Android 截圖功能源碼的分析
一般沒有修改rom的android原生系統(tǒng)截圖功能的組合鍵是音量減+開機鍵；今天我們從源碼角度來分析截圖功能是如何在源碼中實現的。
在android系統(tǒng)中，由于我們的每一個Android界面都是一個Activity，而界面的顯示都是通過Window對象實現的，每個Window對象實際上都是PhoneWindow的實例，而每個PhoneWindow對象都對應一個PhoneWindowManager對象，當我們在Activity界面執(zhí)行按鍵操作的時候，在將按鍵的處理操作分發(fā)到App之前，首先會回調PhoneWindowManager中的dispatchUnhandledKey方法，該方法主要用于執(zhí)行當前App處理按鍵之前的操作，我們具體看一下該方法的實現。
/** {@inheritDoc} */ @Override public KeyEvent dispatchUnhandledKey(WindowState win, KeyEvent event, int policyFlags) { ... KeyEvent fallbackEvent = null; if ((event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) { final KeyCharacterMap kcm = event.getKeyCharacterMap(); final int keyCode = event.getKeyCode(); final int metaState = event.getMetaState(); final boolean initialDown = event.getAction() == KeyEvent.ACTION_DOWN && event.getRepeatCount() == 0; // Check for fallback actions specified by the key character map. final FallbackAction fallbackAction; if (initialDown) { fallbackAction = kcm.getFallbackAction(keyCode, metaState); } else { fallbackAction = mFallbackActions.get(keyCode); } if (fallbackAction != null) { ... final int flags = event.getFlags() | KeyEvent.FLAG_FALLBACK; fallbackEvent = KeyEvent.obtain( event.getDownTime(), event.getEventTime(), event.getAction(), fallbackAction.keyCode, event.getRepeatCount(), fallbackAction.metaState, event.getDeviceId(), event.getScanCode(), flags, event.getSource(), null); if (!interceptFallback(win, fallbackEvent, policyFlags)) { fallbackEvent.recycle(); fallbackEvent = null; } if (initialDown) { mFallbackActions.put(keyCode, fallbackAction); } else if (event.getAction() == KeyEvent.ACTION_UP) { mFallbackActions.remove(keyCode); fallbackAction.recycle(); } } } ... return fallbackEvent; }這里我們關注一下方法體中調用的：interceptFallback方法，通過調用該方法將處理按鍵的操作下發(fā)到該方法中，我們繼續(xù)看一下該方法的實現邏輯。
private boolean interceptFallback(WindowState win, KeyEvent fallbackEvent, int policyFlags) { int actions = interceptKeyBeforeQueueing(fallbackEvent, policyFlags); if ((actions & ACTION_PASS_TO_USER) != 0) { long delayMillis = interceptKeyBeforeDispatching( win, fallbackEvent, policyFlags); if (delayMillis == 0) { return true; } } return false; }然后我們看到在interceptFallback方法中我們調用了interceptKeyBeforeQueueing方法，通過閱讀我們我們知道該方法主要實現了對截屏按鍵的處理流程，這樣我們繼續(xù)看一下interceptKeyBeforeWueueing方法的處理：
@Override public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags) { if (!mSystemBooted) { // If we have not yet booted, don't let key events do anything. return 0; } ... // Handle special keys. switch (keyCode) { case KeyEvent.KEYCODE_VOLUME_DOWN: case KeyEvent.KEYCODE_VOLUME_UP: case KeyEvent.KEYCODE_VOLUME_MUTE: { if (mUseTvRouting) { // On TVs volume keys never go to the foreground app result &= ~ACTION_PASS_TO_USER; } if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) { if (down) { if (interactive && !mScreenshotChordVolumeDownKeyTriggered && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) { mScreenshotChordVolumeDownKeyTriggered = true; mScreenshotChordVolumeDownKeyTime = event.getDownTime(); mScreenshotChordVolumeDownKeyConsumed = false; cancelPendingPowerKeyAction(); interceptScreenshotChord(); } } else { mScreenshotChordVolumeDownKeyTriggered = false; cancelPendingScreenshotChordAction(); } } ... return result; }可以發(fā)現這里首先判斷當前系統(tǒng)是否已經boot完畢，若尚未啟動完畢，則所有的按鍵操作都將失效，若啟動完成，則執(zhí)行后續(xù)的操作，這里我們只是關注音量減少按鍵和電源按鍵組合的處理事件。另外這里多說一句想安卓系統(tǒng)的HOME按鍵事件，MENU按鍵事件，進程列表按鍵事件等等都是在這里實現的，后續(xù)中我們會陸續(xù)介紹這方面的內容。
