一、启动窗口的类型

 private static final int STARTING_WINDOW_TYPE_NONE = 0; //不使用启动窗口private static final int STARTING_WINDOW_TYPE_SNAPSHOT = 1; //应用热启动，启动窗口是activity的一张截图private static final int STARTING_WINDOW_TYPE_SPLASH_SCREEN = 2; //应用冷启动，启动窗口是应用配置的一张图片

二、启动窗口的意义

应用有三种启动状态：冷启动、温启动、热启动
冷启动：应用自设备开机后首次启动；系统终止应用后，应用首次启动。时间开销最大。
热启动：应用的所有Activity仍驻留在内存中，系统的全部工作只是将Activity重新带到前台。时间开销最小。
温启动：应用进程已经存在，但应用必须通过调用onCreate()从头重新创建Activity。时间开销中间。

应用冷启动时，自点击home launcher的应用Icon起，到应用完成第一次绘制，系统进程、应用进程要完成大量的初始化工作，非常费时。假如没有starting window，这一过程给用户的感受就是：这应用好卡，点击之后3秒钟过去了，还没反应；或者，我是不是没点到应用图标上。starting window的意义就在于，用户点击应用Icon之后，启动窗口会立即打开，等应用完成第一次绘制后，启动窗口被系统销毁，纵享丝滑。

三、启动窗口的流程

3.1 ActivityStack.SHOW_APP_STARTING_PREVIEW

    // Set to false to disable the preview that is shown while a new activity// is being started.private static final boolean SHOW_APP_STARTING_PREVIEW = true; // 1. starting window的总开关...
void startActivityLocked(ActivityRecord r, ActivityRecord focusedTopActivity,boolean newTask, boolean keepCurTransition, ActivityOptions options) {Task rTask = r.getTask();final boolean allowMoveToFront = options == null || !options.getAvoidMoveToFront();final boolean isOrhasTask = rTask == this || hasChild(rTask);...} else if (SHOW_APP_STARTING_PREVIEW && doShow) {// Figure out if we are transitioning from another activity that is// "has the same starting icon" as the next one.  This allows the// window manager to keep the previous window it had previously// created, if it still had one.Task prevTask = r.getTask();ActivityRecord prev = prevTask.topActivityWithStartingWindow();if (prev != null) {// We don't want to reuse the previous starting preview if:// (1) The current activity is in a different task.if (prev.getTask() != prevTask) {prev = null;}// (2) The current activity is already displayed.else if (prev.nowVisible) {prev = null;}}r.showStartingWindow(prev, newTask, isTaskSwitch(r, focusedTopActivity)); // 2. 调用ActivityRecord默认方法，两个类在同一个package，无需import}} else {// If this is the first activity, don't do any fancy animations,// because there is nothing for it to animate on top of.ActivityOptions.abort(options);}}

注释1：该boolean值是ActivityStack类的全局变量，正如英文注释，此布尔值是startingwindow的总开关。
注释2：r是startActivityLocked方法的一个形参，是ActivityRecord类型的变量，调用ActivityRecord.showStartingWindow()方法。

3.1 ActivityRecord.showStartingWindow()

    static final int STARTING_WINDOW_NOT_SHOWN = 0;static final int STARTING_WINDOW_SHOWN = 1;static final int STARTING_WINDOW_REMOVED = 2;int mStartingWindowState = STARTING_WINDOW_NOT_SHOWN; // 1. 启动窗口的状态void showStartingWindow(ActivityRecord prev, boolean newTask, boolean taskSwitch) {if (mTaskOverlay) {// We don't show starting window for overlay activities.return;}if (pendingOptions != null&& pendingOptions.getAnimationType() == ActivityOptions.ANIM_SCENE_TRANSITION) {// Don't show starting window when using shared element transition.return;}final CompatibilityInfo compatInfo =mAtmService.compatibilityInfoForPackageLocked(info.applicationInfo);final boolean shown = addStartingWindow(packageName, theme,prev != null ? prev.appToken : null, newTask, taskSwitch, isProcessRunning(),allowTaskSnapshot(),mState.ordinal() >= STARTED.ordinal() && mState.ordinal() <= STOPPED.ordinal()); // 2. 调用本类方法if (shown) {mStartingWindowState = STARTING_WINDOW_SHOWN; // 3. 置启动窗口的状态}}

