xref: /frameworks/base/docs/html/guide/topics/graphics/prop-animation.jd

page.title=Property Animation
parent.title=Animation
parent.link=animation.html
@jd:body

  <div id="qv-wrapper">
    <div id="qv">
      <h2>In this document</h2>

      <ol>
        <li><a href="#how">How Property Animation Works</a></li>

        <li><a href="#value-animator">Animating with ValueAnimator</a></li>

        <li><a href="#object-animator">Animating with ObjectAnimator</a></li>

        <li><a href="#choreography">Choreographing Multiple Animations with
        AnimatorSet</a></li>

        <li><a href="#listeners">Animation Listeners</a></li>

        <li><a href="#type-evaluator">Using a TypeEvaluator</a></li>

        <li><a href="#interpolators">Using Interpolators</a></li>

        <li><a href="#keyframes">Specifying Keyframes</a></li>

        <li><a href="#layout">Animating Layout Changes to ViewGroups</a></li>

        <li><a href="#views">Animating Views</a>
          <ol>
            <li><a href="#view-prop-animator">ViewPropertyAnimator</a></li>
          </ol>
        </li>

        <li><a href="#declaring-xml">Declaring Animations in XML</a></li>
      </ol>

      <h2>Key classes</h2>

      <ol>
        <li><code><a href=
        "/reference/android/animation/ValueAnimator.html">ValueAnimator</a></code></li>

        <li><code><a href=
        "/reference/android/animation/ObjectAnimator.html">ObjectAnimator</a></code></li>

        <li><code><a href=
        "/reference/android/animation/TypeEvaluator.html">TypeEvaluator</a></code></li>
      </ol>

      <h2>Related samples</h2>

      <ol>
        <li><a href=
        "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/index.html">API
        Demos</a></li>
      </ol>
    </div>
  </div>
  <p>The property animation system is a robust framework that allows you
  to animate almost anything. You can define an animation to change any object property over time,
  regardless of whether it draws to the screen or not. A property animation changes a property's
  (a field in an object) value over a specified length of time. To animate something, you specify the
  object property that you want to animate, such as an object's position on the screen, how long
  you want to animate it for, and what values you want to animate between. </p>

  <p>The property animation system lets you define the following characteristics of an
  animation:</p>

  <ul>
    <li>Duration: You can specify the duration of an animation. The default length is 300 ms.</li>

    <li>Time interpolation: You can specify how the values for the property are calculated as a
    function of the animation's current elapsed time.</li>

    <li>Repeat count and behavior: You can specify whether or not to have an animation repeat when
    it reaches the end of a duration and how many times to repeat the animation. You can also
    specify whether you want the animation to play back in reverse. Setting it to reverse plays
    the animation forwards then backwards repeatedly, until the number of repeats is reached.</li>

    <li>Animator sets: You can group animations into logical sets that play together or
    sequentially or after specified delays.</li>

    <li>Frame refresh delay: You can specify how often to refresh frames of your animation. The
    default is set to  refresh every 10 ms, but the speed in which your application can refresh frames is
    ultimately dependent on how busy the system is overall and how fast the system can service the underlying timer.</li>
  </ul>


  <h2 id="how">How Property Animation Works</h2>

  <p>First, let's go over how an animation works with a simple example. Figure 1 depicts a
  hypothetical object that is animated with its <code>x</code> property, which represents its
  horizontal location on a screen. The duration of the animation is set to 40 ms and the distance
  to travel is 40 pixels. Every 10 ms, which is the default frame refresh rate, the object moves
  horizontally by 10 pixels. At the end of 40ms, the animation stops, and the object ends at
  horizontal position 40. This is an example of an animation with linear interpolation, meaning the
  object moves at a constant speed.</p><img src="{@docRoot}images/animation/animation-linear.png">

  <p class="img-caption"><strong>Figure 1.</strong> Example of a linear animation</p>

  <p>You can also specify animations to have a non-linear interpolation. Figure 2 illustrates a
  hypothetical object that accelerates at the beginning of the animation, and decelerates at the
  end of the animation. The object still moves 40 pixels in 40 ms, but non-linearly. In the
  beginning, this animation accelerates up to the halfway point then decelerates from the
  halfway point until the end of the animation. As Figure 2 shows, the distance traveled
  at the beginning and end of the animation is less than in the middle.</p><img src=
  "{@docRoot}images/animation/animation-nonlinear.png">

  <p class="img-caption"><strong>Figure 2.</strong> Example of a non-linear animation</p>

  <p>Let's take a detailed look at how the important components of the property animation system
  would calculate animations like the ones illustrated above. Figure 3 depicts how the main classes
  work with one another.</p><img src="{@docRoot}images/animation/valueanimator.png">

  <p class="img-caption"><strong>Figure 3.</strong> How animations are calculated</p>

  <p>The {@link android.animation.ValueAnimator} object keeps track of your animation's timing,
  such as how long the animation has been running, and the current value of the property that it is
  animating.</p>

  <p>The {@link android.animation.ValueAnimator} encapsulates a {@link
  android.animation.TimeInterpolator}, which defines animation interpolation, and a {@link
  android.animation.TypeEvaluator}, which defines how to calculate values for the property being
  animated. For example, in Figure 2, the {@link android.animation.TimeInterpolator} used would be
  {@link android.view.animation.AccelerateDecelerateInterpolator} and the {@link
  android.animation.TypeEvaluator} would be {@link android.animation.IntEvaluator}.</p>

