right-to-left, and rows run top-to-bottom. For a Container with
621 vertical orientation, columns run top-to-bottom and rows run
622 left-to-right or right-to-left. All columns in a row are fully
623 traversed before proceeding to the next row.
624 <br>
625 In addition, the fitness test is extended to exclude JComponents
626 that have or inherit empty InputMaps.
627 </ol>
628 <p>
629 The figure below shows an implicit focus transfer:
630 <br><img src="ImplicitFocusTransfer.gif" align=middle alt="Implicit focus transfer."><br>
631
632 Assume the following:
633 <ul>
634 <li><b>A</b>, <b>B</b> and <b>C</b> are components in some window (a container)
635 <li><b>R</b> is a container in the window and it is a parent of <b>B</b> and <b>C</b>.
636 Besides, <b>R</b> is a focus cycle root.
637 <li><b>B</b> is the default component in the focul traversal cycle of <b>R</b>
638 <li><b>R</b> is a traversable Container in the pic.1, and it is a non-traversable
639 Container in the pic.2.
640 <li>In such a case a forward traversal will look as follows:
641 <ul>
642 <li> pic.1 : <b>A</b> -> <b>R</b> -> <b>B</b> -> <b>C</b>
643 <li> pic.2 : <b>A</b> -> <b>B</b> -> <b>C</b>
644 </ul>
645 </ul>
646
647 <p>
648 Swing applications, or mixed Swing/AWT applications, that use one of
649 the standard look and feels, or any other look and feel derived from
650 BasicLookAndFeel, will use LayoutFocusTraversalPolicy for all
651 Containers by default.
652 <p>
653 All other applications, including pure AWT applications, will use
654 <code>DefaultFocusTraversalPolicy</code> by default.
655
656 <a name="FocusTraversalPolicyProviders"></a>
657 <h3>Focus Traversal Policy Providers</h3>
658 <p>
659 A Container that isn't a focus cycle root has an option to provide a
660 FocusTraversalPolicy of its own. To do so, one needs to set Container's focus
661 traversal policy provider property to <code>true</code> with the call to
662
663 <blockquote>
664 <code>Container.setFocusTraversalPolicyProvider(boolean)</code>
665 </blockquote>
666
667 To determine whether a Container is a focus traversal policy provider, the
668 following method should be used:
669
670 <blockquote>
671 <code>Container.isFocusTraversalPolicyProvider()</code>
672 </blockquote>
673
674 If focus traversal policy provider property is set on a focus cycle root, it
675 isn't considered a focus traversal policy provider and behaves just like any
676 other focus cycle root.
677
678 <p>
679 The main difference between focus cycle roots and focus traversal policy
680 providers is that the latter allow focus to enter and leave them just as all other
681 Containers. However, children inside focus traversal policy provider are
682 traversed in the order determined by provider's FocusTraversalPolicy. In order
683 to enable focus traversal policy providers to behave this way,
684 FocusTraversalPolicies treat them in the following manner:
685
686 <ul>
687 <li> Focus traversal policy providers can be passed to FocusTraversalPolicy
688 methods instead of focus cycle roots.
689 <li> When calculating next or previous Component in
690 <code>FocusTraversalPolicy.getComponentAfter</code> or
691 <code>FocusTraversalPolicy.getComponentBefore</code>,
692 <ul>
693 <li>if a Component is a child of a focus traversal policy provider,
694 the next and previous for this Component are determined using this
695 focus traversal policy provider's FocusTraversalPolicy. However,
696 in order for focus to leave the provider, the following rules are
697 applied:
698 <ul>
699 <li> if at some point the <code>next</code> found Component is
700 the <code>first</code> Component of focus traversal policy
701 provider, the Component after the focus traversal policy
702 provider is returned
703 <li> if at some point the <code>previous</code> found Component is
704 the <code>last</code> Component of focus traversal policy
705 provider, the Component before the focus traversal policy
706 provider is returned
707 </ul>
708 <li> When calculating the next Component in
709 <code>FocusTraversalPolicy.getComponentAfter</code>,
710 <ul>
711 <li> if an obtained Component is a non-traversable Container and
712 it is a focus traversal policy provider, then the default Component
713 of that provider is returned
714 <li> if the Component passed to the <code>FocusTraversalPolicy.getComponentAfter</code>
715 method is a traversable Container and it is a focus
716 traversal policy provider, then the default Component of this provider
717 is returned
718 </ul>
719 <li> When calculating the previous Component in
720 <code>FocusTraversalPolicy.getComponentBefore</code>,
721 <ul>
722 <li> if an obtained Component is a Container (traversable or
723 non-traversable) and it is a focus traversal policy provider, then