注释1：全局变量，访问权限为default。描述启动窗口的状态，一共有3种状态：默认的not_shown；添加成功后的shown；移除后的removed。
注释2：调用本类的addStartingWindow方法，返回值类型为boolean值，代表启动窗口是否添加成功。
注释3：启动窗口添加成功后，其状态设置为shown。

3.3 ActivityRecord.addStartingWindow()

boolean addStartingWindow(String pkg, int theme, CompatibilityInfo compatInfo,CharSequence nonLocalizedLabel, int labelRes, int icon, int logo, int windowFlags,IBinder transferFrom, boolean newTask, boolean taskSwitch, boolean processRunning,boolean allowTaskSnapshot, boolean activityCreated) {...final int type = getStartingWindowType(newTask, taskSwitch, processRunning,allowTaskSnapshot, activityCreated, snapshot);if (type == STARTING_WINDOW_TYPE_SNAPSHOT) {if (isActivityTypeHome()) {// The snapshot of home is only used once because it won't be updated while screen// is on (see {@link TaskSnapshotController#screenTurningOff}).mWmService.mTaskSnapshotController.removeSnapshotCache(task.mTaskId);if ((mDisplayContent.mAppTransition.getTransitFlags()& WindowManager.TRANSIT_FLAG_KEYGUARD_GOING_AWAY_NO_ANIMATION) == 0) {// Only use snapshot of home as starting window when unlocking directly.return false;}}return createSnapshot(snapshot); // 1. 应用热启动}// If this is a translucent window, then don't show a starting window -- the current// effect (a full-screen opaque starting window that fades away to the real contents// when it is ready) does not work for this.ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Checking theme of starting window: 0x%x", theme);if (theme != 0) {AttributeCache.Entry ent = AttributeCache.instance().get(pkg, theme,com.android.internal.R.styleable.Window,mWmService.mCurrentUserId);if (ent == null) {// Whoops!  App doesn't exist. Um. Okay. We'll just pretend like we didn't// see that.return false;}final boolean windowIsTranslucent = ent.array.getBoolean(com.android.internal.R.styleable.Window_windowIsTranslucent, false); // 2. 查看启动Activity主题的windowIsTranslucent属性final boolean windowIsFloating = ent.array.getBoolean(com.android.internal.R.styleable.Window_windowIsFloating, false); // 3. 查看windowIsFloating属性final boolean windowShowWallpaper = ent.array.getBoolean(com.android.internal.R.styleable.Window_windowShowWallpaper, false);final boolean windowDisableStarting = ent.array.getBoolean(com.android.internal.R.styleable.Window_windowDisablePreview, false); // 4.查看windowDisablePreview属性ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Translucent=%s Floating=%s ShowWallpaper=%s",windowIsTranslucent, windowIsFloating, windowShowWallpaper);if (windowIsTranslucent) {return false;}if (windowIsFloating || windowDisableStarting) {return false;}if (windowShowWallpaper) {if (getDisplayContent().mWallpaperController.getWallpaperTarget() == null) {// If this theme is requesting a wallpaper, and the wallpaper// is not currently visible, then this effectively serves as// an opaque window and our starting window transition animation// can still work.  We just need to make sure the starting window// is also showing the wallpaper.windowFlags |= FLAG_SHOW_WALLPAPER;} else {return false;}}}if (transferStartingWindow(transferFrom)) {return true;}// There is no existing starting window, and we don't want to create a splash screen, so// that's it!if (type != STARTING_WINDOW_TYPE_SPLASH_SCREEN) { // 5. 代码走到这里，说明启动窗口的类型不是snapshot。如果也不是splash，那就只能是none.return false;}ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Creating SplashScreenStartingData");mStartingData = new SplashScreenStartingData(mWmService, pkg, // 6. mStartingData赋值theme, compatInfo, nonLocalizedLabel, labelRes, icon, logo, windowFlags,// getMergedOverrideConfiguration());getMergedOverrideConfiguration(), overrideTheme);scheduleAddStartingWindow(); // 7. 发送消息return true;}

注释1：return createSnapshot(snapshot);启动窗口类型为snapshot，给mStartingData赋值snapshotdata，然后发送消息。

    private boolean createSnapshot(ActivityManager.TaskSnapshot snapshot) {if (snapshot == null) {return false;}ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Creating SnapshotStartingData");mStartingData = new SnapshotStartingData(mWmService, snapshot); // 给mStartingData赋值snapshotdata，类比注释6scheduleAddStartingWindow(); // 对应注释7return true;}

