Applying a Transition

In the transitions framework, animations create a series of frames that depict a change between the view hierarchies in the starting and ending scenes. The framework represents these animations as transition objects, which contain information about an animation. To run an animation, you provide the transition to use and the ending scene to a transition manager.

This lesson teaches you run an animation between two scenes using built-in transitions to move, resize, and fade views. The next lesson shows you how to define custom transitions.

Create a Transition

In the previous lesson, you learned how to create scenes that represent the state of different view hierarchies. Once you have defined the starting scene and the ending scene you want to change between, you need to create a Transition object that defines an animation. The framework enables you to specify a built-in transition in a resource file and inflate it in your code or to create an instance of a built-in transition directly in your code.

Table 1. Built-in transition types.

Class	Tag	Attributes	Effect
AutoTransition	<autoTransition/>	-	Default transition. Fade out, move and resize, and fade in views, in that order.
Fade	<fade/>	android:fadingMode="[fade_in | fade_out | fade_in_out]"	fade_in fades in views fade_out fades out views fade_in_out (default) does a fade_out followed by a fade_in.
ChangeBounds	<changeBounds/>	-	Moves and resizes views.

Create a transition instance from a resource file

This technique enables you to modify your transition definition without having to change the code of your activity. This technique is also useful to separate complex transition definitions from your application code, as shown in Specify Multiple Transitions.

To specify a built-in transition in a resource file, follow these steps:

1. Add the res/transition/ directory to your project.
2. Create a new XML resource file inside this directory.
3. Add an XML node for one of the built-in transitions.

For example, the following resource file specifies the Fade transition:

res/transition/fade_transition.xml

<fade xmlns:android="http://schemas.android.com/apk/res/android" />

The following code snippet shows how to inflate a Transition instance inside your activity from a resource file:

Transition mFadeTransition =
        TransitionInflater.from(this).
        inflateTransition(R.transition.fade_transition);

Create a transition instance in your code

This technique is useful for creating transition objects dynamically if you modify the user interface in your code, and to create simple built-in transition instances with few or no parameters.

To create an instance of a built-in transition, invoke one of the public constructors in the subclasses of the Transition class. For example, the following code snippet creates an instance of the Fade transition:

Transition mFadeTransition = new Fade();

Apply a Transition

You typically apply a transition to change between different view hierarchies in response to an event, such as a user action. For example, consider a search app: when the user enters a search term and clicks the search button, the app changes to the scene that represents the results layout while applying a transition that fades out the search button and fades in the search results.

To make a scene change while applying a transition in response to some event in your activity, call the TransitionManager.go() static method with the ending scene and the transition instance to use for the animation, as shown in the following snippet:

TransitionManager.go(mEndingScene, mFadeTransition);

The framework changes the view hierarchy inside the scene root with the view hierarchy from the ending scene while running the animation specified by the transition instance. The starting scene is the ending scene from the last transition. If there was no previous transition, the starting scene is determined automatically from the current state of the user interface.

If you do not specify a transition instance, the transition manager can apply an automatic transition that does something reasonable for most situations. For more information, see the API reference for the TransitionManager class.

Choose Specific Target Views

The framework applies transitions to all views in the starting and ending scenes by default. In some cases, you may only want to apply an animation to a subset of views in a scene. For example, the framework does not support animating changes to ListView objects, so you should not try to animate them during a transition. The framework enables you to select specific views you want to animate.

Each view that the transition animates is called a target. You can only select targets that are part of the view hierarchy associated with a scene.

To remove one or more views from the list of targets, call the removeTarget() method before starting the transition. To add only the views you specify to the list of targets, call the addTarget() method. For more information, see the API reference for the Transition class.

Specify Multiple Transitions

To get the most impact from an animation, you should match it to the type of changes that occur between the scenes. For example, if you are removing some views and adding others between scenes, a fade out/fade in animation provides a noticeable indication that some views are no longer available. If you are moving views to different points on the screen, a better choice would be to animate the movement so that users notice the new location of the views.

