Android快速启动窗口技术

以Android7.0为例，我们知道Android应用在启动时候，PhoneWindowManager会添加一个空白启动窗口，叫做addStartingWindow，在界面加载完毕后，会removeStartingWindow。

    /** {@inheritDoc} */@Overridepublic View addStartingWindow(IBinder appToken, String packageName, int theme,CompatibilityInfo compatInfo, CharSequence nonLocalizedLabel, int labelRes,int icon, int logo, int windowFlags) {if (!SHOW_STARTING_ANIMATIONS) {return null;}if (packageName == null) {return null;}WindowManager wm = null;View view = null;try {Context context = mContext;if (DEBUG_STARTING_WINDOW) Slog.d(TAG, "addStartingWindow " + packageName+ ": nonLocalizedLabel=" + nonLocalizedLabel + " theme="+ Integer.toHexString(theme));if (theme != context.getThemeResId() || labelRes != 0) {try {context = context.createPackageContext(packageName, 0);context.setTheme(theme);} catch (PackageManager.NameNotFoundException e) {// Ignore}}PhoneWindow win = new PhoneWindow(context);win.setIsStartingWindow(true);final TypedArray ta = win.getWindowStyle();if (ta.getBoolean(com.android.internal.R.styleable.Window_windowDisablePreview, false)|| ta.getBoolean(com.android.internal.R.styleable.Window_windowShowWallpaper,false)) {return null;}Resources r = context.getResources();win.setTitle(r.getText(labelRes, nonLocalizedLabel));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.if (mKeyguardHidden) {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.PRIVATE_FLAG_SHOW_FOR_ALL_USERS;if (!compatInfo.supportsScreen()) {params.privateFlags |= WindowManager.LayoutParams.PRIVATE_FLAG_COMPATIBLE_WINDOW;}params.setTitle("Starting " + packageName);wm = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);view = win.getDecorView();if (win.isFloating()) {// Whoops, there is no way to display an animation/preview// of such a thing!  After all that work...  let's skip it.// (Note that we must do this here because it is in// getDecorView() where the theme is evaluated...  maybe// we should peek the floating attribute from the theme// earlier.)return null;}if (DEBUG_STARTING_WINDOW) Slog.d(TAG, "Adding starting window for " + packageName+ " / " + appToken + ": "+ (view.getParent() != null ? view : null));wm.addView(view, params);// 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 ? view : 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;}

空白启动窗口给用户的操作体验是很差的，尤其是低端手机，或者应用冷启动时间较长时。所以就想到能不能在应用冷启动的时候，对启动窗口进行截屏，然后将截图缓存起来。当下一次应用冷启动时候，用截图的窗口代替原来的空白启动窗口呢？答案是可以的。流程如下图所示。问题的关键是截屏时机在哪里？冷启动过程中，WindowState中的mAttrs(WindowManager.LayoutParams)的type，有一个切换过程，从WindowManager.LayoutParams.TYPE_APPLICATION_STARTING --->WindowManager.LayoutParams.FIRST_APPLICATION_WINDOW，

            if (mService.isFastStartingWindowSupport() && mWin.mAttrs.type == WindowManager.LayoutParams.FIRST_APPLICATION_WINDOW  && mService.lastLayoutParamsType== android.view.WindowManager.LayoutParams.TYPE_APPLICATION_STARTING  && mWin.mAppToken != null) {mCanScreenshot = true;doCacheBitmap();}mService.lastLayoutParamsType = mWin.mAttrs.type;//application start , mWin.mAttrs.type from  3--->1

lastLayoutParamsType是在 WMS中新声明的

public int lastLayoutParamsType = -1;

当然这个变化还有你的应用androidmanifest中application的设置有关。

上面截屏的逻辑是定义在WindowStateAnimator中performShowLocked的，在此时截屏，其实整个界面还是没有绘制完成的，看下面的doCacheBitmap，需要子线程延时一定时间，同时，对截屏的图像进行判断。

    void doCacheBitmap() {AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>() {@Overrideprotected Void doInBackground(Void... para) {//android.os.Process.setThreadPriority//    (android.os.Process.THREAD_PRIORITY_FOREGROUND);try {if (mWmSleep != -1) {Thread.sleep(mWmSleep);} else {Thread.sleep(250);}if(mWin.mWinAnimator.mSurfaceController == null){// in some case , mSurfaceController is nullreturn null;}if(!mCanScreenshot){return null;}Trace.traceBegin(Trace.TRACE_TAG_WINDOW_MANAGER, "AsyncScreenshot");Bitmap bmShot = SurfaceControl.screenshot(new Rect(),(int) mWin.mWinAnimator.mSurfaceController.mSurfaceW,(int) mWin.mWinAnimator.mSurfaceController.mSurfaceH,mSurfaceController.mSurfaceLayer,mSurfaceController.mSurfaceLayer, false, 0);Slog.i(TAG, "doCacheBitmap, mToken =" + mWin.mToken);int leftTopColor = bmShot.getPixel(0, 0);int rightTopColor = bmShot.getPixel(mContext.getResources().getDisplayMetrics().widthPixels - 1, 0);int leftBottomColor = bmShot.getPixel(0, mContext.getResources().getDisplayMetrics().heightPixels - 1);int rightBottomColor = bmShot.getPixel(mContext.getResources().getDisplayMetrics().widthPixels - 1, mContext.getResources().getDisplayMetrics().heightPixels - 1);int averageColor = (getAverageRGB(leftTopColor) + getAverageRGB(rightTopColor) + getAverageRGB(leftBottomColor) + getAverageRGB(rightBottomColor)) / 4;if (averageColor < 5) {//in some case, the bitmap is a starting window(it is a picture with black boder,or the picture is whole black) , not a started window , so returnreturn null;}if (bmShot != null && mWin.mToken != null) {mService.setBitmapByToken(mWin.mToken.token, bmShot.copy(bmShot.getConfig(), true));bmShot.recycle();}Trace.traceEnd(Trace.TRACE_TAG_WINDOW_MANAGER);} catch (Exception e) {e.printStackTrace();}return null;}};task.execute();}

应用快速启动流程如下图所示：

(1) 用户在点击应用的时候，ActivityManagerService会执行startActivity方法，然后ActivityStack执行startActivityLocked方法，然后ActivityRecord执行showStartingWindow方法，最后调用WindowManagerService的setAppStartingWindow来设置一个启动窗口。

(2) WindowManagerService会判断当前系统是否支持FSW并且当前窗口是否有Bitmap缓存，如果是开机后第一次启动，则执行Android系统原有方法addStartingWindow，添加一个空白窗口。如果系统支持FSW并且当前窗口有Bitmap缓存，则执行addFastStartingWindow方法，添加一个非空白窗口。

(3) 应用冷启动时，WindowMangerService通过WindowStateAnimator进行窗口缓存。WindowMangerService收到configChange广播，则清除bitmap缓存。

加入该技术后，给用户的感觉是应用秒开，对比测试友商手机，冷启动速度都要比我们的差，加入该技术的应用，冷启动速度只有300ms。

该技术的局限性在于，仅适用于启动界面变化较小的APP，变化较大的应用不适合，第三方应用基本都不适合。

第一篇博客就写到这里把。

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