1 桌面壁纸
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先看桌面情况下壁纸，这个情况应该属于我们最为熟悉的，那么就不用多说，大概就是因为桌面Activity的配置主题xml设置一个类似showallpaper的属性既可以，就可以让桌面后面显示壁纸了
2 aosp编译后版本锁屏后点亮屏幕也可以看到桌面壁纸（注意这里还不是专门锁屏壁纸）
大家发现锁屏点亮后确实看到的壁纸和桌面的壁纸一模一样
3 疑惑
这里是不是大家就开始比较疑惑了，请问普通Activity是通过属性配置的，而且壁纸窗口又是在wms属于最底层的状态，他依附于桌面这个可以理解，但是为啥锁屏时候又可以看到壁纸呢？锁屏理论上也只是一个window盖在最顶层，理论即使这个锁屏window透明，那么透看到的也是有桌面Activity的情况，但现实情况是我们只看到了壁纸，并没有看到桌面
针对以上的疑惑，那接下来我们就需要去解答这个疑惑。
首先我们知道壁纸属于系统中一个特殊窗口，一直处于系统最底层。这个窗口在系统中有专门类进行他的显示情况，那就我们的WallpaperController类
这个WallpaperController.java中有log打印，但需要我们将对应标志位给设置一下：
if (DEBUG_WALLPAPER) {Slog.v(TAG, "Wallpaper visibility: " + visible + " at display "+ mDisplayContent.getDisplayId());}
这里我们把DEBUG_WALLPAPER就可以把log打开了
10-23 22:55:41.115 5313 6475 V WindowManager: Win Window{60c77f1 u0 ScreenDecorOverlayBottom}: isOnScreen=true mDrawState=410-23 22:55:41.115 5313 6475 V WindowManager: Win Window{df0a78b u0 ScreenDecorOverlay}: isOnScreen=true mDrawState=410-23 22:55:41.115 5313 6475 V WindowManager: Win Window{b7a35a u0 NavigationBar0}: isOnScreen=true mDrawState=410-23 22:55:41.115 5313 6475 V WindowManager: Win Window{3cd76e8 u0 NotificationShade}: isOnScreen=true mDrawState=410-23 22:55:41.115 5313 6475 V WindowManager: Found wallpaper target: Window{3cd76e8 u0 NotificationShade}10-23 22:55:41.115 5313 6475 V WindowManager: New wallpaper target: Window{3cd76e8 u0 NotificationShade} prevTarget: null10-23 22:55:41.116 5313 6475 V WindowManager: Report new wp offset Window{803c2b8 u0 com.android.systemui.ImageWallpaper} x=0.0 y=0.5 zoom=0.010-23 22:55:41.116 5313 6475 V WindowManager: Wallpaper visibility: true at display 010-23 22:55:41.116 5313 6475 D WindowManager: Wallpaper token android.os.Binder@3a6eda2 visible=true10-23 22:55:41.118 5313 6475 D WindowManager: New wallpaper: target=Window{3cd76e8 u0 NotificationShade} prev=null10-23 22:55:41.163 5313 5313 D WindowManager: powerPress: eventTime=118823847 interactive=true count=0 beganFromNonInteractive=true mShortPressOnPowerBehavior=1
打开后我们点亮锁屏和在桌面锁屏发现都有类似以下打印
WindowManager: Found wallpaper target: Window{3cd76e8 u0 NotificationShade}
看名字大家大概就知道这个log是在寻找一个wallpaper target:，而且找到的是NotificationShade即锁屏窗口
前面疑惑中就写到正常应该是桌面
10-24 00:18:50.543 10429 10450 V WindowManager: Found wallpaper target: Window{8d79f3 u0 com.android.launcher3/com.android.launcher3.uioverrides.QuickstepLauncher}
这日志其实就可以给我们非常重要线索，可以找出对应代码在哪
private final ToBooleanFunction<WindowState> mFindWallpaperTargetFunction = w -> {if ((w.mAttrs.type == TYPE_WALLPAPER)) {if (mFindResults.topWallpaper == null || mFindResults.resetTopWallpaper) {mFindResults.setTopWallpaper(w);mFindResults.resetTopWallpaper = false;}return false;}mFindResults.resetTopWallpaper = true;if (mService.mAtmService.getTransitionController().getTransitionPlayer() == null) {if (w.mActivityRecord != null && !w.mActivityRecord.isVisible()&& !w.mActivityRecord.isAnimating(TRANSITION | PARENTS)) {// If this window's app token is hidden and not animating, it is of no interest.if (DEBUG_WALLPAPER) Slog.v(TAG, "Skipping hidden and not animating token: " + w);return false;}} else {if (w.mActivityRecord != null && !w.mActivityRecord.isVisibleRequested()) {// An activity that is not going to remain visible shouldn't be the target.return false;}}if (DEBUG_WALLPAPER) Slog.v(TAG, "Win " + w + ": isOnScreen=" + w.isOnScreen()+ " mDrawState=" + w.mWinAnimator.mDrawState + " w.mWillReplaceWindow = " + w.mWillReplaceWindow);if (w.toString().contains("NotificationShade")) {Slog.v(TAG, "1111 Win " + w + ": isOnScreen=" + w.isOnScreen()+ " mDrawState=" + w.mWinAnimator.mDrawState + " w.mWillReplaceWindow = " + w.mWillReplaceWindow);}if (w.mWillReplaceWindow && mWallpaperTarget == null&& !mFindResults.useTopWallpaperAsTarget) {// When we are replacing a window and there was wallpaper before replacement, we want to// keep the window until the new windows fully appear and can determine the visibility,// to avoid flickering.mFindResults.setUseTopWallpaperAsTarget(true);}final