回到我們的interceptKeyBeforeQueueing方法，當我用按下音量減少按鍵的時候回進入到：case KeyEvent.KEYCODE_VOLUME_MUTE分支并執(zhí)行相應的邏輯，然后同時判斷用戶是否按下了電源鍵，若同時按下了電源鍵，則執(zhí)行：
if (interactive && !mScreenshotChordVolumeDownKeyTriggered && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) { mScreenshotChordVolumeDownKeyTriggered = true; mScreenshotChordVolumeDownKeyTime = event.getDownTime(); mScreenshotChordVolumeDownKeyConsumed = false; cancelPendingPowerKeyAction(); interceptScreenshotChord(); }可以發(fā)現這里的interceptScreenshotChrod方法就是系統(tǒng)準備開始執(zhí)行截屏操作的開始，我們繼續(xù)看一下interceptcreenshotChord方法的實現。
private void interceptScreenshotChord() { if (mScreenshotChordEnabled && mScreenshotChordVolumeDownKeyTriggered && mScreenshotChordPowerKeyTriggered && !mScreenshotChordVolumeUpKeyTriggered) { final long now = SystemClock.uptimeMillis(); if (now <= mScreenshotChordVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS && now <= mScreenshotChordPowerKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) { mScreenshotChordVolumeDownKeyConsumed = true; cancelPendingPowerKeyAction(); mHandler.postDelayed(mScreenshotRunnable, getScreenshotChordLongPressDelay()); } } }在方法體中我們最終會執(zhí)行發(fā)送一個延遲的異步消息，請求執(zhí)行截屏的操作而這里的延時時間，若當前輸入框是打開狀態(tài)，則延時時間為輸入框關閉時間加上系統(tǒng)配置的按鍵超時時間，若當前輸入框沒有打開則直接是系統(tǒng)配置的按鍵超時處理時間，可看一下getScreenshotChordLongPressDelay方法的具體實現。
private long getScreenshotChordLongPressDelay() { if (mKeyguardDelegate.isShowing()) { // Double the time it takes to take a screenshot from the keyguard return (long) (KEYGUARD_SCREENSHOT_CHORD_DELAY_MULTIPLIER * ViewConfiguration.get(mContext).getDeviceGlobalActionKeyTimeout()); } return ViewConfiguration.get(mContext).getDeviceGlobalActionKeyTimeout(); }回到我們的interceptScreenshotChord方法，發(fā)送了異步消息之后系統(tǒng)最終會被我們發(fā)送的Runnable對象的run方法執(zhí)行；這樣我們看一下Runnable類型的mScreenshotRunnable的run方法的實現:
private final Runnable mScreenshotRunnable = new Runnable() { @Override public void run() { takeScreenshot(); } };好吧，方法體中并未執(zhí)行其他操作，直接就是調用了takeScreenshot方法，這樣我們繼續(xù)看一下takeScreenshot方法的實現。
private void takeScreenshot() { synchronized (mScreenshotLock) { if (mScreenshotConnection != null) { return; } ComponentName cn = new ComponentName("com.android.systemui", "com.android.systemui.screenshot.TakeScreenshotService"); Intent intent = new Intent(); intent.setComponent(cn); ServiceConnection conn = new ServiceConnection() { @Override public void onServiceConnected(ComponentName name, IBinder service) { synchronized (mScreenshotLock) { if (mScreenshotConnection != this) { return; } Messenger messenger = new Messenger(service); Message msg = Message.obtain(null, 1); final ServiceConnection myConn = this; Handler h = new Handler(mHandler.getLooper()) { @Override public void handleMessage(Message msg) { synchronized (mScreenshotLock) { if (mScreenshotConnection == myConn) { mContext.unbindService(mScreenshotConnection); mScreenshotConnection = null; mHandler.removeCallbacks(mScreenshotTimeout); } } } }; msg.replyTo = new Messenger(h); msg.arg1 = msg.arg2 = 0; if (mStatusBar != null && mStatusBar.isVisibleLw()) msg.arg1 = 1; if (mNavigationBar != null && mNavigationBar.isVisibleLw()) msg.arg2 = 1; try { messenger.send(msg); } catch (RemoteException e) { } } } @Override public void onServiceDisconnected(ComponentName name) {} }; if (mContext.bindServiceAsUser( intent, conn, Context.BIND_AUTO_CREATE, UserHandle.CURRENT)) { mScreenshotConnection = conn; mHandler.postDelayed(mScreenshotTimeout, 10000); } } }可以發(fā)現這里通過反射機制創(chuàng)建了一個TakeScreenshotService對象然后調用了bindServiceAsUser，這樣就創(chuàng)建了TakeScreenshotService服務并在服務創(chuàng)建之后發(fā)送了一個異步消息。好了，我們看一下TakeScreenshotService的實現邏輯。
