1 桌面壁纸

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=4
10-23 22:55:41.115  5313  6475 V WindowManager: Win Window{df0a78b u0 ScreenDecorOverlay}: isOnScreen=true mDrawState=4
10-23 22:55:41.115  5313  6475 V WindowManager: Win Window{b7a35a u0 NavigationBar0}: isOnScreen=true mDrawState=4
10-23 22:55:41.115  5313  6475 V WindowManager: Win Window{3cd76e8 u0 NotificationShade}: isOnScreen=true mDrawState=4
10-23 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: null
10-23 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=0.0
10-23 22:55:41.116  5313  6475 V WindowManager: Wallpaper visibility: true at display 0
10-23 22:55:41.116  5313  6475 D WindowManager: Wallpaper token android.os.Binder@3a6eda2 visible=true
10-23 22:55:41.118  5313  6475 D WindowManager: New wallpaper: target=Window{3cd76e8 u0 NotificationShade} prev=null
10-23 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;