---

Transition Animation

Transition animation is used when it is impossible to mathematically interpolate the affect of changing the value of a layer property, or the state of a layer in the layer tree.

This chapter discusses the transition animation functionality provided by Core Animation.

Contents:

Creating a Transition Animation
Configuring a Transition

Creating a Transition Animation

The CATransition class provides transition functionality to Core Animation using Core Image filters. It is a direct subclass of CAAnimation as it affects an entire layer, rather than a specific property of a layer.

A new instance of CATransition is created using the inherited class method animation. This will create a transition animation with the default values shown in Table 1:

Configuring a Transition

Once created, you can configure the transition animation using one of the predefined transition types or create a custom transition using a Core Image filter.

The predefined transitions are used by setting the type property to one of the constants in Table 2.

With the exception of kCATransitionFade, the predefined transition types also allow you to specify a direction for the transition by setting the subType property to one of the constants in Table 3.

The startProgress property allows you to change the start point of the transition by setting a value that represents a fraction of the entire animation. For example, to start a transition half way through its progress the startProgress value would be set to 0.5. Similarly, you can specify the endProgress value for the transition. The endProgress is the fraction of the entire transition that the transition should stop at. The default values are 0.0 and 1.0, respectively.

If the predefined transitions don’t provide the desired visual effect, you can specify a custom Core Image filter object that is used to display the transition. A custom filter must support both the kCIInputImageKey and the kCIInputTargetImageKey input keys, and the kCIOutputImageKey output key. The filter may optionally support the kCIInputExtentKey input key, which is set to a rectangle describing the region in which the transition should run.

---