public class TakeScreenshotService extends Service { private static final String TAG = "TakeScreenshotService"; private static GlobalScreenshot mScreenshot; private Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { switch (msg.what) { case 1: final Messenger callback = msg.replyTo; if (mScreenshot == null) { mScreenshot = new GlobalScreenshot(TakeScreenshotService.this); } mScreenshot.takeScreenshot(new Runnable() { @Override public void run() { Message reply = Message.obtain(null, 1); try { callback.send(reply); } catch (RemoteException e) { } } }, msg.arg1 > 0, msg.arg2 > 0); } } }; @Override public IBinder onBind(Intent intent) { return new Messenger(mHandler).getBinder(); }}可以發(fā)現在在TakeScreenshotService類的定義中有一個Handler成員變量，而我們在啟動TakeScreentshowService的時候回發(fā)送一個異步消息，這樣就會執(zhí)行mHandler的handleMessage方法，然后在handleMessage方法中我們創(chuàng)建了一個GlobalScreenshow對象，然后執(zhí)行了takeScreenshot方法，好吧，繼續(xù)看一下takeScreentshot方法的執(zhí)行邏輯。
/** * Takes a screenshot of the current display and shows an animation. */ void takeScreenshot(Runnable finisher, boolean statusBarVisible, boolean navBarVisible) { // We need to orient the screenshot correctly (and the Surface api seems to take screenshots // only in the natural orientation of the device :!) mDisplay.getRealMetrics(mDisplayMetrics); float[] dims = {mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels}; float degrees = getDegreesForRotation(mDisplay.getRotation()); boolean requiresRotation = (degrees > 0); if (requiresRotation) { // Get the dimensions of the device in its native orientation mDisplayMatrix.reset(); mDisplayMatrix.preRotate(-degrees); mDisplayMatrix.mapPoints(dims); dims[0] = Math.abs(dims[0]); dims[1] = Math.abs(dims[1]); } // Take the screenshot mScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]); if (mScreenBitmap == null) { notifyScreenshotError(mContext, mNotificationManager); finisher.run(); return; } if (requiresRotation) { // Rotate the screenshot to the current orientation Bitmap ss = Bitmap.createBitmap(mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels, Bitmap.Config.ARGB_8888); Canvas c = new Canvas(ss); c.translate(ss.getWidth() / 2, ss.getHeight() / 2); c.rotate(degrees); c.translate(-dims[0] / 2, -dims[1] / 2); c.drawBitmap(mScreenBitmap, 0, 0, null); c.setBitmap(null); // Recycle the previous bitmap mScreenBitmap.recycle(); mScreenBitmap = ss; } // Optimizations mScreenBitmap.setHasAlpha(false); mScreenBitmap.prepareToDraw(); // Start the post-screenshot animation startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels, statusBarVisible, navBarVisible); }可以看到這里后兩個參數：statusBarVisible，navBarVisible是否可見，而這兩個參數在我們
PhoneWindowManager.takeScreenshot方法傳遞的：if (mStatusBar != null && mStatusBar.isVisibleLw()) msg.arg1 = 1; if (mNavigationBar != null && mNavigationBar.isVisibleLw()) msg.arg2 = 1;
可見若果mStatusBar可見，則傳遞的statusBarVisible為true，若mNavigationBar可見，則傳遞的navBarVisible為true。然后我們在截屏的時候判斷nStatusBar是否可見，mNavigationBar是否可見，若可見的時候則截屏同樣將其截屏出來。繼續(xù)回到我們的takeScreenshot方法，然后調用了：
// Take the screenshotmScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]);
方法，看注釋，這里就是執(zhí)行截屏事件的具體操作了，然后我看一下SurfaceControl.screenshot方法的具體實現，另外這里需要注意的是，截屏之后返回的是一個Bitmap對象，其實熟悉android繪制機制的童鞋應該知道android中所有顯示能夠顯示的東西，在內存中表現都是Bitmap對象。
