Crossfading Two Views 淡入淡出的两种观点

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.

Here's an example of a crossfade from a progress indicator to some text content.http://blog.youkuaiyun.com/sergeycao

If you want to jump ahead and see a full working example, download and run the sample app and select the Crossfade example. See the following files for the code implementation:

• src/CrossfadeActivity.java
• layout/activity_crossfade.xml
• menu/activity_crossfade.xml

Create the Views

Create the two views that you want to crossfade. The following example creates a progress indicator and a scrollable text view:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <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" />

    </ScrollView>

    <ProgressBar android:id="@+id/loading_spinner"
        style="?android:progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center" />

</FrameLayout>

Set up the Animation

To set up the animation:

1. Create member variables for the views that you want to crossfade. You need these references later when modifying the views during the animation.
2. For the view that is being faded in, set its visibility to GONE. This prevents the view from taking up layout space and omits it from layout calculations, speeding up processing.
3. Cache the config_shortAnimTime 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. config_longAnimTime and config_mediumAnimTime are also available if you wish to use them.

Here's an example using the layout from the previous code snippet as the activity content view:

public class CrossfadeActivity extends Activity {

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

    ...

    @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);
    }

Crossfade the Views

Now that the views are properly set up, crossfade them by doing the following:

1. For the view that is fading in, set the alpha value to 0 and the visibility to VISIBLE. (Remember that it was initially set to GONE.) This makes the view visible but completely transparent.
2. For the view that is fading in, animate its alpha value from 0 to 1. At the same time, for the view that is fading out, animate the alpha value from 1 to 0.
3. Using onAnimationEnd() in an Animator.AnimatorListener, set the visibility of the view that was fading out to GONE. Even though the alpha value is 0, setting the view's visibility to GONE prevents the view from taking up layout space and omits it from layout calculations, speeding up processing.

The following method shows an example of how to do this:

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() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    mHideView.setVisibility(View.GONE);
                }
            });
}