  <p>To start an animation, create a {@link android.animation.ValueAnimator} and give it the
  starting and ending values for the property that you want to animate, along with the duration of
  the animation. When you call {@link android.animation.ValueAnimator#start start()} the animation
  begins. During the whole animation, the {@link android.animation.ValueAnimator} calculates an <em>elapsed fraction</em>
  between 0 and 1, based on the duration of the animation and how much time has elapsed. The
  elapsed fraction represents the percentage of time that the animation has completed, 0 meaning 0%
  and 1 meaning 100%. For example, in Figure 1, the elapsed fraction at t = 10 ms would be .25
  because the total duration is t = 40 ms.</p>

  <p>When the {@link android.animation.ValueAnimator} is done calculating an elapsed fraction, it
  calls the {@link android.animation.TimeInterpolator} that is currently set, to calculate an
  <em>interpolated fraction</em>. An interpolated fraction maps the elapsed fraction to a new
  fraction that takes into account the time interpolation that is set. For example, in Figure 2,
  because the animation slowly accelerates, the interpolated fraction, about .15, is less than the
  elapsed fraction, .25, at t = 10 ms. In Figure 1, the interpolated fraction is always the same as
  the elapsed fraction.</p>

  <p>When the interpolated fraction is calculated, {@link android.animation.ValueAnimator} calls
  the appropriate {@link android.animation.TypeEvaluator}, to calculate the value of the
  property that you are animating, based on the interpolated fraction, the starting value, and the
  ending value of the animation. For example, in Figure 2, the interpolated fraction was .15 at t =
  10 ms, so the value for the property at that time would be .15 X (40 - 0), or 6.</p>

 <!-- <p>When the final value is calculated, the {@link android.animation.ValueAnimator} calls the
  {@link android.animation.ValueAnimator.AnimatorUpdateListener#onAnimationUpdate
  onAnimationUpdate()} method. Implement this callback to obtain the property value by
  calling {@link android.animation.ValueAnimator#getAnimatedValue getAnimatedValue()} and set the
  value for the property in the object that you are animating. Setting the property doesn't redraw
  the object on the screen, so you need to call {@link
  android.view.View#invalidate invalidate()} to refresh the View that the object
  resides in. If the object is actually a View object, then the system calls {@link
  android.view.View#invalidate invalidate()} when the property is changed.
  The system redraws the window and the {@link android.animation.ValueAnimator}
  repeats the process.</p>-->

  <p>The <code>com.example.android.apis.animation</code> package in the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/index.html">API
  Demos</a> sample project provides many examples on how to use the property
  animation system.</p>
  
  <h2 id="property-vs-view">How Property Animation Differs from View Animation</h2>
  
  <p>The view animation system provides the capability to only animate {@link android.view.View}
  objects, so if you wanted to animate non-{@link android.view.View} objects, you have to implement
  your own code to do so. The view animation system is also constrained in the fact that it only
  exposes a few aspects of a {@link android.view.View} object to animate, such as the scaling and
  rotation of a View but not the background color, for instance.</p>

  <p>Another disadvantage of the view animation system is that it only modified where the
  View was drawn, and not the actual View itself. For instance, if you animated a button to move
  across the screen, the button draws correctly, but the actual location where you can click the
  button does not change, so you have to implement your own logic to handle this.</p>

  <p>With the property animation system, these constraints are completely removed, and you can animate
  any property of any object (Views and non-Views) and the object itself is actually modified.
  The property animation system is also more robust in the way it carries out animation. At
  a high level, you assign animators to the properties that you want to animate, such as color,
  position, or size and can define aspects of the animation such as interpolation and
  synchronization of multiple animators.</p>

  <p>The view animation system, however, takes less time to setup and requires less code to write.
  If view animation accomplishes everything that you need to do, or if your existing code already
  works the way you want, there is no need to use the property animation system. It also might
  make sense to use both animation systems for different situations if the use case arises.</p>

  <h2>API Overview</h2>

  <p>You can find most of the property animation system's APIs in {@link android.animation
  android.animation}. Because the view animation system already
  defines many interpolators in {@link android.view.animation android.view.animation}, you can use
  those interpolators in the property animation system as well. The following tables describe the main
  components of the property animation system.</p>

  <p>The {@link android.animation.Animator} class provides the basic structure for creating
  animations. You normally do not use this class directly as it only provides minimal
  functionality that must be extended to fully support animating values. The following
  subclasses extend {@link android.animation.Animator}:
  </p>
  <p class="table-caption"><strong>Table 1.</strong> Animators</p>
      <table>
        <tr>
          <th>Class</th>

          <th>Description</th>
        </tr>

        <tr>
          <td>{@link android.animation.ValueAnimator}</td>

          <td>The main timing engine for property animation that also computes the values for the
          property to be animated. It has all of the core functionality that calculates animation
          values and contains the timing details of each animation, information about whether an
          animation repeats, listeners that receive update events, and the ability to set custom
          types to evaluate. There are two pieces to animating properties: calculating the animated
          values and setting those values on the object and property that is being animated. {@link
          android.animation.ValueAnimator} does not carry out the second piece, so you must listen
          for updates to values calculated by the {@link android.animation.ValueAnimator} and
          modify the objects that you want to animate with your own logic. See the section about
          <a href="#value-animator">Animating with ValueAnimator</a> for more information.</td>
        </tr>