WindowContainer animatingContainer = w.mActivityRecord != null? w.mActivityRecord.getAnimatingContainer() : null;final boolean keyguardGoingAwayWithWallpaper = (animatingContainer != null&& animatingContainer.isAnimating(TRANSITION | PARENTS)&& AppTransition.isKeyguardGoingAwayTransitOld(animatingContainer.mTransit)&& (animatingContainer.mTransitFlags& TRANSIT_FLAG_KEYGUARD_GOING_AWAY_WITH_WALLPAPER) != 0);boolean needsShowWhenLockedWallpaper = false;if ((w.mAttrs.flags & FLAG_SHOW_WHEN_LOCKED) != 0&& mService.mPolicy.isKeyguardLocked()&& (mService.mPolicy.isKeyguardOccluded()|| mService.mPolicy.isKeyguardUnoccluding())) {// The lowest show when locked window decides whether we need to put the wallpaper// behind.needsShowWhenLockedWallpaper = !isFullscreen(w.mAttrs)|| (w.mActivityRecord != null && !w.mActivityRecord.fillsParent());}if (keyguardGoingAwayWithWallpaper || needsShowWhenLockedWallpaper) {// Keep the wallpaper during Keyguard exit but also when it's needed for a// non-fullscreen show when locked activity.mFindResults.setUseTopWallpaperAsTarget(true);}final RecentsAnimationController recentsAnimationController =mService.getRecentsAnimationController();final boolean animationWallpaper = animatingContainer != null&& animatingContainer.getAnimation() != null&& animatingContainer.getAnimation().getShowWallpaper();final boolean hasWallpaper = w.hasWallpaper() || animationWallpaper;final boolean isRecentsTransitionTarget = (recentsAnimationController != null&& recentsAnimationController.isWallpaperVisible(w));if (isRecentsTransitionTarget) {if (DEBUG_WALLPAPER) Slog.v(TAG, "Found recents animation wallpaper target: " + w);mFindResults.setWallpaperTarget(w);return true;} else if (hasWallpaper && w.isOnScreen()&& (mWallpaperTarget == w || w.isDrawFinishedLw())) {if (DEBUG_WALLPAPER) Slog.v(TAG, "Found wallpaper target: " + w);mFindResults.setWallpaperTarget(w);if (w == mWallpaperTarget && w.isAnimating(TRANSITION | PARENTS)) {// The current wallpaper target is animating, so we'll look behind it for// another possible target and figure out what is going on later.if (DEBUG_WALLPAPER) Slog.v(TAG,"Win " + w + ": token animating, looking behind.");}// Found a target! End search.return true;}return false;};
上面最关键判断代码：
if (hasWallpaper && w.isOnScreen()&& (mWallpaperTarget == w || w.isDrawFinishedLw()))
这里的 hasWallpaper非常关键
final boolean hasWallpaper = w.hasWallpaper() || animationWallpaper;
主要调用是WindowState的方法
boolean hasWallpaper() {return (mAttrs.flags & FLAG_SHOW_WALLPAPER) != 0|| (mActivityRecord != null && mActivityRecord.hasWallpaperBackgroudForLetterbox());}
这里其实也就是判断window中是否有FLAG_SHOW_WALLPAPER属性，而且经过额外加log打印发现hasWallpaper值变化在锁屏window解锁前后
那么其实我们可以猜测是不是锁屏window会去动态改变自己的FLAG_SHOW_WALLPAPER属性，在有桌面显示时候锁屏的window实际是没有这个属性，在锁屏状态下是有这个属性。根据猜想去systemui代码中grep相关FLAG_SHOW_WALLPAPER关键字
果然发现有如下：
frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/NotificationShadeWindowControllerImpl.java
private void applyKeyguardFlags(State state) {final boolean scrimsOccludingWallpaper =state.mScrimsVisibility == ScrimController.OPAQUE || state.mLightRevealScrimOpaque;final boolean keyguardOrAod = state.mKeyguardShowing|| (state.mDozing && mDozeParameters.getAlwaysOn());if ((keyguardOrAod && !state.mBackdropShowing && !scrimsOccludingWallpaper)|| mKeyguardViewMediator.isAnimatingBetweenKeyguardAndSurfaceBehindOrWillBe()) {// Show the wallpaper if we're on keyguard/AOD and the wallpaper is not occluded by a// solid backdrop or scrim. Also, show it if we are currently animating between the// keyguard and the surface behind the keyguard - we want to use the wallpaper as a// backdrop for this animation.mLpChanged.flags |= LayoutParams.FLAG_SHOW_WALLPAPER;} else {mLpChanged.flags &= ~LayoutParams.FLAG_SHOW_WALLPAPER;}
这里其实就是代表有锁屏时候需要有 mLpChanged.flags |= LayoutParams.FLAG_SHOW_WALLPAPER;
锁屏退出时清除锁屏的FLAG_SHOW_WALLPAPER
mLpChanged.flags &= ~LayoutParams.FLAG_SHOW_WALLPAPER;
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