public static Bitmap screenshot(int width, int height) { // TODO: should take the display as a parameter IBinder displayToken = SurfaceControl.getBuiltInDisplay( SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN); return nativeScreenshot(displayToken, new Rect(), width, height, 0, 0, true, false, Surface.ROTATION_0); }好吧，這里調用的是nativeScreenshot方法，它是一個native方法，具體的實現在JNI層，這里就不做過多的介紹了。繼續(xù)回到我們的takeScreenshot方法，在調用了截屏方法screentshot之后，判斷是否截屏成功：
if (mScreenBitmap == null) { notifyScreenshotError(mContext, mNotificationManager); finisher.run(); return; }若截屏之后，截屏的bitmap對象為空，這里判斷截屏失敗，調用了notifyScreenshotError方法，發(fā)送截屏失敗的notification通知。
static void notifyScreenshotError(Context context, NotificationManager nManager) { Resources r = context.getResources(); // Clear all existing notification, compose the new notification and show it Notification.Builder b = new Notification.Builder(context) .setTicker(r.getString(R.string.screenshot_failed_title)) .setContentTitle(r.getString(R.string.screenshot_failed_title)) .setContentText(r.getString(R.string.screenshot_failed_text)) .setSmallIcon(R.drawable.stat_notify_image_error) .setWhen(System.currentTimeMillis()) .setVisibility(Notification.VISIBILITY_PUBLIC) // ok to show outside lockscreen .setCategory(Notification.CATEGORY_ERROR) .setAutoCancel(true) .setColor(context.getColor( com.android.internal.R.color.system_notification_accent_color)); Notification n = new Notification.BigTextStyle(b) .bigText(r.getString(R.string.screenshot_failed_text)) .build(); nManager.notify(R.id.notification_screenshot, n); }然后繼續(xù)看takeScreenshot方法，判斷截屏的圖像是否需要旋轉，若需要的話，則旋轉圖像：
if (requiresRotation) { // Rotate the screenshot to the current orientation Bitmap ss = Bitmap.createBitmap(mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels, Bitmap.Config.ARGB_8888); Canvas c = new Canvas(ss); c.translate(ss.getWidth() / 2, ss.getHeight() / 2); c.rotate(degrees); c.translate(-dims[0] / 2, -dims[1] / 2); c.drawBitmap(mScreenBitmap, 0, 0, null); c.setBitmap(null); // Recycle the previous bitmap mScreenBitmap.recycle(); mScreenBitmap = ss; }在takeScreenshot方法的最后若截屏成功，我們調用了：
// Start the post-screenshot animation startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels, statusBarVisible, navBarVisible);
開始截屏的動畫，好吧，看一下動畫效果的實現：
/** * Starts the animation after taking the screenshot */ private void startAnimation(final Runnable finisher, int w, int h, boolean statusBarVisible, boolean navBarVisible) { // Add the view for the animation mScreenshotView.setImageBitmap(mScreenBitmap); mScreenshotLayout.requestFocus(); // Setup the animation with the screenshot just taken if (mScreenshotAnimation != null) { mScreenshotAnimation.end(); mScreenshotAnimation.removeAllListeners(); } mWindowManager.addView(mScreenshotLayout, mWindowLayoutParams); ValueAnimator screenshotDropInAnim = createScreenshotDropInAnimation(); ValueAnimator screenshotFadeOutAnim = createScreenshotDropOutAnimation(w, h, statusBarVisible, navBarVisible); mScreenshotAnimation = new AnimatorSet(); mScreenshotAnimation.playSequentially(screenshotDropInAnim, screenshotFadeOutAnim); mScreenshotAnimation.addListener(new AnimatorListenerAdapter() { @Override public void onAnimationEnd(Animator animation) { // Save the screenshot once we have a bit of time now saveScreenshotInWorkerThread(finisher); mWindowManager.removeView(mScreenshotLayout); // Clear any references to the bitmap mScreenBitmap = null; mScreenshotView.setImageBitmap(null); } }); mScreenshotLayout.post(new Runnable() { @Override public void run() { // Play the shutter sound to notify that we've taken a screenshot mCameraSound.play(MediaActionSound.SHUTTER_CLICK); mScreenshotView.setLayerType(View.LAYER_TYPE_HARDWARE, null); mScreenshotView.buildLayer(); mScreenshotAnimation.start(); } }); }好吧，經過著一些列的操作之后我們實現了截屏之后的動畫效果了，這里暫時不分析動畫效果，我們看一下動畫效果之后做了哪些？還記不記的一般情況下我們截屏之后都會收到一個截屏的notification通知？這里應該也是在其AnimatorListenerAdapter的onAnimationEnd方法中實現的，也就是動畫執(zhí)行完成之后，我們看一下其saveScreenshotInWorkerThread方法的實現：
