Programming iOS 6, 3rd Edition

View and Layer

A UIView instance has an accompanying CALayer instance, accessible as the view’s layer property. This layer has a special status: it is partnered with this view to embody all of the view’s drawing. The layer has no corresponding view property, but the view is the layer’s delegate. The documentation sometimes speaks of this layer as the view’s “underlying layer.”

By default, when a UIView is instantiated, its layer is an instance of CALayer. But if you subclass UIView and you want your subclass’s underlying layer to be an instance of a CALayer subclass (built-in or your own), implement the UIView subclass’s layerClass class method.

That, for example, is how the compass in Figure 16-1 is created. We have a UIView subclass, CompassView, and a CALayer subclass, CompassLayer. CompassView contains these lines:

+ (Class) layerClass {
    return [CompassLayer class];
}

Thus, when CompassView is instantiated, its underlying layer is a CompassLayer. In this example, there is no drawing in CompassView; its job is to give CompassLayer a place in the visible interface, because a layer cannot appear without a view.

Because every view has an underlying layer, there is a tight integration between the two. The layer portrays all the view’s drawing; if the view draws, it does so by contributing to the layer’s drawing. The view is the layer’s delegate. And the view’s properties are often merely a convenience for accessing the layer’s properties. For example, when you set the view’s backgroundColor ...