Android launcher 桌面抽屉切换动画
浏览数：24 /
时间：2015年06月11日
版本：1.0
日期：2014.11.10 2014.11.11
版权：© 2014 kince 转载注明出处
一、概述
桌面抽屉之间的切换时Android用户经常触发的行为，好的交互会给用户一个舒适的体验。百度桌面默认是随机切换不同的动画，Android默认是一个大小和透明的渐变的动画，如下：
下面开始分析在Launcher2（KitKat）的源码里面是如何实现这种效果的。
二、下面列举相关的方法和变量
4082：
interface LauncherTransitionable {
View getContent();
void onLauncherTransitionPrepare(Launcher l, boolean animated, boolean toWorkspace);
void onLauncherTransitionStart(Launcher l, boolean animated, boolean toWorkspace);
void onLauncherTransitionStep(Launcher l, float t);
void onLauncherTransitionEnd(Launcher l, boolean animated, boolean toWorkspace);
}
2948：
exitSpringLoadedDragMode()
2926：
exitSpringLoadedDragModeDelayed()
2918：
enterSpringLoadedDragMode()
2899：
showAllApps()
2864：
showWorkspace()
2860：
showWorkspace()
2740：
hideAppsCustomizeHelper()
2573：
showAppsCustomizeHelper()
2498：
dispatchOnLauncherTransitionPrepare()
2097：
onClickAllAppsButton()
2048：
onTouch()
2008：
onClick()
1976：
onBackPressed()
1456：
onNewIntent
1273：
mReceiver
749：
onResume()三、分析
首先从最直观的方式开始，就是Dock栏进入抽屉的按钮.png)
。点击它会从桌面到抽屉，进入抽屉后再按返回键会从抽屉到桌面。这个按钮在Launcher类中对应的变量是mAllAppsButton，因为Launcher类继承了View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks,View.OnTouchListener这几个接口，所以点击事情需要由onClick(View v)方法来处理。在Launcher类的onClick(View v)方法中，
。点击它会从桌面到抽屉，进入抽屉后再按返回键会从抽屉到桌面。这个按钮在Launcher类中对应的变量是mAllAppsButton，因为Launcher类继承了View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks,View.OnTouchListener这几个接口，所以点击事情需要由onClick(View v)方法来处理。在Launcher类的onClick(View v)方法中，/**
* Launches the intent referred by the clicked shortcut.
*
* @param v The view representing the clicked shortcut.
*/
public void onClick(View v) {
// Make sure that rogue clicks don‘t get through while allapps is launching, or after the
// view has detached (it‘s possible for this to happen if the view is removed mid touch).
if (v.getWindowToken() == null) {
return;
}
if (!mWorkspace .isFinishedSwitchingState()) {
return;
}
Object tag = v.getTag();
if (tag instanceof ShortcutInfo) {
// Open shortcut
final Intent intent = ((ShortcutInfo) tag).intent;
int[] pos = new int[2];
v.getLocationOnScreen(pos);
intent.setSourceBounds( new Rect(pos[0], pos[1],
pos[0] + v.getWidth(), pos[1] + v.getHeight()));
boolean success = startActivitySafely(v, intent, tag);
if (success && v instanceof BubbleTextView) {
mWaitingForResume = (BubbleTextView) v;
mWaitingForResume.setStayPressed(true);
}
} else if (tag instanceof FolderInfo) {
if (v instanceof FolderIcon) {
FolderIcon fi = (FolderIcon) v;
handleFolderClick(fi);
}
} else if (v == mAllAppsButton ) {
if (isAllAppsVisible()) {
showWorkspace( true);
} else {
onClickAllAppsButton(v);
}
}
}
从标注地方可以看出，首先对View进行一个判断，如果是mAllAppsButton则开始下面的判断。如果是在抽屉里面，则进入到桌面；如果不是抽屉，则调用onClickAllAppsButton(v)方法。而onClickAllAppsButton(v)方法就是调用showAllApps方法，顾名思义就是进入后抽屉显示所有的app。接着在抽屉里面，如果要返回桌面，按Back键的话会调用onKeyDown或者onBackPressed()方法。
@Override
public boolean onKeyDown( int keyCode, KeyEvent event) {
final int uniChar = event.getUnicodeChar();
final boolean handled = super.onKeyDown(keyCode, event);
final boolean isKeyNotWhitespace = uniChar > 0 && !Character.isWhitespace(uniChar);
if (!handled && acceptFilter() && isKeyNotWhitespace) {
boolean gotKey = TextKeyListener.getInstance().onKeyDown( mWorkspace, mDefaultKeySsb,
keyCode, event);
if (gotKey && mDefaultKeySsb != null && mDefaultKeySsb.length() > 0) {
// something usable has been typed - start a search
// the typed text will be retrieved and cleared by
// showSearchDialog()
// If there are multiple keystrokes before the search dialog takes focus,
// onSearchRequested() will be called for every keystroke,
// but it is idempotent , so it‘s fine.