724 the last Component of that provider is returned
725 </ul>
726 </ul>
727 <li> When calculating the first Component in FocusTraversalPolicy.getFirstComponent,
728 <ul>
729 <li> if an obtained Component is a non-traversable Container and it is a focus
730 traversal policy provider, then the default Component of that provider is
731 returned
732 <li> if an obtained Component is a traversable Container and it is a focus traversal
733 policy provider, then that Container itself is returned
734 </ul>
735 <li> When calculating the last Component in FocusTraversalPolicy.getLastComponent,
736 <ul>
737 <li> if an obtained Component is a Container (traversable or non-traversable)
738 and it is a focus traversal policy provider, then the last Component of
739 that provider is returned
740 </ul>
741 </ul>
742
743 <a name="ProgrammaticTraversal"></a>
744 <h3>Programmatic Traversal</h3>
745 <p>
746 In addition to user-initiated focus traversal, client code can
747 initiate a focus traversal operation programmatically. To client code,
748 programmatic traversals are indistinguishable from user-initiated
749 traversals. The preferred way to initiate a programmatic traversal is
750 to use one of the following methods on <code>KeyboardFocusManager</code>:
751
752 <ul>
753 <li><code>KeyboardFocusManager.focusNextComponent()</code>
754 <li><code>KeyboardFocusManager.focusPreviousComponent()</code>
755 <li><code>KeyboardFocusManager.upFocusCycle()</code>
756 <li><code>KeyboardFocusManager.downFocusCycle()</code>
757 </ul>
758
759 <p>
760 Each of these methods initiates the traversal operation with the
761 current focus owner. If there is currently no focus owner, then no
762 traversal operation occurs. In addition, if the focus owner is not a
763 focus cycle root, then downFocusCycle() performs no traversal
764 operation.
765 <p>
766 <code>KeyboardFocusManager</code> also supports the following variants
767 of these methods:
768
769 <ul>
770 <li><code>KeyboardFocusManager.focusNextComponent(Component)</code>
771 <li><code>KeyboardFocusManager.focusPreviousComponent(Component)</code>
772 <li><code>KeyboardFocusManager.upFocusCycle(Component)</code>
773 <li><code>KeyboardFocusManager.downFocusCycle(Container)</code>
774 </ul>
775
776 Each of these methods initiates the traversal operation with the
777 specified Component rather than the focus owner. That is, the
778 traversal occurs as though the specified Component is the focus owner,
779 though it need not be.
780 <p>
781 Alternate, but equivalent, APIs are defined on the Component and
782 Container classes themselves:
783
784 <ul>
785 <li><code>Component.transferFocus()</code>
786 <li><code>Component.transferFocusBackward()</code>
787 <li><code>Component.transferFocusUpCycle()</code>
788 <li><code>Container.transferFocusDownCycle()</code>
789 </ul>
790
791 As with the <code>KeyboardFocusManager</code> variants, each of these methods
792 initiates the traversal operation as though the Component is the focus
793 owner, though it need not be.
794 <p>
795 Also note that hiding or disabling the focus owner, directly or
796 indirectly via an ancestor, or making the focus owner non-displayable
797 or non-focusable, initiates an automatic, forward focus traversal.
798 While hiding any ancestor, lightweight or heavyweight, will always
799 indirectly hide its children, only disabling a heavyweight ancestor
800 will disable its children. Thus, disabling a lightweight ancestor of
801 the focus owner does not automatically initiate a focus traversal.
802 <p>
803 If client code initiates a focus traversal, and there is no other
804 Component to focus, then the focus owner remains unchanged. If client
805 code initiates an automatic focus traversal by hiding the focus owner,
806 directly or indirectly, or by making the focus owner non-displayable or
807 non-focusable, and there is no other Component to focus, then the
808 global focus owner is cleared. If client code initiates an automatic
809 focus traversal by disabling the focus owner, directly or indirectly,
810 and there is no other Component to focus, then the focus owner remains
811 unchanged.
812
813
814 <a name="Focusability"></a>
815 <h3>Focusability</h3>
816 <p>
817 A focusable Component can become the focus owner ("focusability") and
818 participates in keyboard focus traversal ("focus traversability") with
819 a FocusTraversalPolicy. There is no separation of these two concepts;
820 a Component must either be both focusable and focus traversable, or
821 neither.
822
823 A Component expresses this state via the isFocusable() method. By
824 default, all Components return true from this method. Client code can
825 change this default by calling Component.setFocusable(boolean).
