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What Is a View Hierarchy?

In addition to being responsible for drawing and handling user events, a view instance can act as a container, enclosing other view instances. Those views are linked together creating a view hierarchy. Unlike a class hierarchy, which defines the lineage of a class, the view hierarchy defines the layout of views relative to other views.

The window instance maintains a reference to a single top-level view instance, call the content view. The content view acts as the root of the visible view hierarchy in a window. The view instances enclosed within a view are called subviews. The parent view that encloses a view is referred to as its superview. While a view instance can have multiple subviews, it can have only one superview. In order for a view and its subviews to be visible to the user, the view must be inserted into a window's view hierarchy.

Figure 3-1 shows a sample application window and its view hierarchy.

This window's view hierarchy has these parts.

• The window is represented by an NSWindow instance.

• The content view serves as the root of the window's view hierarchy.

• The content view contains a single subview, an instance of the NSBox class.

• The NSBox instance in turn has two subviews, an NSButton instance, and an NSTextField instance.

• The superview for both the button and text field is the NSBox object. The NSBox container actually encloses the button and text field views.

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