return onSearchRequested();
}
}
// Eat the long press event so the keyboard doesn‘t come up.
if (keyCode == KeyEvent.KEYCODE_MENU && event.isLongPress()) {
return true ;
}
return handled;
}
可以看到在onKeyDown方法中，没有任何关于进出抽屉或者桌面的方法。那么再来看一下onBackPressed()方法： @Override
public void onBackPressed() {
if (isAllAppsVisible()) {
showWorkspace( true);
} else if (mWorkspace .getOpenFolder() != null) {
Folder openFolder = mWorkspace.getOpenFolder();
if (openFolder.isEditingName()) {
openFolder.dismissEditingName();
} else {
closeFolder();
}
} else {
mWorkspace.exitWidgetResizeMode();
// Back button is a no-op here, but give at least some feedback for the button press
mWorkspace.showOutlinesTemporarily();
}
}
发现在这里处理了切换的过程。现在可以确定是显示桌面调用的是showWorkspace()方法，进入抽屉调用的是showAllApps()方法。在这两个方法中，又各自调用不同的方法实现各自的逻辑。
那么这两个方法都是在上面情况下调用的呢？先看showAllApps()方法。
launcher.java类：
1、onResume()
2、onClickAllAppsButton
再看一下showWorkspace()的情况，
1、onResume()
2、mReceiver()
3、onNewIntent()
4、startSearch()
5、startWallpaper()
6、onBackPressed()
7、onClick()
8、onTouch()
9、showWorkspace()
10、exitSpringLoadedDragModeDelayed()
可以发现showAllApps()方法只有被调用两次，而showWorkspace()有十次之多。这说明返回桌面的情形比返回抽屉的情况要多很多，而实际的使用情况也确实是这样的。
接着分别看一下showAllApps()和showWorkspace()各自的具体实现。
void showAllApps( boolean animated) {
if (mState != State.WORKSPACE) return;
showAppsCustomizeHelper(animated, false);
mAppsCustomizeTabHost.requestFocus();
// Change the state *after* we‘ve called all the transition code
mState = State. APPS_CUSTOMIZE;
// Pause the auto-advance of widgets until we are out of AllApps
mUserPresent = false ;
updateRunning();
closeFolder();
// Send an accessibility event to announce the context change
getWindow().getDecorView()
.sendAccessibilityEvent(AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED);
}
逐行分析下，第一行是一个判断，如果当前的状态不是在桌面，那么就退出这个方法。第二行代码就是进入抽屉的动画方法。第三行代码是给抽屉界面焦点。第四行代码进入抽屉后更改当前的状态。之后就不细说了。重点还是showAppsCustomizeHelper(animated, false );这个方法，它就是实现进入抽屉动画的方法。来到这个方法，发现其上面有很长的注释：
/**
* Things to test when changing the following seven functions.
* - Home from workspace
* - from center screen
* - from other screens
* - Home from all apps
* - from center screen
* - from other screens
* - Back from all apps
* - from center screen
* - from other screens
* - Launch app from workspace and quit
* - with back
* - with home
* - Launch app from all apps and quit
* - with back
* - with home
* - Go to a screen that‘s not the default, then all
* apps, and launch and app, and go back
* - with back
* -with home
* - On workspace, long press power and go back
* - with back
* - with home
* - On all apps, long press power and go back
* - with back
* - with home
* - On workspace, power off
* - On all apps, power off
* - Launch an app and turn off the screen while in that app
* - Go back with home key
* - Go back with back key TODO: make this not go to workspace
* - From all apps
* - From workspace
* - Enter and exit car mode (becuase it causes an extra configuration changed)
* - From all apps
* - From the center workspace
* - From another workspace
*/
注释下面是showAppsCustomizeHelper()和hideAppsCustomizeHelper()两个方法，顾名思义hideAppsCustomizeHelper就是离开抽屉的动画实现方法，这两个方法是相对立的。
/**
* Zoom the camera out from the workspace to reveal ‘toView‘.