826
827
828 <a name="FocusableWindows"></a>
829 <h3>Focusable Windows</h3>
830 <p>
831 To support palette windows and input methods, client code can prevent
832 a Window from becoming the focused Window. By transitivity, this
833 prevents the Window or any of its descendants from becoming the focus
834 owner. Non-focusable Windows may still own Windows that are
835 focusable. By default, every Frame and Dialog is focusable. Every
836 Window which is not a Frame or Dialog, but whose nearest owning Frame
837 or Dialog is showing on the screen, and which has at least one
838 Component in its focus traversal cycle, is also focusable by
839 default. To make a Window non-focusable, use
840 Window.setFocusableWindowState(false).
841 <p>
842 If a Window is non-focusable, this restriction is enforced when the
843 <code>KeyboardFocusManager</code> sees a <code>WINDOW_GAINED_FOCUS</code>
844 event for the Window. At this point, the focus change is rejected and
845 focus is reset to a different Window. The rejection recovery scheme
846 is the same as if a <code>VetoableChangeListener</code> rejected the
847 focus change. See <a href="#FocusAndVetoableChangeListener">Focus
848 and VetoableChangeListener</a>.
849 <p>
850 Because the new focus implementation requires that KeyEvents intended
851 for a Window or its descendants be proxied through a child of the
852 Window's owner, and because this proxy must be mapped on X11 in order
853 to receive events, a Window whose nearest owning Frame or Dialog is
854 not showing could never receive KeyEvents on X11. To support this
855 restriction, we have made a distinction between a Window's "window
856 focusability" and its "window focusability state". A Window's
857 focusability state is combined with the showing state of the Window's
858 nearest owning Frame or Dialog to determine the Window's focusability.
859 By default, all Windows have a focusability state of true. Setting a
860 Window's focusability state to false ensures that it will not become
861 the focused Window regardless of the showing state of its nearest
862 owning Frame or Dialog.
863 <p>
864 Swing allows applications to create JWindows with null owners. Swing
865 constructs all such JWindows so that they are owned by a private,
866 hidden Frame. Because the showing state of this Frame will always be
867 false, a JWindow constructed will a null owner can never be the
868 focused Window, even if it has a Window focusability state of true.
869 <p>
870 If the focused Window is made non-focusable, then the AWT will attempt
871 to focus the most recently focused Component of the Window's
872 owner. The Window's owner will thus become the new focused Window. If
873 the Window's owner is also a non-focusable Window, then the focus
874 change request will proceed up the ownership hierarchy recursively.
875 Since not all platforms support cross-Window focus changes (see
876 <a href=#RequestingFocus>Requesting Focus</a>), it is possible that
877 all such focus change requests will fail. In this case, the global
878 focus owner will be cleared and the focused Window will remain unchanged.
879
880 <a name="RequestingFocus"></a>
881 <h3>Requesting Focus</h3>
882
883 <p>
884 A Component can request that it become the focus owner by calling
885 <code>Component.requestFocus()</code>. This initiates a permanent
886 focus transfer to the Component only if the Component is displayable,
887 focusable, visible and all of its ancestors (with the exception of the
888 top-level Window) are visible. The request will be immediately denied if
889 any of these conditions is not met. A disabled Component may be
890 the focus owner; however, in this case, all KeyEvents will be discarded.
891 <p>
892 The request will also be denied if the Component's top-level Window is
893 not the focused Window and the platform does not support requesting
894 focus across Windows. If the request is denied for this reason, the
895 request is remembered and will be granted when the Window is later
896 focused by the user. Otherwise, the focus change request changes the
897 focused Window as well.
898 <p>
899 There is no way to determine synchronously whether a focus change
900 request has been granted. Instead, client code must install a
901 FocusListener on the Component and watch for the delivery of a
902 <code>FOCUS_GAINED</code> event. Client code must not assume that
903 the Component is the focus owner until it receives this event.
904 The event may or may not be delivered before <code>requestFocus()</code>
905 returns. Developers must not assume one behavior or the other.
906 <p>
907 The AWT supports type-ahead if all focus change requests are made on
908 the EventDispatchThread. If client code requests a focus change, and
909 the AWT determines that this request might be granted by the native
910 windowing system, then the AWT will notify the current
911 KeyboardFocusManager that is should enqueue all KeyEvents with a
912 timestamp later than the that of the event currently being handled.
913 These KeyEvents will not be dispatched until the new Component becomes
914 the focus owner. The AWT will cancel the delayed dispatching request
915 if the focus change does not succeed at the native level, if the
916 Component's peer is disposed, or if the focus change is vetoed by a
917 VetoableChangeListener. KeyboardFocusManagers are not required to
918 support type-ahead if a focus change request is made from a thread
919 other than the EventDispatchThread.