注释2、3、4：代码走到这，说明是应用冷启动。查看应用launcher activity的主题，如果其主题将这3个属性中的任何一个置为true，冷启动的时候，不会创建splash screen。
注释5：代码走到这，说明启动窗口的type既不是snapshot，也不是splashscreen，只能是none，直接return false即可，即根本没有启动窗口。
注释6 、7类比注释1。

3.4 ActivityRecord.scheduleAddStartingWindow()

 private final AddStartingWindow mAddStartingWindow = new AddStartingWindow();...void scheduleAddStartingWindow() {// Note: we really want to do sendMessageAtFrontOfQueue() because we// want to process the message ASAP, before any other queued// messages.if (!mWmService.mAnimationHandler.hasCallbacks(mAddStartingWindow)) {ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Enqueueing ADD_STARTING");mWmService.mAnimationHandler.postAtFrontOfQueue(mAddStartingWindow); // 1. Handler机制，跨线程通信。}}private class AddStartingWindow implements Runnable {@Overridepublic void run() {// Can be accessed without holding the global lockfinal StartingData startingData;synchronized (mWmService.mGlobalLock) {// There can only be one adding request, silly caller!mWmService.mAnimationHandler.removeCallbacks(this);if (mStartingData == null) {// Animation has been canceled... do nothing.ProtoLog.v(WM_DEBUG_STARTING_WINDOW,"startingData was nulled out before handling"+ " mAddStartingWindow: %s", ActivityRecord.this);return;}///M: Check starting surface aleady add or not.if (startingSurface != null) {Slog.v(TAG_WM, "already has a starting surface!!!");return;}startingData = mStartingData; // 2. 可能有两种类型。}ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Add starting %s: startingData=%s",this, startingData);WindowManagerPolicy.StartingSurface surface = null;try {surface = startingData.createStartingSurface(ActivityRecord.this); // 3. 可能两种类型} catch (Exception e) {Slog.w(TAG, "Exception when adding starting window", e);}if (surface != null) {boolean abort = false;synchronized (mWmService.mGlobalLock) {// If the window was successfully added, then we need to remove it.if (mStartingData == null) {ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Aborted starting %s: startingData=%s",ActivityRecord.this, mStartingData);startingWindow = null;mStartingData = null;abort = true;} else {startingSurface = surface;}if (!abort) {ProtoLog.v(WM_DEBUG_STARTING_WINDOW,"Added starting %s: startingWindow=%s startingView=%s",ActivityRecord.this, startingWindow, startingSurface);}}if (abort) {surface.remove();}} else {ProtoLog.v(WM_DEBUG_STARTING_WINDOW, "Surface returned was null: %s",ActivityRecord.this);}}}

注释1： mAddStartingWindow是AddStartingWindow类型的变量，AddStartingWindow实现了Runnable接口，执行mAddStartingWindow的run()方法。
注释2、3：可能有snapshot、splash两种类型，下面主要介绍splash的情况。

3.5 SplashScreenStartingData.createStartingSurface()

    StartingSurface createStartingSurface(ActivityRecord activity) {return mService.mPolicy.addSplashScreen(activity.token, mPkg, mTheme, mCompatInfo,mNonLocalizedLabel, mLabelRes, mIcon, mLogo, mWindowFlags,// mMergedOverrideConfiguration, activity.getDisplayContent().getDisplayId());mMergedOverrideConfiguration, activity.getDisplayContent().getDisplayId(),mOverrideTheme);}