        <tr>
          <td>{@link android.animation.ObjectAnimator}</td>

          <td>A subclass of {@link android.animation.ValueAnimator} that allows you to set a target
          object and object property to animate. This class updates the property accordingly when
          it computes a new value for the animation. You want to use
          {@link android.animation.ObjectAnimator} most of the time,
          because it makes the process of animating values on target objects much easier. However,
          you sometimes want to use {@link android.animation.ValueAnimator} directly because {@link
          android.animation.ObjectAnimator} has a few more restrictions, such as requiring specific
          acessor methods to be present on the target object.</td>
        </tr>

        <tr>
          <td>{@link android.animation.AnimatorSet}</td>

          <td>Provides a mechanism to group animations together so that they run in
          relation to one another. You can set animations to play together, sequentially, or after
          a specified delay. See the section about <a href="#choreography">Choreographing multiple
          animations with Animator Sets</a> for more information.</td>
        </tr>
      </table>


      <p>Evaluators tell the property animation system how to calculate values for a given
      property. They take the timing data that is provided by an {@link android.animation.Animator}
      class, the animation's start and end value, and calculate the animated values of the property
      based on this data. The property animation system provides the following evaluators:</p>
      <p class="table-caption"><strong>Table 2.</strong> Evaluators</p>
      <table>
        <tr>
          <th>Class/Interface</th>

          <th>Description</th>
        </tr>

        <tr>
          <td>{@link android.animation.IntEvaluator}</td>

          <td>The default evaluator to calculate values for <code>int</code> properties.</td>
        </tr>

        <tr>
          <td>{@link android.animation.FloatEvaluator}</td>

          <td>The default evaluator to calculate values for <code>float</code> properties.</td>
        </tr>

        <tr>
          <td>{@link android.animation.ArgbEvaluator}</td>

          <td>The default evaluator to calculate values for color properties that are represented
          as hexidecimal values.</td>
        </tr>

        <tr>
          <td>{@link android.animation.TypeEvaluator}</td>

          <td>An interface that allows you to create your own evaluator. If you are animating an
          object property that is <em>not</em> an <code>int</code>, <code>float</code>, or color,
          you must implement the {@link android.animation.TypeEvaluator} interface to specify how
          to compute the object property's animated values. You can also specify a custom {@link
          android.animation.TypeEvaluator} for <code>int</code>, <code>float</code>, and color
          values as well, if you want to process those types differently than the default behavior.
          See the section about <a href="#type-evaluator">Using a TypeEvaluator</a> for more
          information on how to write a custom evaluator.</td>
        </tr>
      </table>




      <p>A time interpolator defines how specific values in an animation are calculated as a
      function of time. For example, you can specify animations to happen linearly across the whole
      animation, meaning the animation moves evenly the entire time, or you can specify animations
      to use non-linear time, for example, accelerating at the beginning and decelerating at the
      end of the animation. Table 3 describes the interpolators that are contained in {@link
      android.view.animation android.view.animation}. If none of the provided interpolators suits
      your needs, implement the {@link android.animation.TimeInterpolator} interface and create your own. See <a href=
  "#interpolators">Using interpolators</a> for more information on how to write a custom
  interpolator.</p>
      <p class="table-caption"><strong>Table 3.</strong> Interpolators</p>
      <table>
        <tr>
          <th>Class/Interface</th>

          <th>Description</th>
        </tr>

        <tr>
          <td>{@link android.view.animation.AccelerateDecelerateInterpolator}</td>

          <td>An interpolator whose rate of change starts and ends slowly but accelerates
          through the middle.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.AccelerateInterpolator}</td>

          <td>An interpolator whose rate of change starts out slowly and then
          accelerates.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.AnticipateInterpolator}</td>

          <td>An interpolator whose change starts backward then flings forward.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.AnticipateOvershootInterpolator}</td>

          <td>An interpolator whose change starts backward, flings forward and overshoots
          the target value, then finally goes back to the final value.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.BounceInterpolator}</td>

          <td>An interpolator whose change bounces at the end.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.CycleInterpolator}</td>

          <td>An interpolator whose animation repeats for a specified number of cycles.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.DecelerateInterpolator}</td>

          <td>An interpolator whose rate of change starts out quickly and and then
          decelerates.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.LinearInterpolator}</td>

          <td>An interpolator whose rate of change is constant.</td>
        </tr>

        <tr>
          <td>{@link android.view.animation.OvershootInterpolator}</td>

          <td>An interpolator whose change flings forward and overshoots the last value then
          comes back.</td>
        </tr>

        <tr>
          <td>{@link android.animation.TimeInterpolator}</td>

          <td>An interface that allows you to implement your own interpolator.</td>
        </tr>
      </table>

