• Native层 –> ViewRootImpl层 –> DecorView层 –> Activity层 –> ViewGroup层 –> View层

从ViewRootImpl层开始看下back事件

frameworks/base/core/java/android/view/ViewRootImpl.java
private int processKeyEvent(QueuedInputEvent q) {final KeyEvent event = (KeyEvent)q.mEvent;// Deliver the key to the view hierarchy.if (mView.dispatchKeyEvent(event)) {return FINISH_HANDLED;}...return FORWARD;}

这里的mView是DecorView，直接调用DecorView的dispatchKeyEvent分发事件

frameworks/base/core/java/com/android/internal/policy/DecorView.java
public boolean dispatchKeyEvent(KeyEvent event) {...if (!mWindow.isDestroyed()) {final Window.Callback cb = mWindow.getCallback();final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event): super.dispatchKeyEvent(event);if (handled) {return true;}}...}

分发back事件时mWindow还玩destroy，因此会调用Callback的dispatchKeyEvent分发，这里Callback为Activity。

frameworks/base/core/java/android/app/Activity.java
public boolean dispatchKeyEvent(KeyEvent event) {...return event.dispatch(this, decor != null? decor.getKeyDispatcherState() : null, this);}

调用了KeyEvent的dispatch继续分发

frameworks/base/core/java/android/view/KeyEvent.java
public final boolean dispatch(Callback receiver, DispatcherState state,Object target) {switch (mAction) {case ACTION_DOWN: {mFlags &= ~FLAG_START_TRACKING;if (DEBUG) Log.v(TAG, "Key down to " + target + " in " + state+ ": " + this);boolean res = receiver.onKeyDown(mKeyCode, this);if (state != null) {if (res && mRepeatCount == 0 && (mFlags&FLAG_START_TRACKING) != 0) {if (DEBUG) Log.v(TAG, "  Start tracking!");state.startTracking(this, target);} else if (isLongPress() && state.isTracking(this)) {try {if (receiver.onKeyLongPress(mKeyCode, this)) {if (DEBUG) Log.v(TAG, "  Clear from long press!");state.performedLongPress(this);res = true;}} catch (AbstractMethodError e) {}}}return res;}case ACTION_UP:if (DEBUG) Log.v(TAG, "Key up to " + target + " in " + state+ ": " + this);if (state != null) {state.handleUpEvent(this);}return receiver.onKeyUp(mKeyCode, this);case ACTION_MULTIPLE:final int count = mRepeatCount;final int code = mKeyCode;if (receiver.onKeyMultiple(code, count, this)) {return true;}if (code != KeyEvent.KEYCODE_UNKNOWN) {mAction = ACTION_DOWN;mRepeatCount = 0;boolean handled = receiver.onKeyDown(code, this);if (handled) {mAction = ACTION_UP;receiver.onKeyUp(code, this);}mAction = ACTION_MULTIPLE;mRepeatCount = count;return handled;}return false;}return false;}

主要看下ACTION_DOWN事件，这里receiver是activity。如果是长按，则会调用activity的onKeyLongPress

frameworks/base/core/java/android/app/Activity.java
public boolean onKeyLongPress(int keyCode, KeyEvent event) {return false;}

可以看到直接返回false，忽略掉的长按事件

frameworks/base/core/java/android/app/Activity.javapublic boolean onKeyDown(int keyCode, KeyEvent event)  {if (keyCode == KeyEvent.KEYCODE_BACK) {if (getApplicationInfo().targetSdkVersion>= Build.VERSION_CODES.ECLAIR) {event.startTracking();} else {onBackPressed();}return true;}...}}

可以看到按下back键会调用onBackPressed()方法，该方法最终会调用到finish()方法。

frameworks/base/core/java/android/app/Activity.javapublic void onBackPressed() {if (mActionBar != null && mActionBar.collapseActionView()) {return;}if (!mFragments.getFragmentManager().popBackStackImmediate()) {finishAfterTransition();}}public void finishAfterTransition() {if (!mActivityTransitionState.startExitBackTransition(this)) {finish();}}

home事件

home事件是不经过app统一由系统处理的，在PhoneWindowManager端拦截掉了

frameworks/base/services/core/java/com/android/server/policy/PhoneWindowManager.java
// First we always handle the home key here, so applications// can never break it, although if keyguard is on, we do let// it handle it, because that gives us the correct 5 second// timeout.
public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags) {
if (keyCode == KeyEvent.KEYCODE_HOME) {...// If we have released the home key, and didn't do anything else// while it was pressed, then it is time to go home!if (!down) {cancelPreloadRecentApps();mHomePressed = false;...handleShortPressOnHome();return -1;}...}

private void handleShortPressOnHome() {// Turn on the connected TV and switch HDMI input if we're a HDMI playback device.getHdmiControl().turnOnTv();// If there's a dream running then use home to escape the dream// but don't actually go home.if (mDreamManagerInternal != null && mDreamManagerInternal.isDreaming()) {mDreamManagerInternal.stopDream(false /*immediate*/);return;}// Go home!launchHomeFromHotKey();}

launchHomeFromHotKey就回到launcher了。