3.6 PhoneWindowManager.addSplashScreen()

public StartingSurface addSplashScreen(IBinder appToken, String packageName, int theme,CompatibilityInfo compatInfo, CharSequence nonLocalizedLabel, int labelRes, int icon,int logo, int windowFlags, Configuration overrideConfig, int displayId) {if (!SHOW_SPLASH_SCREENS) {return null;}if (packageName == null) {return null;}WindowManager wm = null;View view = null;try {Context context = mContext;if (DEBUG_SPLASH_SCREEN) Slog.d(TAG, "addSplashScreen " + packageName+ ": nonLocalizedLabel=" + nonLocalizedLabel + " theme="+ Integer.toHexString(theme));// Obtain proper context to launch on the right display.final Context displayContext = getDisplayContext(context, displayId);if (displayContext == null) {// Can't show splash screen on requested display, so skip showing at all.return null;}context = displayContext;if (theme != context.getThemeResId() || labelRes != 0) {try {context = context.createPackageContext(packageName, CONTEXT_RESTRICTED);context.setTheme(theme);} catch (PackageManager.NameNotFoundException e) {// Ignore}}if (overrideConfig != null && !overrideConfig.equals(EMPTY)) {if (DEBUG_SPLASH_SCREEN) Slog.d(TAG, "addSplashScreen: creating context based"+ " on overrideConfig" + overrideConfig + " for splash screen");final Context overrideContext = context.createConfigurationContext(overrideConfig);overrideContext.setTheme(theme);final TypedArray typedArray = overrideContext.obtainStyledAttributes(com.android.internal.R.styleable.Window);final int resId = typedArray.getResourceId(R.styleable.Window_windowBackground, 0);if (resId != 0 && overrideContext.getDrawable(resId) != null) {// We want to use the windowBackground for the override context if it is// available, otherwise we use the default one to make sure a themed starting// window is displayed for the app.if (DEBUG_SPLASH_SCREEN) Slog.d(TAG, "addSplashScreen: apply overrideConfig"+ overrideConfig + " to starting window resId=" + resId);context = overrideContext;}typedArray.recycle();}final PhoneWindow win = new PhoneWindow(context);win.setIsStartingWindow(true);CharSequence label = context.getResources().getText(labelRes, null);// Only change the accessibility title if the label is localizedif (label != null) {win.setTitle(label, true);} else {win.setTitle(nonLocalizedLabel, false);}win.setType(WindowManager.LayoutParams.TYPE_APPLICATION_STARTING);synchronized (mWindowManagerFuncs.getWindowManagerLock()) {// Assumes it's safe to show starting windows of launched apps while// the keyguard is being hidden. This is okay because starting windows never show// secret information.// TODO(b/113840485): Occluded may not only happen on default displayif (displayId == DEFAULT_DISPLAY && mKeyguardOccluded) {windowFlags |= FLAG_SHOW_WHEN_LOCKED;}}// Force the window flags: this is a fake window, so it is not really// touchable or focusable by the user.  We also add in the ALT_FOCUSABLE_IM// flag because we do know that the next window will take input// focus, so we want to get the IME window up on top of us right away.win.setFlags(windowFlags|WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE|WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE|WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM,windowFlags|WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE|WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE|WindowManager.LayoutParams.FLAG_ALT_FOCUSABLE_IM);win.setDefaultIcon(icon);win.setDefaultLogo(logo);win.setLayout(WindowManager.LayoutParams.MATCH_PARENT,WindowManager.LayoutParams.MATCH_PARENT);final WindowManager.LayoutParams params = win.getAttributes();params.token = appToken;params.packageName = packageName;params.windowAnimations = win.getWindowStyle().getResourceId(com.android.internal.R.styleable.Window_windowAnimationStyle, 0);params.privateFlags |=WindowManager.LayoutParams.PRIVATE_FLAG_FAKE_HARDWARE_ACCELERATED;params.privateFlags |= WindowManager.LayoutParams.SYSTEM_FLAG_SHOW_FOR_ALL_USERS;if (!compatInfo.supportsScreen()) {params.privateFlags |= WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;}params.setTitle("Splash Screen " + packageName);// addSplashscreenContent(win, context);boolean contentAdded = addSplashscreenContent(win, context);wm = (WindowManager) context.getSystemService(WINDOW_SERVICE);view = win.getDecorView();if ((context.getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK)!= Configuration.UI_MODE_NIGHT_YES) {PhoneWindowManagerInjector.addStartingWindow(context, view, win, label != null ? label : nonLocalizedLabel);}if (DEBUG_SPLASH_SCREEN) Slog.d(TAG, "Adding splash screen window for "+ packageName + " / " + appToken + ": " + (view.getParent() != null ? view : null));wm.addView(view, params); // 1. 添加view// Only return the view if it was successfully added to the// window manager... which we can tell by it having a parent.return view.getParent() != null ? new SplashScreenSurface(view, appToken) : null;} catch (WindowManager.BadTokenException e) {// ignoreLog.w(TAG, appToken + " already running, starting window not displayed. " +e.getMessage());} catch (RuntimeException e) {// don't crash if something else bad happens, for example a// failure loading resources because we are loading from an app// on external storage that has been unmounted.Log.w(TAG, appToken + " failed creating starting window", e);} finally {if (view != null && view.getParent() == null) {Log.w(TAG, "view not successfully added to wm, removing view");wm.removeViewImmediate(view);}}return null;}