* Assumes that the view to show is anchored at either the very top or very bottom
* of the screen.
*/
private void showAppsCustomizeHelper(final boolean animated, final boolean springLoaded) {
if (mStateAnimation != null) {
mStateAnimation.setDuration(0);
mStateAnimation.cancel();
mStateAnimation = null ;
}
final Resources res = getResources();
final int duration = res.getInteger(R.integer.config_appsCustomizeZoomInTime );
final int fadeDuration = res.getInteger(R.integer.config_appsCustomizeFadeInTime );
final float scale = (float) res.getInteger(R.integer.config_appsCustomizeZoomScaleFactor );
final View fromView = mWorkspace ;
final AppsCustomizeTabHost toView = mAppsCustomizeTabHost;
final int startDelay =
res.getInteger(R.integer.config_workspaceAppsCustomizeAnimationStagger );
setPivotsForZoom(toView, scale);
// Shrink workspaces away if going to AppsCustomize from workspace
Animator workspaceAnim =
mWorkspace.getChangeStateAnimation(Workspace.State.SMALL, animated);
if (animated) {
toView.setScaleX(scale);
toView.setScaleY(scale);
final LauncherViewPropertyAnimator scaleAnim = new LauncherViewPropertyAnimator(toView);
scaleAnim.
scaleX(1f).scaleY(1f).
setDuration(duration).
setInterpolator( new Workspace.ZoomOutInterpolator());
toView.setVisibility(View. VISIBLE);
toView.setAlpha(0f);
final ObjectAnimator alphaAnim = LauncherAnimUtils
. ofFloat(toView, "alpha", 0f, 1f)
.setDuration(fadeDuration);
alphaAnim.setInterpolator( new DecelerateInterpolator(1.5f));
alphaAnim.addUpdateListener( new AnimatorUpdateListener() {
@Override
public void onAnimationUpdate(ValueAnimator animation) {
if (animation == null) {
throw new RuntimeException("animation is null" );
}
float t = (Float) animation.getAnimatedValue();
dispatchOnLauncherTransitionStep(fromView, t);
dispatchOnLauncherTransitionStep(toView, t);
}
});
// toView should appear right at the end of the workspace shrink
// animation
mStateAnimation = LauncherAnimUtils.createAnimatorSet();
mStateAnimation.play(scaleAnim).after(startDelay);
mStateAnimation.play(alphaAnim).after(startDelay);
mStateAnimation.addListener(new AnimatorListenerAdapter() {
boolean animationCancelled = false;
@Override
public void onAnimationStart(Animator animation) {
updateWallpaperVisibility( true);
// Prepare the position
toView.setTranslationX(0.0f);
toView.setTranslationY(0.0f);
toView.setVisibility(View. VISIBLE);
toView.bringToFront();
}
@Override
public void onAnimationEnd(Animator animation) {
dispatchOnLauncherTransitionEnd(fromView, animated, false);
dispatchOnLauncherTransitionEnd(toView, animated, false);
if (mWorkspace != null && !springLoaded && !LauncherApplication.isScreenLarge()) {
// Hide the workspace scrollbar
mWorkspace.hideScrollingIndicator(true);
hideDockDivider();
}
if (!animationCancelled ) {
updateWallpaperVisibility( false);
}
// Hide the search bar
if (mSearchDropTargetBar != null) {
mSearchDropTargetBar.hideSearchBar(false);
}
}
@Override
public void onAnimationCancel(Animator animation) {
animationCancelled = true ;
}
});
if (workspaceAnim != null) {
mStateAnimation.play(workspaceAnim);
}
boolean delayAnim = false;
dispatchOnLauncherTransitionPrepare(fromView, animated, false);