/** * Creates a new worker thread and saves the screenshot to the media store. */ private void saveScreenshotInWorkerThread(Runnable finisher) { SaveImageInBackgroundData data = new SaveImageInBackgroundData(); data.context = mContext; data.image = mScreenBitmap; data.iconSize = mNotificationIconSize; data.finisher = finisher; data.previewWidth = mPreviewWidth; data.previewheight = mPreviewHeight; if (mSaveInBgTask != null) { mSaveInBgTask.cancel(false); } mSaveInBgTask = new SaveImageInBackgroundTask(mContext, data, mNotificationManager, R.id.notification_screenshot).execute(data); }好吧，這里主要邏輯就是構造了一個SaveImageInBackgroundTask對象，看樣子發(fā)送截屏成功的通知應該是在這里實現的，我們看一下SaveImageInBackgroundTask構造方法的實現邏輯：
SaveImageInBackgroundTask(Context context, SaveImageInBackgroundData data, NotificationManager nManager, int nId) { ... // Show the intermediate notification mTickerAddSpace = !mTickerAddSpace; mNotificationId = nId; mNotificationManager = nManager; final long now = System.currentTimeMillis(); mNotificationBuilder = new Notification.Builder(context) .setTicker(r.getString(R.string.screenshot_saving_ticker) + (mTickerAddSpace ? " " : "")) .setContentTitle(r.getString(R.string.screenshot_saving_title)) .setContentText(r.getString(R.string.screenshot_saving_text)) .setSmallIcon(R.drawable.stat_notify_image) .setWhen(now) .setColor(r.getColor(com.android.internal.R.color.system_notification_accent_color)); mNotificationStyle = new Notification.BigPictureStyle() .bigPicture(picture.createAshmemBitmap()); mNotificationBuilder.setStyle(mNotificationStyle); // For "public" situations we want to show all the same info but // omit the actual screenshot image. mPublicNotificationBuilder = new Notification.Builder(context) .setContentTitle(r.getString(R.string.screenshot_saving_title)) .setContentText(r.getString(R.string.screenshot_saving_text)) .setSmallIcon(R.drawable.stat_notify_image) .setCategory(Notification.CATEGORY_PROGRESS) .setWhen(now) .setColor(r.getColor( com.android.internal.R.color.system_notification_accent_color)); mNotificationBuilder.setPublicVersion(mPublicNotificationBuilder.build()); Notification n = mNotificationBuilder.build(); n.flags |= Notification.FLAG_NO_CLEAR; mNotificationManager.notify(nId, n); // On the tablet, the large icon makes the notification appear as if it is clickable (and // on small devices, the large icon is not shown) so defer showing the large icon until // we compose the final post-save notification below. mNotificationBuilder.setLargeIcon(icon.createAshmemBitmap()); // But we still don't set it for the expanded view, allowing the smallIcon to show here. mNotificationStyle.bigLargeIcon((Bitmap) null); }可以發(fā)現在構造方法的后面狗仔了一個NotificationBuilder對象，然后發(fā)送了一個截屏成功的Notification，這樣我們在截屏動畫之后就收到了Notification的通知了。
總結：
一般默認情況下按下音量減少鍵和開機鍵會執(zhí)行截圖動作，程序執(zhí)行的入口就在在PhoneWindowManager的dispatchUnhandledKey方法中；然后通過TakeScreenshotService服務執(zhí)行截圖邏輯；通過nativie方法獲取截圖的bitmap，如果失敗調用失敗通知欄消息，如果成功調用截圖動畫后發(fā)送成功通知欄消息。
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