920 <p>
921 Because <code>Component.requestFocus()</code> cannot be implemented
922 consistently across platforms, developers are encouraged to use
923 <code>Component.requestFocusInWindow()</code> instead. This method
924 denies cross-Window focus transfers on all platforms automatically.
925 By eliminating the only platform-specific element of the focus transfer,
926 this method achieves consistent cross-platform behavior.
927 <p>
928 In addition, <code>requestFocusInWindow()</code> returns a boolean value.
929 If 'false' is returned, the request is guaranteed to fail. If 'true' is
930 returned, the request will succeed unless it is vetoed, or an
931 extraordinary event, such as disposal of the Component's peer, occurs
932 before the request can be granted by the native windowing
933 system. Again, while a return value of 'true' indicates that the
934 request is likely to succeed, developers must never assume that this
935 Component is the focus owner until this Component receives a
936 <code>FOCUS_GAINED</code> event.
937 <p>
938 If client code wants no Component in the application to be the focus
939 owner, it can call the method <code>KeyboardFocusManager</code>.
940 <code>clearGlobalFocusOwner()</code> on the current
941 <code>KeyboardFocusManager</code>. If there exists a focus owner
942 when this method is called, the focus owner will receive a permanent
943 <code>FOCUS_LOST</code> event. After this point, the AWT
944 focus implementation will discard all KeyEvents until the user or
945 client code explicitly sets focus to a Component.
946 <p>
947 The Component class also supports variants of <code>requestFocus</code> and
948 <code>requestFocusInWindow</code> that allow client code to specify
949 a temporary state.
950 See <a href="#TemporaryFocusEvents">Temporary FocusEvents</a>
951
952 <a name="FocusAndPropertyChangeListener"></a>
953 <h3>Focus and PropertyChangeListener</h3>
954 <p>
955 Client code can listen to changes in context-wide focus state, or to
956 changes in focus-related state in Components, via
957 PropertyChangeListeners.
958 <p>
959 The <code>KeyboardFocusManager</code> supports the following properties:
960
961 <ol>
962 <li><code>focusOwner</code>: the focus owner
963 <li><code>focusedWindow</code>: the focused Window
964 <li><code>activeWindow</code>: the active Window
965 <li><code>defaultFocusTraversalPolicy</code>: the default focus
966 traversal policy
967 <li><code>forwardDefaultFocusTraversalKeys</code>: the Set of default
968 <code>FORWARD_TRAVERSAL_KEYS</code>
969 <li><code>backwardDefaultFocusTraversalKeys</code>: the Set of default
970 <code>BACKWARD_TRAVERSAL_KEYS</code>
971 <li><code>upCycleDefaultFocusTraversalKeys</code>: the Set of default
972 <code>UP_CYCLE_TRAVERSAL_KEYS</code>
973 <li><code>downCycleDefaultFocusTraversalKeys</code>: the Set of default
974 <code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
975 <li><code>currentFocusCycleRoot</code>: the current focus cycle root
976 </ol>
977 <p>
978 A <code>PropertyChangeListener</code> installed on the current
979 <code>KeyboardFocusManager</code> will only see these changes within
980 the <code>KeyboardFocusManager</code>'s context, even though the
981 focus owner, focused Window, active Window, and current focus cycle
982 root comprise the global focus state shared by all contexts.
983 We believe this is less intrusive than requiring client code to pass
984 a security check before installing a <code>PropertyChangeListener</code>.
985 <p>
986 Component supports the following focus-related properties:
987
988 <ol>
989 <li><code>focusable</code>: the Component's focusability
990 <li><code>focusTraversalKeysEnabled</code>: the Component's
991 focus traversal keys enabled state
992 <li><code>forwardFocusTraversalKeys</code>: the Component's Set of
993 <code>FORWARD_TRAVERSAL_KEYS</code>
994 <li><code>backwardFocusTraversalKeys</code>: the Component's Set of
995 <code>BACKWARD_TRAVERSAL_KEYS</code>
996 <li><code>upCycleFocusTraversalKeys</code>: the Component's Set of
997 <code>UP_CYCLE_TRAVERSAL_KEYS</code>
998 </ol>
999 <p>
1000 In addition to the Component properties, Container supports the
1001 following focus-related properties:
1002
1003 <ol>
1004 <li><code>downCycleFocusTraversalKeys</code>: the Container's Set of
1005 <code>DOWN_CYCLE_TRAVERSAL_KEYS</code>
1006 <li><code>focusTraversalPolicy</code>: the Container's focus
1007 traversal policy
1008 <li><code>focusCycleRoot</code>: the Container's focus-cycle-root state
1009 </ol>
1010 <p>
1011 In addition to the Container properties, Window supports the following
1012 focus-related property:
1013
1014 <ol>
1015 <li><code>focusableWindow</code>: the Window's focusable Window state
1016 </ol>
1017 <p>
1018 Also note that a <code>PropertyChangeListener</code> installed on a
1019 Window will never see a <code>PropertyChangeEvent</code> for the
1020 <code>focusCycleRoot</code> property.