  <h2 id="value-animator">Animating with ValueAnimator</h2>

  <p>The {@link android.animation.ValueAnimator} class lets you animate values of some type for the
  duration of an animation by specifying a set of <code>int</code>, <code>float</code>, or color
  values to animate through. You obtain a {@link android.animation.ValueAnimator} by calling one of
  its factory methods: {@link android.animation.ValueAnimator#ofInt ofInt()}, {@link
  android.animation.ValueAnimator#ofFloat ofFloat()}, or {@link
  android.animation.ValueAnimator#ofObject ofObject()}. For example:</p>
  <pre>
ValueAnimator animation = ValueAnimator.ofFloat(0f, 1f);
animation.setDuration(1000);
animation.start();
</pre>

  <p>In this code, the {@link android.animation.ValueAnimator} starts calculating the values of the
  animation, between 0 and 1, for a duration of 1000 ms, when the <code>start()</code> method
  runs.</p>

  <p>You can also specify a custom type to animate by doing the following:</p>
  <pre>
ValueAnimator animation = ValueAnimator.ofObject(new MyTypeEvaluator(), startPropertyValue, endPropertyValue);
animation.setDuration(1000);
animation.start();
</pre>

  <p>In this code, the {@link android.animation.ValueAnimator} starts calculating the values of the
  animation, between <code>startPropertyValue</code> and <code>endPropertyValue</code> using the
  logic supplied by <code>MyTypeEvaluator</code> for a duration of 1000 ms, when the {@link
  android.animation.ValueAnimator#start start()} method runs.</p>

  <p>The previous code snippets, however, has no real effect on an object, because the {@link
  android.animation.ValueAnimator} does not operate on objects or properties directly. The most likely thing
  that you want to do is modify the objects that you want to animate with these calculated values. You do
  this by defining listeners in the {@link android.animation.ValueAnimator} to appropriately handle important events
  during the animation's lifespan, such as frame updates. When implementing the listeners, you can
  obtain the calculated value for that specific frame refresh by calling {@link
  android.animation.ValueAnimator#getAnimatedValue getAnimatedValue()}. For more information on listeners,
  see the section about <a href="#listeners">Animation Listeners</a>.

  <h2 id="object-animator">Animating with ObjectAnimator</h2>

  <p>The {@link android.animation.ObjectAnimator} is a subclass of the {@link
  android.animation.ValueAnimator} (discussed in the previous section) and combines the timing
  engine and value computation of {@link android.animation.ValueAnimator} with the ability to
  animate a named property of a target object. This makes animating any object much easier, as you
  no longer need to implement the {@link android.animation.ValueAnimator.AnimatorUpdateListener},
  because the animated property updates automatically.</p>

  <p>Instantiating an {@link android.animation.ObjectAnimator} is similar to a {@link
  android.animation.ValueAnimator}, but you also specify the object and the name of that object's property (as
  a String) along with the values to animate between:</p>
  <pre>
ObjectAnimator anim = ObjectAnimator.ofFloat(foo, "alpha", 0f, 1f);
anim.setDuration(1000);
anim.start();
</pre>

  <p>To have the {@link android.animation.ObjectAnimator} update properties correctly, you must do
  the following:</p>

  <ul>
    <li>The object property that you are animating must have a setter function (in camel case) in the form of
    <code>set&lt;propertyName&gt;()</code>. Because the {@link android.animation.ObjectAnimator}
    automatically updates the property during animation, it must be able to access the property
    with this setter method. For example, if the property name is <code>foo</code>, you need to
    have a <code>setFoo()</code> method. If this setter method does not exist, you have three
    options:

      <ul>
        <li>Add the setter method to the class if you have the rights to do so.</li>

        <li>Use a wrapper class that you have rights to change and have that wrapper receive the
        value with a valid setter method and forward it to the original object.</li>

        <li>Use {@link android.animation.ValueAnimator} instead.</li>
      </ul>
    </li>

    <li>If you specify only one value for the <code>values...</code> parameter in one of the {@link
    android.animation.ObjectAnimator} factory methods, it is assumed to be the ending value of the
    animation. Therefore, the object property that you are animating must have a getter function
    that is used to obtain the starting value of the animation. The getter function must be in the
    form of <code>get&lt;propertyName&gt;()</code>. For example, if the property name is
    <code>foo</code>, you need to have a <code>getFoo()</code> method.</li>

    <li>The getter (if needed) and setter methods of the property that you are animating must
    operate on the same type as the starting and ending values that you specify to {@link
    android.animation.ObjectAnimator}. For example, you must have
    <code>targetObject.setPropName(float)</code> and <code>targetObject.getPropName(float)</code>
    if you construct the following {@link android.animation.ObjectAnimator}:
      <pre>
ObjectAnimator.ofFloat(targetObject, "propName", 1f)
</pre>
    </li>

    <li>Depending on what property or object you are animating, you might need to call the {@link
    android.view.View#invalidate invalidate()} method on a View force the screen to redraw itself with the
    updated animated values. You do this in the
    {@link android.animation.ValueAnimator.AnimatorUpdateListener#onAnimationUpdate onAnimationUpdate()}
    callback. For example, animating the color property of a Drawable object only cause updates to the
    screen when that object redraws itself. All of the property setters on View, such as
    {@link android.view.View#setAlpha setAlpha()} and {@link android.view.View#setTranslationX setTranslationX()}
    invalidate the View properly, so you do not need to invalidate the View when calling these
    methods with new values. For more information on listeners, see the section about <a href="#listeners">Animation Listeners</a>.
    </li>
  </ul>