dispatchOnLauncherTransitionPrepare(toView, animated, false);
// If any of the objects being animated haven‘t been measured/laid out
// yet, delay the animation until we get a layout pass
if ((((LauncherTransitionable) toView).getContent().getMeasuredWidth() == 0) ||
( mWorkspace.getMeasuredWidth() == 0) ||
(toView.getMeasuredWidth() == 0)) {
delayAnim = true;
}
final AnimatorSet stateAnimation = mStateAnimation;
final Runnable startAnimRunnable = new Runnable() {
public void run() {
// Check that mStateAnimation hasn‘t changed while
// we waited for a layout/draw pass
if (mStateAnimation != stateAnimation)
return;
setPivotsForZoom(toView, scale);
dispatchOnLauncherTransitionStart(fromView, animated, false);
dispatchOnLauncherTransitionStart(toView, animated, false);
LauncherAnimUtils.startAnimationAfterNextDraw( mStateAnimation, toView);
}
};
if (delayAnim) {
final ViewTreeObserver observer = toView.getViewTreeObserver();
observer.addOnGlobalLayoutListener( new OnGlobalLayoutListener() {
public void onGlobalLayout() {
startAnimRunnable.run();
toView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
});
} else {
startAnimRunnable.run();
}
} else {
toView.setTranslationX(0.0f);
toView.setTranslationY(0.0f);
toView.setScaleX(1.0f);
toView.setScaleY(1.0f);
toView.setVisibility(View. VISIBLE);
toView.bringToFront();
if (!springLoaded && !LauncherApplication.isScreenLarge()) {
// Hide the workspace scrollbar
mWorkspace.hideScrollingIndicator(true);
hideDockDivider();
// Hide the search bar
if (mSearchDropTargetBar != null) {
mSearchDropTargetBar.hideSearchBar(false);
}
}
dispatchOnLauncherTransitionPrepare(fromView, animated, false);
dispatchOnLauncherTransitionStart(fromView, animated, false);
dispatchOnLauncherTransitionEnd(fromView, animated, false);
dispatchOnLauncherTransitionPrepare(toView, animated, false);
dispatchOnLauncherTransitionStart(toView, animated, false);
dispatchOnLauncherTransitionEnd(toView, animated, false);
updateWallpaperVisibility( false);
}
}
变量mStateAnimation的类型是AnimatorSet，是专门负责上述两个方法里面的动画执行。变量duration、fadeDuration、scale分别是进入抽屉动画的伸缩的时间、透明度改变的时间以及伸缩的大小。从下面两行代码可以看出final View fromView = mWorkspace; final AppsCustomizeTabHost toView = mAppsCustomizeTabHost ;fromView就是桌面，而toView就是抽屉。startDelay是动画之前的准备时间。
setPivotsForZoom(toView, scale);方法是对View进行一个缩放，scale是缩放的参数。
workspaceAnim是桌面消失的动画，先不去看具体实现。如果有动画需求进入if判断，在这里有一个scaleAnim和alphaAnim，这就是抽屉出现的动画，从代码
mStateAnimation = LauncherAnimUtils.createAnimatorSet(); mStateAnimation.play(scaleAnim).after(startDelay); mStateAnimation.play(alphaAnim).after(startDelay);可以看出二者同时执行。接着是mStateAnimation动画的回调接口，具体逻辑不再分析。然后如果workspaceAnim不为空的话，就执行说明消失的动画。再看delayAnim变量，这是用来判断是否需要延迟动画执行。如果需要的话就监听View树是绘制，绘制完毕之后再执行动画；否则执行进入抽屉的动画。还有一个重要的地方是在在上面的方法中出现了
dispatchOnLauncherTransitionPrepare(fromView, animated, false); dispatchOnLauncherTransitionStart(fromView, animated, false); dispatchOnLauncherTransitionEnd(fromView, animated, false);等方法。其实这是在Launcher类中定义的接口里面的方法，具体如下：
interface LauncherTransitionable {
View getContent();
void onLauncherTransitionPrepare(Launcher l, boolean animated,
boolean toWorkspace);
void onLauncherTransitionStart(Launcher l, boolean animated,
boolean toWorkspace);
void onLauncherTransitionStep(Launcher l, float t);