1021 A Window is always a focus cycle root; this property cannot change.
1022 <p>
1023 <a name="FocusAndVetoableChangeListener"></a>
1024 <h3>Focus and VetoableChangeListener</h3>
1025 <p>
1026 The <code>KeyboardFocusManager</code> also supports
1027 <code>VetoableChangeListener</code>s for the following properties:
1028
1029 <ol>
1030 <li>"focusOwner": the focus owner
1031 <li>"focusedWindow": the focused Window
1032 <li>"activeWindow": the active Window
1033 </ol>
1034
1035 If a VetoableChangeListener vetoes a focus or activation change by
1036 throwing a PropertyVetoException, the change is aborted. Any
1037 VetoableChangeListeners which had already approved the change will
1038 asynchronously receive PropertyChangeEvents indicating a reversion of
1039 state to the previous value.
1040 <p>
1041 VetoableChangeListeners are notified of the state change before the
1042 change is reflected in the KeyboardFocusManager. Conversely,
1043 PropertyChangeListeners are notified after the change is reflected.
1044 It follows that all VetoableChangeListeners will be notified before
1045 any PropertyChangeListener.
1046 <p>
1047 VetoableChangeListeners must be idempotent, and must veto both loss
1048 and gain events for a particular focus change (e.g., both
1049 <code>FOCUS_LOST</code> and <code>FOCUS_GAINED</code>). For example,
1050 if a <code>VetoableChangeListener</code> vetoes a <code>FOCUS_LOST</code>
1051 event, a <code>KeyboardFocusManager</code> is not required to search the
1052 <code>EventQueue</code> and remove the associated pending
1053 <code>FOCUS_GAINED</code> event. Instead, the
1054 <code>KeyboardFocusManager</code> is free to attempt to
1055 dispatch this event and it is the responsibility of the
1056 <code>VetoableChangeListener</code> to veto it as well. In addition,
1057 during processing of the <code>FOCUS_GAINED</code> event, the
1058 <code>KeyboardFocusManager</code> may attempt to resync the global
1059 focus state by synthesizing another <code>FOCUS_LOST</code> event.
1060 This event must be vetoed just as the first <code>FOCUS_LOST</code> event was.
1061 <p>
1062 A <code>KeyboardFocusManager</code> may not hold any locks while
1063 notifying <code>PropertyChangeListener</code>s of a state change.
1064 This requirement is relaxed for <code>VetoableChangeListeners</code>,
1065 however. Therefore, client-definied <code>VetoableChangeListener</code>s
1066 should avoid acquiring additional locks inside
1067 <code>vetoableChange(PropertyChangeEvent)</code> as this may lead to deadlock.
1068
1069 If a focus or activation change is rejected, the KeyboardFocusManager
1070 will initiate rejection recovery as follows:
1071
1072 <ul>
1073 <li>If a focused or active Window change was rejected, then the
1074 focused or active Window will be reset to the Window which was
1075 previously the focused or active Window. If there is no such
1076 Window, then the <code>KeyboardFocusManager</code> will clear
1077 the global focus owner.
1078 <li>If a focus owner change was rejected, then the focus owner will be
1079 reset to the Component which was previously the focus owner. If
1080 that is not possible, then it will be reset to the next Component
1081 in the focus traversal cycle after the previous focus owner. If
1082 that is also not possible, then the <code>KeyboardFocusManager</code>
1083 will clear the global focus owner.
1084 </ul>
1085
1086 <code>VetoableChangeListener</code>s must be careful to avoid vetoing focus
1087 changes initiated as a result of veto rejection recovery. Failure
1088 to anticipate this situation could lead to an infinite cycle of
1089 vetoed focus changes and recovery attempts.
