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If a responder creates persistent objects while handling events during a multi-touch sequence, it should implement touchesCancelled:withEvent: to dispose of those objects when the system cancels the sequence.

Note that a responder object should also dispose of those same objects when it receives the last touchesEnded:withEvent: message for a multi-touch sequence.

The best places to find this value are the methods touchesBegan:withEvent: and touchesEnded:withEvent:. In many cases, the latter method is preferred because it corresponds to the touch phase in which the user lifts a finger from a tap.

Each view object in UIKit is backed by a Core Animation layer object, which is an instance of the CALayer class. This layer class provides the fundamental support for the layout and rendering of a view’s contents and for compositing and animating that content.

Although Core Animation has a central role, UIKit streamlines the programming experience by providing a transparent layer on top of Core Animation. This transparent layer eliminates the need to access Core Animation layers directly most of the time, instead letting you access similar behaviors using the methods and properties of the UIView class.

Where Core Animation becomes important, however, is when the UIView class does not provide everything you need. At that point, you can dive down into the Core Animation layers and do some pretty sophisticated rendering for your application.

Every window and view object maintains its own local coordinate system.

The frame property contains a rectangle, the frame rectangle, that specifies the view’s location and size relative to its parent view’s coordinate system

The center point represents the known center point of your view and is always the best way to manipulate the position of your view. The frame rectangle is a convenience value that is computed using the bounds and center point and is valid only when the view’s transform is set to the identity transform.

Scaling of your view’s content occurs because the first time a view is shown, its rendered contents are cached in the underlying layer. Rather than force the view to redraw itself every time its bounds change or a scaling factor is applied, UIKit uses the view’s content mode to determine how to display the cached content.

Do consider using content modes when you want to implement resizable controls in your application; by doing so you can avoid both control distortion and the writing of custom drawing code. Buttons and segmented controls are particularly suitable for content mode–based drawing.

The parent-child relationships in the view hierarchy help define the chain of objects that is responsible for handling touch events in your application.

When the user rotates the device, parent-child relationships also help define how each view’s size and position are altered by changes to the user interface orientation.

You set the delegate using the setAnimationDelegate: class method of UIView, and use the setAnimationWillStartSelector: and setAnimationDidStopSelector: methods to specify the selectors that will receive the messages.

When you hide a view, it remains in the view hierarchy, but its contents are not drawn and it does not receive touch events. Because it remains in the view hierarchy, a hidden view continues to participate in autoresizing and other layout operations. If you hide a view that is currently the first responder, the view does not automatically resign its first responder status. Events targeted at the first responder are still delivered to the hidden view.