void onLauncherTransitionEnd(Launcher l, boolean animated,
boolean toWorkspace);
}
分别在Workspace、AppsCustomizePagedView、AppsCustomizeTabHost、PagedView中继承这个接口。最后回过头看一下桌面消失动画的实现，它是在Workspace类里面处理的。
Animator getChangeStateAnimation (final SizeState state, boolean animated, int delay) {
Log.i(TAG, "getChangeStateAnimation");
if (mSizeState == state) {
return null ;
}
// Initialize animation arrays for the first time if necessary
initAnimationArrays();
AnimatorSet anim = animated ? LauncherAnimUtils.createAnimatorSet() : null;
// Stop any scrolling, move to the current page right away
setCurrentPage(getNextPage());
final boolean isEditViewMode = isEditViewMode(state);
final SizeState oldState = mSizeState ;
final boolean oldStateIsNormal = (oldState == SizeState.NORMAL);
final boolean oldStateIsSpringLoaded = (oldState == SizeState.SPRING_LOADED );
final boolean oldStateIsSmall = (oldState == SizeState.SMALL);
mSizeState = state;
final boolean stateIsNormal = (state == SizeState.NORMAL);
final boolean stateIsSpringLoaded = (state == SizeState.SPRING_LOADED );
final boolean stateIsSmall = (state == SizeState.SMALL);
float finalScaleFactor = 1.0f;
float finalBackgroundAlpha = (stateIsSpringLoaded || isEditViewMode) ? 1.0f: 0f;
float translationX = 0;
float translationY = 0;
boolean zoomIn = true;
if (state != SizeState.NORMAL) {
if (isEditViewMode) {
finalScaleFactor = getCellLayoutScale(state);
} else {
finalScaleFactor = mSpringLoadedShrinkFactor- (state == SizeState.SMALL ? 0.1f : 0);
}
finalScaleFactor = mSpringLoadedShrinkFactor - (stateIsSmall ? 0.1f : 0);
setPageSpacing(mSpringLoadedPageSpacing);
if (oldStateIsNormal && stateIsSmall) {
zoomIn = false;
setLayoutScale(finalScaleFactor);
updateChildrenLayersEnabled( false);
} else {
finalBackgroundAlpha = 1.0f;
setLayoutScale(finalScaleFactor);
}
} else {
setPageSpacing(mOriginalPageSpacing);
setLayoutScale(1.0f);
}
final int duration;
if (isEditViewMode) {
duration = getResources().getInteger(R.integer.config_overviewTransitionTime );
} else if (zoomIn) {
duration = getResources().getInteger(R.integer.config_workspaceUnshrinkTime );
} else {
duration = getResources().getInteger(R.integer.config_appsCustomizeWorkspaceShrinkTime );
}
for (int i = 0; i < getChildCount(); i++) {
final CellLayout cl = (CellLayout) getChildAt(i);
float finalAlpha = (!mWorkspaceFadeInAdjacentScreens || stateIsSpringLoaded ||(i == mCurrentPage)) ? 1f : 0f;
float currentAlpha = cl.getShortcutsAndWidgets().getAlpha();
float initialAlpha = currentAlpha;
// Determine the pages alpha during the state transition
if ((oldStateIsSmall && stateIsNormal) || (oldStateIsNormal && stateIsSmall)) {
// To/from workspace - only show the current page unless the transition is not
// animated and the animation end callback below doesn‘t run;
// or, if we‘re in spring-loaded mode
if (i == mCurrentPage || !animated || oldStateIsSpringLoaded) {
finalAlpha = 1f;
} else {
initialAlpha = 0f;
finalAlpha = 0f;
}
}
mOldAlphas[i] = initialAlpha;
mNewAlphas[i] = finalAlpha;
if (animated) {
mOldTranslationXs[i] = cl.getTranslationX();
mOldTranslationYs[i] = cl.getTranslationY();
mOldScaleXs[i] = cl.getScaleX();
mOldScaleYs[i] = cl.getScaleY();