1090
1091
1092 <a name="ZOrder"></a>
1093 <h3>Z-Order</h3>
1094 <p>
1095 On some native windowing systems, the Z-order of a Window can affect
1096 its focused or active (if applicable) state. On Microsoft Windows, the
1097 top-most Window is naturally the focused Window as well. However, on
1098 Solaris, many window managers use a point-to-focus model that ignores
1099 Z-order in determining the focused Window.
1100
1101 When focusing or activating Windows, the AWT adheres to the UI
1102 requirements of the native platform. Therefore, the focus behavior of
1103 Z-order-related methods such as:
1104 <ul>
1105 <li><code>Window.toFront()</code>
1106 <li><code>Window.toBack()</code>
1107 <li><code>Window.show()</code>
1108 <li><code>Window.hide()</code>
1109 <li><code>Window.setVisible(boolean)</code>
1110 <li><code>Window.dispose()</code>
1111 <li><code>Frame.setState(int)</code>
1112 </ul>
1113 is platform-dependent. In JDK 1.4, the behavior of these methods on
1114 Microsoft Windows and Solaris is as follows:
1115 <ul>
1116 <li><code>Window.toFront()</code>:<br>
1117 <b>Microsoft Windows</b>: The Window is moved to front, if possible.
1118 While we will always be able to move this Window in front of other
1119 Windows in the same VM, Windows 98 and Windows 2000 do not allow an
1120 application to bring any of its windows to the front unless one
1121 of that application's windows is already in the foreground. In
1122 this case, Windows will instead flash the Window's icon in the
1123 taskbar. If the Window is moved to the front, it will be made
1124 the focused and (if applicable) active Window.
1125 <br>
1126 <b>Solaris</b>: The Window is moved to front. In a point-to-focus
1127 window manager, the Window will become the focused Window if it
1128 is the top-most Window underneath the cursor. In a click-to-focus
1129 window manager, the focused Window will remain unchanged.
1130
1131 <li><code>Window.toBack()</code>:<br>
1132 <b>Microsoft Windows</b>: The Window is moved to back. Note however
1133 that Microsoft Windows insists that an owned Window always be in
1134 front of all of its recursive owners. Thus, after the completion of
1135 this operation, the Window may not be the lowest Java Window in the
1136 Z-order. If the Window, or any of its owners, was the focused Window,
1137 then the focused Window is reset to the top-most Window in the VM.
1138 <br>
1139 <b>Solaris</b>: The Window is moved to back. Like Microsoft Windows,
1140 some window managers insist than an owned Window always be in front
1141 of all of its recursive owners. Thus, after the completion of this
1142 operation, the Window may not be the lowest Java Window in the
1143 Z-order. If the Window was the focused Window, it will lose
1144 focus in a point-to-focus window manager if it is no longer the
1145 top-most Window under the cursor. In a click-to-focus window
1146 manager, the focused Window will remain unchanged.
1147
1148 <li><code>Window.show()/Window.setVisible(true)/Frame.setState(NORMAL)</code>:<br>
1149 <b>Microsoft Windows</b>: The Window is moved to front and becomes the focused
1150 Window.
1151 <br>
1152 <b>Solaris</b>: The Window is moved to front. In a point-to-focus focus
1153 window manager, the Window will be focused if it is now the
1154 top-most Window under the cursor. In a click-to-focus window
1155 manager, the Window will become the focused Window.
1156
1157 <li><code>Window.hide()/Window.setVisible(false)/Window.dispose()/
1158 Frame.setState(ICONIFIED)</code>:<br>
1159 <b>Microsoft Windows</b>: If the Window was the focused Window, the focused
1160 Window is reset to a window chosen by the OS, or to no window. The
1161 window may be in a native application, or a Java application in
1162 another VM.
1163 <br>
1164 <b>Solaris</b>: If the Window was the focused Window, in a point-to-
1165 focus window manager, the top-most Window under the cursor will
1166 become the focused Window. In a click-to-focus window manager,
1167 the focused Window is reset to a window chosen by the window
1168 manager. The window may be in a native application, or a Java
1169 application in another VM.
1170 </ul>
1171
1172 <a name="ReplacingDefaultKeyboardFocusManager"></a>
1173 <h3>Replacing DefaultKeyboardFocusManager</h3>
1174 <p>
1175 <code>KeyboardFocusManager</code>s are pluggable at the browser context
1176 level. Client code can subclass <code>KeyboardFocusManager</code> or
1177 <code>DefaultKeyboardFocusManager</code> to modify the way that WindowEvents
1178 related to focus, FocusEvents, and KeyEvents are handled and
1179 dispatched, and to examine and modify the global focus state. A custom
1180 <code>KeyboardFocusManager</code> can also reject focus changes at a more
1181 fundamental level then a FocusListener or WindowListener ever could.
1182 <p>
1183 While giving a developer ultimate control over the focus model,
1184 replacing the entire <code>KeyboardFocusManager</code> is a difficult process
1185 requiring a thorough understanding of the peer focus layer.
1186 Fortunately, most applications do not need this much control.
1187 Developers are encouraged to use KeyEventDispatchers,
1188 KeyEventPostProcessors, FocusTraversalPolicies,
1189 VetoableChangeListeners, and other concepts discussed in this document
1190 before resorting to a full replacement of the <code>KeyboardFocusManager</code>.