You do not have to choose only one animation, since the transitions framework enables you to combine animation effects in a transition set that contains a group of individual built-in or custom transitions.

To define a transition set from a collection of transitions in XML, create a resource file in the res/transitions/ directory and list the transitions under the transitionSet element. For example, the following snippet shows how to specify a transition set that has the same behaviour as the AutoTransition class:

<transitionSet xmlns:android="http://schemas.android.com/apk/res/android"
    android:transitionOrdering="sequential">
    <fade android:fadingMode="fade_out" />
    <changeBounds />
    <fade android:fadingMode="fade_in" />
</transitionSet>

To inflate the transition set into a TransitionSet object in your code, call the TransitionInflater.from() method in your activity. The TransitionSet class extends from the Transition class, so you can use it with a transition manager just like any other Transition instance.

Apply a Transition Without Scenes

Changing view hierarchies is not the only way to modify your user interface. You can also make changes by adding, modifying, and removing child views within the current hierarchy. For example, you can implement a search interaction with just a single layout. Start with the layout showing a search entry field and a search icon. To change the user interface to show the results, remove the search button when the user clicks it by calling the ViewGroup.removeView() method, and add the search results by calling ViewGroup.addView() method.

You may want to use this approach if the alternative is to have two hierarchies that are nearly identical. Rather than having to create and maintain two separate layout files for a minor difference in the user interface, you can have one layout file containing a view hierarchy that you modify in code.

If you make changes within the current view hierarchy in this fashion, you do not need to create a scene. Instead, you can create and apply a transition between two states of a view hierarchy using a delayed transition. This feature of the transitions framework starts with the current view hierarchy state, records changes you make to its views, and applies a transition that animates the changes when the system redraws the user interface.

To create a delayed transition within a single view hierarchy, follow these steps:

1. When the event that triggers the transition occurs, call the TransitionManager.beginDelayedTransition() method providing the parent view of all the views you want to change and the transition to use. The framework stores the current state of the child views and their property values.
2. Make changes to the child views as required by your use case. The framework records the changes you make to the child views and their properties.
3. When the system redraws the user interface according to your changes, the framework animates the changes between the original state and the new state.

The following example shows how to animate the addition of a text view to a view hierarchy using a delayed transition. The first snippet shows the layout definition file:

res/layout/activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mainLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <EditText
        android:id="@+id/inputText"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
    ...
</RelativeLayout>

The next snippet shows the code that animates the addition of the text view:

MainActivity.java

private TextView mLabelText;
private Fade mFade;
private ViewGroup mRootView;
...

// Load the layout
this.setContentView(R.layout.activity_main);
...

// Create a new TextView and set some View properties
mLabelText = new TextView();
mLabelText.setText("Label").setId("1");

// Get the root view and create a transition
mRootView = (ViewGroup) findViewById(R.id.mainLayout);
mFade = new Fade(IN);

// Start recording changes to the view hierarchy
TransitionManager.beginDelayedTransition(mRootView, mFade);

// Add the new TextView to the view hierarchy
mRootView.addView(mLabelText);

// When the system redraws the screen to show this update,
// the framework will animate the addition as a fade in

Define Transition Lifecycle Callbacks

The transition lifecycle is similar to the activity lifecycle. It represents the transition states that the framework monitors during the time between a call to the TransitionManager.go() method and the completion of the animation. At important lifecycle states, the framework invokes callbacks defined by the TransitionListener interface.

Transition lifecycle callbacks are useful, for example, for copying a view property value from the starting view hierarchy to the ending view hierarchy during a scene change. You cannot simply copy the value from its starting view to the view in the ending view hierarchy, because the ending view hierarchy is not inflated until the transition is completed. Instead, you need to store the value in a variable and then copy it into the ending view hierarchy when the framework has finished the transition. To get notified when the transition is completed, you can implement the TransitionListener.onTransitionEnd() method in your activity.

For more information, see the API reference for the TransitionListener class.