ZoneView is a View implementation that creates zones for which the child views are not created or stored until they are needed for display or model/view translations. This enables a substantial reduction in memory consumption for situations where the model being represented is very large, by building view objects only for the region being actively viewed/edited. The size of the children can be estimated in some way, or calculated asynchronously with only the result being saved.

ZoneView extends BoxView to provide a box that implements zones for its children. The zones are special View implementations (the children of an instance of this class) that represent only a portion of the model that an instance of ZoneView is responsible for. The zones don't create child views until an attempt is made to display them. A box shaped view is well suited to this because:

• Boxes are a heavily used view, and having a box that provides this behavior gives substantial opportunity to plug the behavior into a view hierarchy from the view factory.
• Boxes are tiled in one direction, so it is easy to divide them into zones in a reliable way.
• Boxes typically have a simple relationship to the model (i.e. they create child views that directly represent the child elements).
• Boxes are easier to estimate the size of than some other shapes.

The default behavior is controlled by two properties, maxZoneSize and maxZonesLoaded. Setting maxZoneSize to Integer.MAX_VALUE would have the effect of causing only one zone to be created. This would effectively turn the view into an implementation of the decorator pattern. Setting maxZonesLoaded to a value of Integer.MAX_VALUE would cause zones to never be unloaded. For simplicity, zones are created on boundaries represented by the child elements of the element the view is responsible for. The zones can be any View implementation, but the default implementation is based upon AsyncBoxView which supports fairly large zones efficiently.