android_frameworks_base/docs/html/training/animation/crossfade.jd

page.title=Crossfading Two Views
trainingnavtop=true


@jd:body

    <div id="tb-wrapper">
      <div id="tb">
        <h2>
          This lesson teaches you to:
        </h2>
        <ol>
          <li>
            <a href="#views">Create the Views</a>
          </li>
          <li>
            <a href="#setup">Set up the Animation</a>
          </li>
          <li>
            <a href="#animate">Crossfade the Views</a>
          </li>
        </ol>
    </div>
    </div>

    <p>
      Crossfade animations (also know as dissolve) gradually fade out one UI component while simultaneously fading in
      another. This animation is useful for situations where you want to switch content or views
      in your app. Crossfades are very subtle and short but offer a fluid transition from one screen to the
      next. When you don't use them, however, transitions often feel abrupt or hurried.
    </p>
    <p>Here's an example of a crossfade from a progress indicator to some text content.
    </p>

<div class="framed-galaxynexus-land-span-8">
  <video class="play-on-hover" autoplay>
    <source src="anim_crossfade.mp4" type="video/mp4">
    <source src="anim_crossfade.webm" type="video/webm">
    <source src="anim_crossfade.ogv" type="video/ogg">
  </video>
</div>
<div class="figure-caption">
Crossfade animation
  <div class="video-instructions">&nbsp;</div>
</div>

    <p>
      If you want to jump ahead and see a full working example,
      <a href="{@docRoot}shareables/training/Animations.zip">download</a>
      and run the sample app and select the Crossfade example.
      See the following files for the code implementation:
    </p>
    <ul>
      <li>
        <code>src/CrossfadeActivity.java</code>
      </li>
      <li>
        <code>layout/activity_crossfade.xml</code>
      </li>
      <li>
        <code>menu/activity_crossfade.xml</code>
      </li>
    </ul>
    <h2 id="views">
      Create the Views
    </h2>
    <p>
      Create the two views that you want to crossfade. The following example creates a progress
      indicator and a scrollable text view:
    </p>
    <pre>
&lt;FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"&gt;

    &lt;ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"&gt;

        &lt;TextView style="?android:textAppearanceMedium"
            android:lineSpacingMultiplier="1.2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@string/lorem_ipsum"
            android:padding="16dp" /&gt;

    &lt;/ScrollView&gt;

    &lt;ProgressBar android:id="@+id/loading_spinner"
        style="?android:progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center" /&gt;

&lt;/FrameLayout&gt;
</pre>
    <h2 id="setup">
      Set up the Animation
    </h2>
    <p>
      To set up the animation:
    </p>
    <ol>
      <li>Create member variables for the views that you want to crossfade. You need
      these references later when modifying the views during the animation.
      </li>
      <li>For the view that is being faded in, set its visibility to {@link
      android.view.View#GONE}. This prevents the view from taking up layout space and omits it
      from layout calculations, speeding up processing.
      </li>
      <li>Cache the <code>{@link android.R.integer#config_shortAnimTime}</code>
      system property in a member variable. This property defines a standard
      "short" duration for the animation. This duration is ideal for subtle animations or
      animations that occur very frequently. {@link android.R.integer#config_longAnimTime} and
      {@link android.R.integer#config_mediumAnimTime} are also available if you wish to use them.
      </li>
    </ol>
    <p>
      Here's an example using the layout from the previous code snippet as the activity content
      view:
    </p>
    <pre>
public class CrossfadeActivity extends Activity {

    private View mContentView;
    private View mLoadingView;
    private int mShortAnimationDuration;

    ...

    &#64;Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_crossfade);

        mContentView = findViewById(R.id.content);
        mLoadingView = findViewById(R.id.loading_spinner);

        // Initially hide the content view.
        mContentView.setVisibility(View.GONE);

        // Retrieve and cache the system's default "short" animation time.
        mShortAnimationDuration = getResources().getInteger(
                android.R.integer.config_shortAnimTime);
    }

</pre>
    <h2 id="animate">
      Crossfade the Views
    </h2>
    <p>
      Now that the views are properly set up, crossfade them by doing the following:
    </p>
    <ol>
      <li>For the view that is fading in, set the alpha value to <code>0</code> and the visibility
      to {@link android.view.View#VISIBLE}. (Remember that it was initially set to {@link
      android.view.View#GONE}.) This makes the view visible but completely transparent.
      </li>
      <li>For the view that is fading in, animate its alpha value from <code>0</code> to
      <code>1</code>. At the same time, for the view that is fading out, animate the alpha value
      from <code>1</code> to <code>0</code>.
      </li>
      <li>Using {@link android.animation.Animator.AnimatorListener#onAnimationEnd onAnimationEnd()}
      in an {@link android.animation.Animator.AnimatorListener}, set the visibility of the view
      that was fading out to {@link android.view.View#GONE}. Even though the alpha value is <code>0</code>,
      setting the view's visibility to {@link android.view.View#GONE} prevents the view from taking
      up layout space and omits it from layout calculations, speeding up processing.
      </li>
    </ol>
    <p>
      The following method shows an example of how to do this:
    </p>
    <pre>
private View mContentView;
private View mLoadingView;
private int mShortAnimationDuration;

...

private void crossfade() {

    // Set the content view to 0% opacity but visible, so that it is visible
    // (but fully transparent) during the animation.
    mContentView.setAlpha(0f);
    mContentView.setVisibility(View.VISIBLE);

    // Animate the content view to 100% opacity, and clear any animation
    // listener set on the view.
    mContentView.animate()
            .alpha(1f)
            .setDuration(mShortAnimationDuration)
            .setListener(null);

    // Animate the loading view to 0% opacity. After the animation ends,
    // set its visibility to GONE as an optimization step (it won't
    // participate in layout passes, etc.)
    mHideView.animate()
            .alpha(0f)
            .setDuration(mShortAnimationDuration)
            .setListener(new AnimatorListenerAdapter() {
                &#64;Override
                public void onAnimationEnd(Animator animation) {
                    mHideView.setVisibility(View.GONE);
                }
            });
}
</pre>