/** {@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;
  }

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;
  }

@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;
  }

if (interactive && !mScreenshotChordVolumeDownKeyTriggered
                && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
              mScreenshotChordVolumeDownKeyTriggered = true;
              mScreenshotChordVolumeDownKeyTime = event.getDownTime();
              mScreenshotChordVolumeDownKeyConsumed = false;
              cancelPendingPowerKeyAction();
              interceptScreenshotChord();
            }

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());
      }
    }
  }

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();
  }

private final Runnable mScreenshotRunnable = new Runnable() {
    @Override
    public void run() {
      takeScreenshot();
    }
  };

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);
      }
    }
  }

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();
  }
}

/**
   * 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);
  }

PhoneWindowManager.takeScreenshot方法傳遞的：

if (mStatusBar != null && mStatusBar.isVisibleLw())
              msg.arg1 = 1;
            if (mNavigationBar != null && mNavigationBar.isVisibleLw())
              msg.arg2 = 1;

// Take the screenshot
mScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]);

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);
  }

if (mScreenBitmap == null) {
      notifyScreenshotError(mContext, mNotificationManager);
      finisher.run();
      return;
    }

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);
  }

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;
    }

// Start the post-screenshot animation
    startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,
        statusBarVisible, navBarVisible);

/**
   * 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();
      }
    });
  }

/**
   * 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);
  }

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);
  }