  <h2 id="choreography">Choreographing Multiple Animations with AnimatorSet</h2>

  <p>In many cases, you want to play an animation that depends on when another animation starts or
  finishes. The Android system lets you bundle animations together into an {@link
  android.animation.AnimatorSet}, so that you can specify whether to start animations
  simultaneously, sequentially, or after a specified delay. You can also nest {@link
  android.animation.AnimatorSet} objects within each other.</p>

  <p>The following sample code taken from the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/BouncingBalls.html">Bouncing
  Balls</a> sample (modified for simplicity) plays the following {@link android.animation.Animator}
  objects in the following manner:</p>

  <ol>
    <li>Plays <code>bounceAnim</code>.</li>

    <li>Plays <code>squashAnim1</code>, <code>squashAnim2</code>, <code>stretchAnim1</code>, and
    <code>stretchAnim2</code> at the same time.</li>

    <li>Plays <code>bounceBackAnim</code>.</li>

    <li>Plays <code>fadeAnim</code>.</li>
  </ol>
  <pre>
AnimatorSet bouncer = new AnimatorSet();
bouncer.play(bounceAnim).before(squashAnim1);
bouncer.play(squashAnim1).with(squashAnim2);
bouncer.play(squashAnim1).with(stretchAnim1);
bouncer.play(squashAnim1).with(stretchAnim2);
bouncer.play(bounceBackAnim).after(stretchAnim2);
ValueAnimator fadeAnim = ObjectAnimator.ofFloat(newBall, "alpha", 1f, 0f);
fadeAnim.setDuration(250);
AnimatorSet animatorSet = new AnimatorSet();
animatorSet.play(bouncer).before(fadeAnim);
animatorSet.start();
</pre>

  <p>For a more complete example on how to use animator sets, see the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/BouncingBalls.html">Bouncing
  Balls</a> sample in APIDemos.</p>

<h2 id="listeners">Animation Listeners</h2>
<p>
You can listen for important events during an animation's duration with the listeners described below.
</p>

  <ul>
    <li>{@link android.animation.Animator.AnimatorListener}

      <ul>
        <li>{@link android.animation.Animator.AnimatorListener#onAnimationStart onAnimationStart()}
        - Called when the animation starts.</li>

        <li>{@link android.animation.Animator.AnimatorListener#onAnimationEnd onAnimationEnd()} -
        Called when the animation ends.</li>

        <li>{@link android.animation.Animator.AnimatorListener#onAnimationRepeat
        onAnimationRepeat()} - Called when the animation repeats itself.</li>

        <li>{@link android.animation.Animator.AnimatorListener#onAnimationCancel
        onAnimationCancel()} - Called when the animation is canceled. A cancelled animation
        also calls {@link android.animation.Animator.AnimatorListener#onAnimationEnd onAnimationEnd()},
        regardless of how they were ended.</li>
      </ul>
    </li>

    <li>{@link android.animation.ValueAnimator.AnimatorUpdateListener}

      <ul>
        <li>
          <p>{@link android.animation.ValueAnimator.AnimatorUpdateListener#onAnimationUpdate
          onAnimationUpdate()} - called on every frame of the animation. Listen to this event to
          use the calculated values generated by {@link android.animation.ValueAnimator} during an
          animation. To use the value, query the {@link android.animation.ValueAnimator} object
          passed into the event to get the current animated value with the {@link
          android.animation.ValueAnimator#getAnimatedValue getAnimatedValue()} method. Implementing this
          listener is required if you use {@link android.animation.ValueAnimator}. </p>

          <p>
          Depending on what property or object you are animating, you might need to call
          {@link android.view.View#invalidate invalidate()} on a View to force that area of the
          screen to redraw itself with the new animated values. For example, animating the
          color property of a Drawable object only cause updates to the screen when that object
          redraws itself. All of the property setters on View,
          such as {@link android.view.View#setAlpha setAlpha()} and
          {@link android.view.View#setTranslationX setTranslationX()} invalidate the View
          properly, so you do not need to invalidate the View when calling these methods with new values.
          </p>

        </li>
      </ul>
    </li>
  </ul>

<p>You can extend the {@link android.animation.AnimatorListenerAdapter} class instead of
implementing the {@link android.animation.Animator.AnimatorListener} interface, if you do not
want to implement all of the methods of the {@link android.animation.Animator.AnimatorListener}
interface. The {@link android.animation.AnimatorListenerAdapter} class provides empty
implementations of the methods that you can choose to override.</p>
  <p>For example, the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/BouncingBalls.html">Bouncing
  Balls</a> sample in the API demos creates an {@link android.animation.AnimatorListenerAdapter}
  for just the {@link android.animation.Animator.AnimatorListener#onAnimationEnd onAnimationEnd()}
  callback:</p>
  <pre>
ValueAnimatorAnimator fadeAnim = ObjectAnimator.ofFloat(newBall, "alpha", 1f, 0f);
fadeAnim.setDuration(250);
fadeAnim.addListener(new AnimatorListenerAdapter() {
public void onAnimationEnd(Animator animation) {
    balls.remove(((ObjectAnimator)animation).getTarget());
}
</pre>


