版本：1.0 日期：.11.10.11.11 版权：© 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()：onClick()1976：onBackPressed()1456：onNewIntent1273：mReceiver749：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。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 shortcutfinal 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。
@Overridepublic 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：
@Overridepublic 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 pressmWorkspace.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 codemState = State. APPS_CUSTOMIZE;// Pause the auto-advance of widgets until we are out of AllAppsmUserPresent = false ;updateRunning();closeFolder();// Send an accessibility event to announce the context changegetWindow().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 workspaceAnimator 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() {@Overridepublic 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// animationmStateAnimation = LauncherAnimUtils.createAnimatorSet();mStateAnimation.play(scaleAnim).after(startDelay);mStateAnimation.play(alphaAnim).after(startDelay);mStateAnimation.addListener(new AnimatorListenerAdapter() {boolean animationCancelled = false;@Overridepublic void onAnimationStart(Animator animation) {updateWallpaperVisibility( true);// Prepare the positiontoView.setTranslationX(0.0f);toView.setTranslationY(0.0f);toView.setVisibility(View. VISIBLE);toView.bringToFront();}@Overridepublic void onAnimationEnd(Animator animation) {dispatchOnLauncherTransitionEnd(fromView, animated, false);dispatchOnLauncherTransitionEnd(toView, animated, false);if (mWorkspace != null && !springLoaded && !LauncherApplication.isScreenLarge()) {// Hide the workspace scrollbarmWorkspace.hideScrollingIndicator(true);hideDockDivider();}if (!animationCancelled ) {updateWallpaperVisibility( false);}// Hide the search barif (mSearchDropTargetBar != null) {mSearchDropTargetBar.hideSearchBar(false);}}@Overridepublic 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 passif ((((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 passif (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 scrollbarmWorkspace.hideScrollingIndicator(true);hideDockDivider();// Hide the search barif (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 necessaryinitAnimationArrays();AnimatorSet anim = animated ? LauncherAnimUtils.createAnimatorSet() : null;// Stop any scrolling, move to the current page right awaysetCurrentPage(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 transitionif ((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 modeif (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 disappearsanimateBackgroundGradient(getResources().getInteger(R.integer.config_appsCustomizeSpringLoadedBgAlpha ) / 100f, false);} else {// Fade the background gradient awayanimateBackgroundGradient(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。
