Transition Animation

Transition animation is used when it is impossible to mathematically interpolate the effect 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.

Creating a Transition Animation

The CATransition class provides transition functionality to Core Animation. 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:

Table 1  Default CATransition property values

Transition Property



Uses a fade transition. The value is kCATransitionFade.


Not applicable.


Uses the duration of the current transaction or 0.25 seconds if the duration has not been set for a transaction. The value is 0.0


Uses linear pacing. The value is nil.





Configuring a Transition

Once created, you can configure the transition animation using one of the predefined transition types or, on OS X, 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.

Table 2  Common transition types

Transition Type



The layer fades as it becomes visible or hidden.


The layer slides into place over any existing content.


The layer pushes any existing content as it slides into place


The layer is gradually revealed in the direction specified by the transition subtype.

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.

Table 3  Common transition subtypes

Transition Subtype Constant



The transition begins at the right side of the layer.


The transition begins at the left side of the layer.


The transition begins at the top of the layer.


The transition begins at the bottom of the layer.

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 and your application is targeted to OS X rather than iOS, 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.