  <h2 id="layout">Animating Layout Changes to ViewGroups</h2>

  <p>The property animation system provides the capability to animate changes to ViewGroup objects
  as well as provide an easy way to animate View objects themselves.</p>

  <p>You can animate layout changes within a ViewGroup with the {@link
  android.animation.LayoutTransition} class. Views inside a ViewGroup can go through an appearing
  and disappearing animation when you add them to or remove them from a ViewGroup or when you call
  a View's {@link android.view.View#setVisibility setVisibility()} method with {@link
  android.view.View#VISIBLE}, android.view.View#INVISIBLE}, or {@link android.view.View#GONE}. The remaining Views in the
  ViewGroup can also animate into their new positions when you add or remove Views. You can define
  the following animations in a {@link android.animation.LayoutTransition} object by calling {@link
  android.animation.LayoutTransition#setAnimator setAnimator()} and passing in an {@link
  android.animation.Animator} object with one of the following {@link
  android.animation.LayoutTransition} constants:</p>

  <ul>
    <li><code>APPEARING</code> - A flag indicating the animation that runs on items that are
    appearing in the container.</li>

    <li><code>CHANGE_APPEARING</code> - A flag indicating the animation that runs on items that are
    changing due to a new item appearing in the container.</li>

    <li><code>DISAPPEARING</code> - A flag indicating the animation that runs on items that are
    disappearing from the container.</li>

    <li><code>CHANGE_DISAPPEARING</code> - A flag indicating the animation that runs on items that
    are changing due to an item disappearing from the container.</li>
  </ul>

  <p>You can define your own custom animations for these four types of events to customize the look
  of your layout transitions or just tell the animation system to use the default animations.</p>

  <p>The <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/LayoutAnimations.html">
  LayoutAnimations</a> sample in API Demos shows you how to define animations for layout
  transitions and then set the animations on the View objects that you want to animate.</p>

  <p>The <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/LayoutAnimationsByDefault.html">
  LayoutAnimationsByDefault</a> and its corresponding <a href=
  "{@docRoot}resources/samples/ApiDemos/res/layout/layout_animations_by_default.html">layout_animations_by_default.xml</a>
  layout resource file show you how to enable the default layout transitions for ViewGroups in XML.
  The only thing that you need to do is to set the <code>android:animateLayoutchanges</code>
  attribute to <code>true</code> for the ViewGroup. For example:</p>
  <pre>
&lt;LinearLayout
    android:orientation="vertical"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:id="@+id/verticalContainer"
    android:animateLayoutChanges="true" /&gt;
</pre>

  <p>Setting this attribute to true automatically animates Views that are added or removed from the
  ViewGroup as well as the remaining Views in the ViewGroup.</p>

  <h2 id="type-evaluator">Using a TypeEvaluator</h2>

  <p>If you want to animate a type that is unknown to the Android system, you can create your own
  evaluator by implementing the {@link android.animation.TypeEvaluator} interface. The types that
  are known by the Android system are <code>int</code>, <code>float</code>, or a color, which are
  supported by the {@link android.animation.IntEvaluator}, {@link
  android.animation.FloatEvaluator}, and {@link android.animation.ArgbEvaluator} type
  evaluators.</p>

  <p>There is only one method to implement in the {@link android.animation.TypeEvaluator}
  interface, the {@link android.animation.TypeEvaluator#evaluate evaluate()} method. This allows
  the animator that you are using to return an appropriate value for your animated property at the
  current point of the animation. The {@link android.animation.FloatEvaluator} class demonstrates
  how to do this:</p>
  <pre>
public class FloatEvaluator implements TypeEvaluator {

    public Object evaluate(float fraction, Object startValue, Object endValue) {
        float startFloat = ((Number) startValue).floatValue();
        return startFloat + fraction * (((Number) endValue).floatValue() - startFloat);
    }
}
</pre>

  <p class="note"><strong>Note:</strong> When {@link android.animation.ValueAnimator} (or {@link
  android.animation.ObjectAnimator}) runs, it calculates a current elapsed fraction of the
  animation (a value between 0 and 1) and then calculates an interpolated version of that depending
  on what interpolator that you are using. The interpolated fraction is what your {@link
  android.animation.TypeEvaluator} receives through the <code>fraction</code> parameter, so you do
  not have to take into account the interpolator when calculating animated values.</p>

  <h2 id="interpolators">Using Interpolators</h2>

  <p>An interpolator define how specific values in an animation are calculated as a function of
  time. For example, you can specify animations to happen linearly across the whole animation,
  meaning the animation moves evenly the entire time, or you can specify animations to use
  non-linear time, for example, using acceleration or deceleration at the beginning or end of the
  animation.</p>

  <p>Interpolators in the animation system receive a fraction from Animators that represent the
  elapsed time of the animation. Interpolators modify this fraction to coincide with the type of
  animation that it aims to provide. The Android system provides a set of common interpolators in
  the {@link android.view.animation android.view.animation package}. If none of these suit your
  needs, you can implement the {@link android.animation.TimeInterpolator} interface and create your
  own.</p>

  <p>As an example, how the default interpolator {@link
  android.view.animation.AccelerateDecelerateInterpolator} and the {@link
  android.view.animation.LinearInterpolator} calculate interpolated fractions are compared below.
  The {@link android.view.animation.LinearInterpolator} has no effect on the elapsed fraction. The {@link
  android.view.animation.AccelerateDecelerateInterpolator} accelerates into the animation and
  decelerates out of it. The following methods define the logic for these interpolators:</p>