1191 <p>
1192 First note that, because unhindered access to Components in other
1193 contexts represents a security hole, the SecurityManager must grant a
1194 new permission, "replaceKeyboardFocusManager", before client code is
1195 permitted to replace the <code>KeyboardFocusManager</code> with an arbitrary
1196 subclass instance. Because of the security check, replacing the
1197 <code>KeyboardFocusManager</code> is not an option for applications that will be
1198 deployed in environments with a SecurityManager, such as applets in a
1199 browser.
1200 <p>
1201 Once installed, a <code>KeyboardFocusManager</code> instance has
1202 access to the global focus state via a set of protected functions.
1203 The <code>KeyboardFocusManager</code> can only call these functions
1204 if it is installed in the calling thread's context. This ensures
1205 that malicious code cannot circumvent the security check in
1206 <code>KeyboardFocusManager.setCurrentFocusManager</code>.
1207 A <code>KeyboardFocusManager</code> should always work with
1208 the global focus state instead of the context focus state.
1209 Failure to do this will lead to incorrect behavior of the
1210 <code>KeyboardFocusManager</code>.
1211 <p>
1212 The primary responsibility of a <code>KeyboardFocusManager</code>
1213 is the dispatch of the following events:
1214
1215 <ul>
1216 <li>all <code>KeyEvent</code>s
1217 <li>all <code>FocusEvent</code>s
1218 <li><code>WindowEvent.WINDOW_GAINED_FOCUS</code>
1219 <li><code>WindowEvent.WINDOW_LOST_FOCUS</code>
1220 <li><code>WindowEvent.WINDOW_ACTIVATED</code>
1221 <li><code>WindowEvent.WINDOW_DEACTIVATED</code>
1222 </ul>
1223
1224 The peer layer will provide the <code>KeyboardFocusManager</code>
1225 with all of the above events except <code>WINDOW_ACTIVATED</code>
1226 and <code>WINDOW_DEACTIVATED</code>. The <code>KeyboardFocusManager</code>
1227 must synthesize <code>WINDOW_ACTIVATED</code> and
1228 <code>WINDOW_DEACTIVATED</code> events when appropriate and target them
1229 accordingly.
1230 <p>
1231 The <code>KeyboardFocusManager</code> may need to retarget the events
1232 provided by the peer layer to its own notion of the focus owner or
1233 focused Window:
1234 <ul>
1235 <li>A KeyEvent must be retargeted to the focus owner. Because the peer
1236 layer is unaware of any lightweight Components, KeyEvents will
1237 arrive from the peer layer targeted to the focus owner's
1238 heavyweight Container, not the focus owner.
1239 <li>A <code>FOCUS_LOST</code> event must be retargeted to the focus
1240 owner. Again, this is necessary because the peer layer is
1241 unaware of lightweight Components.
1242 <li>A <code>WINDOW_LOST_FOCUS</code> event must be retargeted to
1243 the focused Window. The implementation of the Window class
1244 may cause the native focused Window to differ from the Java
1245 focused Window.
1246 </ul>
1247 <p>
1248 A <code>KeyboardFocusManager</code> must ensure proper event ordering,
1249 and a 1-to-1 correspondence between an event and its opposite event type.
1250 The peer layer does not make any of these guarantees. For example, it is
1251 possible for the peer layer to send a <code>FOCUS_GAINED</code>
1252 event before a <code>WINDOW_GAINED_FOCUS</code> event.
1253 The <code>KeyboardFocusManager</code> is responsible for
1254 ensuring that the <code>WINDOW_GAINED_FOCUS</code> event is dispatched
1255 before the <code>FOCUS_GAINED</code> event.
1256 <p>
1257 Before redispatching an event via <code>KeyboardFocusManager</code>.
1258 <code>redispatchEvent</code>, a <code>KeyboardFocusManager</code>
1259 must attempt to update the global focus state. Typically, this
1260 is done using one of the <code>KeyboardFocusManager.setGlobal*</code>
1261 methods; however, an implementation is free to implement its own methods.
1262 After attempting an update, the <code>KeyboardFocusManager</code>
1263 must verify that the global focus state change
1264 was not rejected. A rejection is detected when a call to the
1265 corresponding <code>getGlobal*</code> method returns a value different than the
1266 value just set. Rejections occur in three standard cases:
1267 <ul>
1268 <li>If the <code>KeyboardFocusManager</code> attempts
1269 to set the global focus owner to a non-focusable Component.
1270 <li>If the <code>KeyboardFocusManager</code> attempts
1271 to set the global focused Window to a non-focusable Window.