mOldBackgroundAlphas[i] = cl.getBackgroundAlpha();
mNewTranslationXs[i] = translationX;
mNewTranslationYs[i] = translationY;
mNewScaleXs[i] = finalScaleFactor;
mNewScaleYs[i] = finalScaleFactor;
mNewBackgroundAlphas[i] = finalBackgroundAlpha;
} else {
cl.setTranslationX(translationX);
cl.setTranslationY(translationY);
cl.setScaleX(finalScaleFactor);
cl.setScaleY(finalScaleFactor);
cl.setBackgroundAlpha(finalBackgroundAlpha);
cl.setShortcutAndWidgetAlpha(finalAlpha);
}
cl.isEditViewMode(isEditViewMode);
}
if (animated) {
for (int index = 0; index < getChildCount(); index++) {
final int i = index;
final CellLayout cl = (CellLayout) getChildAt(i);
float currentAlpha = cl.getShortcutsAndWidgets().getAlpha();
if (mOldAlphas [i] == 0 && mNewAlphas[i] == 0) {
cl.setTranslationX(mNewTranslationXs [i]);
cl.setTranslationY(mNewTranslationYs [i]);
cl.setScaleX( mNewScaleXs[i]);
cl.setScaleY( mNewScaleYs[i]);
cl.setBackgroundAlpha(mNewBackgroundAlphas [i]);
cl.setShortcutAndWidgetAlpha(mNewAlphas [i]);
cl.setRotationY( mNewRotationYs[i]);
} else {
LauncherViewPropertyAnimator a = new LauncherViewPropertyAnimator(cl);
a.translationX( mNewTranslationXs[i])
.translationY(mNewTranslationYs [i])
.scaleX( mNewScaleXs[i])
.scaleY( mNewScaleYs[i])
.setDuration(duration)
.setInterpolator(mZoomInInterpolator );
anim.play(a);
if (mOldAlphas [i] != mNewAlphas [i] || currentAlpha != mNewAlphas [i]) {
LauncherViewPropertyAnimator alphaAnim =
new LauncherViewPropertyAnimator(cl.getShortcutsAndWidgets());
alphaAnim.alpha( mNewAlphas[i])
.setDuration(duration)
.setInterpolator(mZoomInInterpolator );
anim.play(alphaAnim);
}
if (mOldBackgroundAlphas [i] != 0 ||
mNewBackgroundAlphas[i] != 0) {
ValueAnimator bgAnim =
LauncherAnimUtils.ofFloat(cl, 0f, 1f).setDuration(duration);
bgAnim.setInterpolator(mZoomInInterpolator );
bgAnim.addUpdateListener( new LauncherAnimatorUpdateListener() {
public void onAnimationUpdate(float a, float b) {
cl.setBackgroundAlpha(
a * mOldBackgroundAlphas[i] +
b * mNewBackgroundAlphas[i]);
}
});
anim.play(bgAnim);
}
}
}
anim.setStartDelay(delay);
}
if (stateIsSpringLoaded) {
// Right now we‘re covered by Apps Customize
// Show the background gradient immediately, so the gradient will
// be showing once AppsCustomize disappears
animateBackgroundGradient(getResources().getInteger(
R.integer.config_appsCustomizeSpringLoadedBgAlpha ) / 100f, false);
} else {
// Fade the background gradient away
animateBackgroundGradient(0f, true);
}
return anim;
}
在说明方法之前先说一下，Launcher2和Launcher3桌面消失动画的区别。Launcher2进入抽屉后是一个黑色的背景，没有壁纸显示效果，而Launcher3可以看到壁纸。所以getChangeStateAnimation 方法也是有一定区别的。关于这个方法大体可以分为三个部分，一是变量初始赋值阶段，二是动画设置阶段，三是动画返回阶段。重点是第二个动画设置阶段，因为桌面的消失、出现都会调用这个方法。同样的关于这个动画最主要的部分还是伸缩和透明度的变化，理论上应该是这样的，桌面消失时开始变小、透明度逐渐不可见；桌面出现时开始变大、透明度逐渐可见。不过从这个方法的实现来看，其性能消耗比较大。它是用一个for循环来遍历Workspace的CellLayout的个数，也就是有几个桌面。然后再对每个CellLayout进行一个动画效果。亲测在低端机器上比如联想s868t是非常卡顿的。
hideAppsCustomizeHelper()的实现就不再赘述，和showAppsCustomizeHelper()实现机制都是一样的。那么最后总结一下，从桌面进入抽屉调用
hideAppsCustomizeHelper()方法，这个方法实现进入抽屉的动画效果，具体说就是对抽屉这个View进行一个动画。在这方法里面还有一个桌面消失的动画getChangeStateAnimation ，在这个动画执行完毕后才会执行进入抽屉的动画。然后从抽屉返回桌面正好是相反的流程。现在很多桌面进入抽屉都会有其他的动画效果，比如反转、上下等效果，其实就是对以上三个方法进行修改添加就可以了。
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