  <p><strong>AccelerateDecelerateInterpolator</strong></p>
  <pre>
public float getInterpolation(float input) {
    return (float)(Math.cos((input + 1) * Math.PI) / 2.0f) + 0.5f;
}
</pre>

  <p><strong>LinearInterpolator</strong></p>
  <pre>
public float getInterpolation(float input) {
    return input;
}
</pre>

  <p>The following table represents the approximate values that are calculated by these
  interpolators for an animation that lasts 1000ms:</p>

  <table>
    <tr>
      <th>ms elapsed</th>

      <th>Elapsed fraction/Interpolated fraction (Linear)</th>

      <th>Interpolated fraction (Accelerate/Decelerate)</th>
    </tr>

    <tr>
      <td>0</td>

      <td>0</td>

      <td>0</td>
    </tr>

    <tr>
      <td>200</td>

      <td>.2</td>

      <td>.1</td>
    </tr>

    <tr>
      <td>400</td>

      <td>.4</td>

      <td>.345</td>
    </tr>

    <tr>
      <td>600</td>

      <td>.6</td>

      <td>.8</td>
    </tr>

    <tr>
      <td>800</td>

      <td>.8</td>

      <td>.9</td>
    </tr>

    <tr>
      <td>1000</td>

      <td>1</td>

      <td>1</td>
    </tr>
  </table>

  <p>As the table shows, the {@link android.view.animation.LinearInterpolator} changes the values
  at the same speed, .2 for every 200ms that passes. The {@link
  android.view.animation.AccelerateDecelerateInterpolator} changes the values faster than {@link
  android.view.animation.LinearInterpolator} between 200ms and 600ms and slower between 600ms and
  1000ms.</p>

  <h2 id="keyframes">Specifying Keyframes</h2>

  <p>A {@link android.animation.Keyframe} object consists of a time/value pair that lets you define
  a specific state at a specific time of an animation. Each keyframe can also have its own
  interpolator to control the behavior of the animation in the interval between the previous
  keyframe's time and the time of this keyframe.</p>

  <p>To instantiate a {@link android.animation.Keyframe} object, you must use one of the factory
  methods, {@link android.animation.Keyframe#ofInt ofInt()}, {@link
  android.animation.Keyframe#ofFloat ofFloat()}, or {@link android.animation.Keyframe#ofObject
  ofObject()} to obtain the appropriate type of {@link android.animation.Keyframe}. You then call
  the {@link android.animation.PropertyValuesHolder#ofKeyframe ofKeyframe()} factory method to
  obtain a {@link android.animation.PropertyValuesHolder} object. Once you have the object, you can
  obtain an animator by passing in the {@link android.animation.PropertyValuesHolder} object and
  the object to animate. The following code snippet demonstrates how to do this:</p>
  <pre>
Keyframe kf0 = Keyframe.ofFloat(0f, 0f);
Keyframe kf1 = Keyframe.ofFloat(.5f, 360f);
Keyframe kf2 = Keyframe.ofFloat(1f, 0f);
PropertyValuesHolder pvhRotation = PropertyValuesHolder.ofKeyframe("rotation", kf0, kf1, kf2);
ObjectAnimator rotationAnim = ObjectAnimator.ofPropertyValuesHolder(target, pvhRotation)
rotationAnim.setDuration(5000ms);
</pre>

  <p>For a more complete example on how to use keyframes, see the <a href=
  "{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/animation/MultiPropertyAnimation.html">
  MultiPropertyAnimation</a> sample in APIDemos.</p>

  <h2 id="views">Animating Views</h2>

  <p>The property animation system allow streamlined animation of View objects and offerse
  a few advantages over the view animation system. The view
  animation system transformed View objects by changing the way that they were drawn. This was
  handled in the container of each View, because the View itself had no properties to manipulate.
  This resulted in the View being animated, but caused no change in the View object itself. This
  led to behavior such as an object still existing in its original location, even though it was
  drawn on a different location on the screen. In Android 3.0, new properties and the corresponding
  getter and setter methods were added to eliminate this drawback.</p>
  <p>The property animation system
  can animate Views on the screen by changing the actual properties in the View objects. In
  addition, Views also automatically call the {@link android.view.View#invalidate invalidate()}
  method to refresh the screen whenever its properties are changed. The new properties in the {@link
  android.view.View} class that facilitate property animations are:</p>

  <ul>
    <li><code>translationX</code> and <code>translationY</code>: These properties control where the
    View is located as a delta from its left and top coordinates which are set by its layout
    container.</li>

    <li><code>rotation</code>, <code>rotationX</code>, and <code>rotationY</code>: These properties
    control the rotation in 2D (<code>rotation</code> property) and 3D around the pivot point.</li>

    <li><code>scaleX</code> and <code>scaleY</code>: These properties control the 2D scaling of a
    View around its pivot point.</li>

    <li><code>pivotX</code> and <code>pivotY</code>: These properties control the location of the
    pivot point, around which the rotation and scaling transforms occur. By default, the pivot
    point is located at the center of the object.</li>

