Animating Views Using Scenes and Transitions ？ - Creating Custom Transitions

A custom transition enables you to create an animation that is not available from any of the built-in transition classes. For example, you can define a custom transition that turns the foreground color of text and
input fields to gray to indicate that the fields are disabled in the new screen. This type of change helps users see the fields you disabled.
A custom transition, like one of the built-in transition types, applies animations to child views of both the starting and ending scenes. Unlike built-in transition types, however, you have to provide the code that
captures property values and generates animations. You may also want to define a subset of target views for your animation.
This lesson teaches you to capture property values and generate animations to create custom transitions.

Extend the Transition Class

To create a custom transition, add a class to your project that extends the

Transition

class
and override the methods shown in the following snippet:

public class CustomTransition extends Transition {

@Override
public void captureStartValues(TransitionValues values) {}

@Override
public void captureEndValues(TransitionValues values) {}

@Override
public Animator createAnimator(ViewGroup sceneRoot,
TransitionValues startValues,
TransitionValues endValues) {}
}

The following sections explain how to override these methods.

Capture View Property Values

Transition animations use the property animation system described in Property
Animation. Property animations change a view property between a starting and ending value over a specified period of time, so the framework needs to have both the starting and ending value of the property to construct the animation.
However, a property animation usually needs only a small subset of all the view's property values. For example, a color animation needs color property values, while a movement animation needs position property values.
Since the property values needed for an animation are specific to a transition, the transitions framework does not provide every property value to a transition. Instead, the framework invokes callback methods that allow a transition to capture only the property
values it needs and store them in the framework.

Capturing Starting Values

To pass the starting view values to the framework, implement the

captureStartValues(transitionValues)

method.
The framework calls this method for every view in the starting scene. The method argument is a

TransitionValues

object
that contains a reference to the view and a

Map

instance
in which you can store the view values you want. In your implementation, retrieve these property values and pass them back to the framework by storing them in the map.
To ensure that the key for a property value does not conflict with other

TransitionValues

keys,
use the following naming scheme:

package_name:transition_name:property_name

The following snippet shows an implementation of the

captureStartValues()

method:

public class CustomTransition extends Transition {

// Define a key for storing a property value in
// TransitionValues.values with the syntax
// package_name:transition_class:property_name to avoid collisions
private static final String PROPNAME_BACKGROUND =
"com.example.android.customtransition:CustomTransition:background";

@Override
public void captureStartValues(TransitionValues transitionValues) {
// Call the convenience method captureValues
captureValues(transitionValues);
}

// For the view in transitionValues.view, get the values you
// want and put them in transitionValues.values
private void captureValues(TransitionValues transitionValues) {
// Get a reference to the view
View view = transitionValues.view;
// Store its background property in the values map
transitionValues.values.put(PROPNAME_BACKGROUND, view.getBackground());
}
...
}

Capture Ending Values

The framework calls the

captureEndValues(TransitionValues)

method
once for every target view in the ending scene. In all other respects,

captureEndValues()

works
the same as

captureStartValues()

.
The following code snippet shows an implementation of the

captureEndValues()

method:

@Override
public void captureEndValues(TransitionValues transitionValues) {
captureValues(transitionValues);
}

In this example, both the

captureStartValues()

and

captureEndValues()

methods
invoke

captureValues()

to retrieve and store values. The view property that

captureValues()

retrieves is the
same, but it has different values in the starting and ending scenes. The framework maintains separate maps for the starting and ending states of a view.

Create a Custom Animator

To animate the changes to a view between its state in the starting scene and its state in the ending scene, you provide an animator by overriding the

createAnimator()

method.
When the framework calls this method, it passes in the scene root view and the

TransitionValues

objects
that contain the starting and ending values you captured.
The number of times the framework calls the

createAnimator()

method
depends on the changes that occur between the starting and ending scenes. For example, consider a fade out/fade in animation implemented as a custom transition. If the starting scene has five targets of which two are removed from the ending scene, and the
ending scene has the three targets from the starting scene plus a new target, then the framework calls

createAnimator()

six
times: three of the calls animate the fading out and fading in of the targets that stay in both scene objects; two more calls animate the fading out of the targets removed from the ending scene; and one call animates the fading in of the new target in the
ending scene.
For target views that exist on both the starting and ending scenes, the framework provides a

TransitionValues

object
for both the

startValues

and

endValues

arguments. For target views that only exist in the starting or the ending
scene, the framework provides a

TransitionValues

object
for the corresponding argument and

null

for the other.
To implement the

createAnimator(ViewGroup,
TransitionValues, TransitionValues)

method when you create a custom transition, use the view property values you captured to create an

Animator

object
and return it to the framework. For an example implementation, see the

ChangeColor

class
in the CustomTransitionsample. For more information about property animators, see Property
Animation.

Apply a Custom Transition

Custom transitions work the same as built-in transitions. You can apply a custom transition using a transition manager, as described in Applying
a Transition.

NEXT CLASS: ADDING ANIMATIONS