1272 <li>If the change is rejected by an installed
1273 <code>VetoableChangeListener</code>.
1274 </ul>
1275 <p>
1276 Client-defined implementations of <code>KeyboardFocusManager</code>
1277 can adjust the set of focus transfers which are rejected by overriding the
1278 accessor and mutator methods for the global focus state.
1279 <p>
1280 If a request to change the global focus state is rejected, the
1281 <code>KeyboardFocusManager</code> must discard the event which prompted
1282 the focus change request. The Component to which the event was targeted
1283 must not receive the event.
1284 <p>
1285 The <code>KeyboardFocusManager</code> is also expected to initiate rejection
1286 recovery as outlined in <a href="#FocusAndVetoableChangeListener">Focus
1287 and VetoableChangeListener</a>.
1288 <p>
1289 Finally, a KeyboardFocusManager must handle the following set of
1290 special cases:
1291 <ul>
1292 <li>When handling a <code>WINDOW_GAINED_FOCUS</code> event, the
1293 <code>KeyboardFocusManager</code> must set focus to the
1294 appropriate child Component of the Window. If a child
1295 Component of the Window previously requested focus,
1296 but the focus change was rejected because the platform
1297 does not support cross-Window focus change requests,
1298 then focus should be set to that child Component.
1299 Otherwise, if the Window has never been focused, focus should be
1300 set to the Window's initial Component to focus. If the Window was
1301 previously focused, focus should be set to the Window's most
1302 recent focus owner.
1303 <li>The <code>KeyboardFocusManager</code> must ensure that the
1304 opposite Component or Window are as accurate as the native
1305 windowing platform permits. For example, the
1306 <code>KeyboardFocusManager</code> may need to
1307 retarget the opposite Component to a lightweight child of the
1308 heavyweight initially specified by the peer layer.
1309 <br>
1310 If the peer layer states that the opposite Component or Window is
1311 <code>null</code>, it is acceptable for the
1312 <code>KeyboardFocusManager</code> to propagate
1313 this value. <code>null</code> indicates that it is highly
1314 probably that no other Component or Window was involved
1315 in the focus or activation change. Because of platform
1316 limitations, this computation may be
1317 subject to a heuristic and could be incorrect. Nevertheless, this
1318 heuristic will be the best possible guess which the peer layer
1319 could make.
1320 <li>Focus and activation changes in which a Component or Window loses
1321 focus or activation to itself must be discarded.
1322 <li>Events posted by the peer layer claiming that the active Window
1323 has lost focus to the focused Window must be discarded. The peer
1324 implementation of the Window class may generate these spurious
1325 events.
1326 </ul>
1327
1328 <a name="Incompatibilities"></a>
1329 <h3>Incompatibilities with Previous Releases</h3>
1330 <p><b>Cross-platform changes:</b>
1331 <ol>
1332 <li>The default focus traversability for all Components is now
1333 'true'. Previously, some Components (in particular, all
1334 lightweights), had a default focus traversability of 'false'. Note
1335 that despite this change, however, the
1336 <code>DefaultFocusTraversalPolicy</code> for all AWT Containers
1337 will preserve the traversal order of previous releases.
1338 <li>A request to focus a non-focus traversable (i.e., non-focusable)
1339 Component will be denied. Previously, such requests were granted.
1340 <li><code>Window.toFront()</code> and <code>Window.toBack()</code>
1341 now perform no operation if the Window is not visible.
1342 Previously, the behavior was platform-dependent.
1343 <li>KeyListeners installed on <code>Component</code>s
1344 will no longer see <code>KeyEvent</code>s that map to focus
1345 traversal operations, and
1346 <code>Component.handleEvent()</code> will no longer be invoked
1347 for such events. Previously, AWT Components saw these events
1348 and had an opportunity to consume them before AWT
1349 initiated focus traversal. Code that requires this
1350 functionality should instead disable focus traversal keys on
1351 its <code>Component</code>s and handle focus traversal
1352 itself. Alternately, the code can use an
1353 <code>AWTEventListener</code> or
1354 <code>KeyEventDispatcher</code> to pre-listen to all
1355 <code>KeyEvent</code>s.
1356 </ol>
1357 <p><b>Changes specific to Microsoft Windows:</b>
1358 <ol>
1359 <li><code>Window.toBack()</code> changes the focused Window to
1360 the top-most Window after the Z-order change.
1361 <li><code>requestFocus()</code> now allows cross-Window focus
1362 change requests in all cases. Previously, requests were granted
1363 for heavyweights, but denied for lightweights.
1364 </ol>
1365
1366 </body>
1367 </html>