    <li><code>x</code> and <code>y</code>: These are simple utility properties to describe the
    final location of the View in its container, as a sum of the left and top values and
    translationX and translationY values.</li>

    <li><code>alpha</code>: Represents the alpha transparency on the View. This value is 1 (opaque)
    by default, with a value of 0 representing full transparency (not visible).</li>
  </ul>

  <p>To animate a property of a View object, such as its color or rotation value, all you need to
  do is create a property animator and specify the View property that you want to
  animate. For example:</p>
  <pre>
ObjectAnimator.ofFloat(myView, "rotation", 0f, 360f);
</pre>

<p>For more information on creating animators, see the sections on animating with
<a href="#value-animator">ValueAnimator</a> and <a href="#object-animator">ObjectAnimator</a>.
</p>

<h3 id="view-prop-animator">Animating with ViewPropertyAnimator</h3>
<p>The {@link android.view.ViewPropertyAnimator} provides a simple way to animate several
properties of a {@link android.view.View} in parallel, using a single underlying {@link
android.animation.Animator}
object. It behaves much like an {@link android.animation.ObjectAnimator}, because it modifies the
actual values of the view's properties, but is more efficient when animating many properties at
once. In addition, the code for using the {@link android.view.ViewPropertyAnimator} is much
more concise and easier to read. The following code snippets show the differences in using multiple
{@link android.animation.ObjectAnimator} objects, a single
{@link android.animation.ObjectAnimator}, and the {@link android.view.ViewPropertyAnimator} when
simultaneously animating the <code>x</code> and <code>y</code> property of a view.</p>

<p><strong>Multiple ObjectAnimator objects</strong></p>
<pre>
ObjectAnimator animX = ObjectAnimator.ofFloat(myView, "x", 50f);
ObjectAnimator animY = ObjectAnimator.ofFloat(myView, "y", 100f);
AnimatorSet animSetXY = new AnimatorSet();
animSetXY.playTogether(animX, animY);
animSetXY.start();
</pre>

<p><strong>One ObjectAnimator</strong></p>
<pre>
PropertyValuesHolder pvhX = PropertyValuesHolder.ofFloat("x", 50f);
PropertyValuesHolder pvhY = PropertyValuesHolder.ofFloat("y", 100f);
ObjectAnimator.ofPropertyValuesHolder(myView, pvhX, pvyY).start();
</pre>

<p><strong>ViewPropertyAnimator</strong></p>
<pre>
myView.animate().x(50f).y(100f);
</pre>

<p>
For more detailed information about {@link
android.view.ViewPropertyAnimator}, see the corresponding Android Developers
<a href="http://android-developers.blogspot.com/2011/05/introducing-viewpropertyanimator.html">blog
post</a>.</p>

<h2 id="declaring-xml">Declaring Animations in XML</h2>

  <p>The property animation system lets you declare property animations with XML instead of doing
  it programmatically. By defining your animations in XML, you can easily reuse your animations
in multiple activities and more easily edit the animation sequence.</p>

<p>To distinguish animation files that use the new property animation APIs from those that use the
legacy <a href="{@docRoot}guide/topics/graphics/view-animation.html">view animation</a> framework,
starting with Android 3.1, you should save the XML files for property animations in the {@code
res/animator/} directory (instead of {@code res/anim/}). Using the {@code animator} directory name
is optional, but necessary if you want to use the layout editor tools in the Eclipse ADT plugin (ADT
11.0.0+), because ADT only searches the {@code res/animator/} directory for property animation
resources.</p>

<p>The following property animation classes have XML declaration support with the
  following XML tags:</p>

  <ul>
    <li>{@link android.animation.ValueAnimator} - <code>&lt;animator&gt;</code></li>

    <li>{@link android.animation.ObjectAnimator} - <code>&lt;objectAnimator&gt;</code></li>

    <li>{@link android.animation.AnimatorSet} - <code>&lt;set&gt;</code></li>
  </ul>

<p>The following example plays the two sets of object animations sequentially, with the first nested
set playing two object animations together:</p>

<pre>
&lt;set android:ordering="sequentially"&gt;
    &lt;set&gt;
        &lt;objectAnimator
            android:propertyName="x"
            android:duration="500"
            android:valueTo="400"
            android:valueType="intType"/&gt;
        &lt;objectAnimator
            android:propertyName="y"
            android:duration="500"
            android:valueTo="300"
            android:valueType="intType"/&gt;
    &lt;/set&gt;
    &lt;objectAnimator
        android:propertyName="alpha"
        android:duration="500"
        android:valueTo="1f"/&gt;
&lt;/set&gt;
</pre>
  <p>In order to run this animation, you must inflate the XML resources in your code to an {@link
  android.animation.AnimatorSet} object, and then set the target objects for all of the animations
  before starting the animation set. Calling {@link android.animation.AnimatorSet#setTarget
  setTarget()} sets a single target object for all children of the {@link
  android.animation.AnimatorSet} as a convenience. The following code shows how to do this:</p>

<pre>
AnimatorSet set = (AnimatorSet) AnimatorInflater.loadAnimator(myContext,
    R.anim.property_animator);
set.setTarget(myObject);
set.start();
</pre>

<p>For information about the XML syntax for defining property animations, see <a
href="{@docRoot}guide/topics/resources/animation-resource.html#Property">Animation Resources</a>.