注释1：wm.addview。wm是WindowManager类型的，addview()方法的真正实现在WindowManagerImpl类中。

3.7 WindowManagerImpl.addview()

    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {applyDefaultToken(params);mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,mContext.getUserId());}

3.8 WindowManagerGlobal.addView()

    public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow, int userId) {if (view == null) {throw new IllegalArgumentException("view must not be null");}if (display == null) {throw new IllegalArgumentException("display must not be null");}if (!(params instanceof WindowManager.LayoutParams)) {throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");}final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;if (parentWindow != null) {parentWindow.adjustLayoutParamsForSubWindow(wparams);} else {// If there's no parent, then hardware acceleration for this view is// set from the application's hardware acceleration setting.final Context context = view.getContext();if (context != null&& (context.getApplicationInfo().flags& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;}}ViewRootImpl root;View panelParentView = null;synchronized (mLock) {// Start watching for system property changes.if (mSystemPropertyUpdater == null) {mSystemPropertyUpdater = new Runnable() {@Override public void run() {synchronized (mLock) {for (int i = mRoots.size() - 1; i >= 0; --i) {mRoots.get(i).loadSystemProperties();}}}};SystemProperties.addChangeCallback(mSystemPropertyUpdater);}int index = findViewLocked(view, false);if (index >= 0) {if (mDyingViews.contains(view)) {// Don't wait for MSG_DIE to make it's way through root's queue.mRoots.get(index).doDie();} else {throw new IllegalStateException("View " + view+ " has already been added to the window manager.");}// The previous removeView() had not completed executing. Now it has.}// If this is a panel window, then find the window it is being// attached to for future reference.if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {final int count = mViews.size();for (int i = 0; i < count; i++) {if (mRoots.get(i).mWindow.asBinder() == wparams.token) {panelParentView = mViews.get(i);}}}root = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);mViews.add(view);mRoots.add(root);mParams.add(wparams);// do this last because it fires off messages to start doing thingstry {root.setView(view, wparams, panelParentView, userId); // 1. 调用} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.index = findViewLocked(view, false);if (index >= 0) {Log.e(TAG, "BadTokenException or InvalidDisplayException, clean up.");removeViewLocked(index, true);}throw e;}}}

注释1：调用ViewRootImpl的setView()方法

3.9 ViewRootImpl.setView()

try {mOrigWindowType = mWindowAttributes.type;mAttachInfo.mRecomputeGlobalAttributes = true;collectViewAttributes();adjustLayoutParamsForCompatibility(mWindowAttributes);res = mWindowSession.addToDisplayAsUser(mWindow, mSeq, mWindowAttributes,getHostVisibility(), mDisplay.getDisplayId(), userId, mTmpFrame,mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,mAttachInfo.mDisplayCutout, inputChannel,mTempInsets, mTempControls); // 1. 跨进程通信setFrame(mTmpFrame);} catch (RemoteException e) {mAdded = false;mView = null;mAttachInfo.mRootView = null;inputChannel = null;mFallbackEventHandler.setView(null);unscheduleTraversals();setAccessibilityFocus(null, null);throw new RuntimeException("Adding window failed", e);} finally {if (restore) {attrs.restore();}}

3.10 Session.addToDisplayAsUser()

public int addToDisplayAsUser(IWindow window, int seq, WindowManager.LayoutParams attrs,int viewVisibility, int displayId, int userId, Rect outFrame,Rect outContentInsets, Rect outStableInsets,DisplayCutout.ParcelableWrapper outDisplayCutout, InputChannel outInputChannel,InsetsState outInsetsState, InsetsSourceControl[] outActiveControls) {return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId, outFrame,outContentInsets, outStableInsets, outDisplayCutout, outInputChannel,outInsetsState, outActiveControls, userId);}

3.11 WindowManagerService.addWindow()

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