上一篇文章中我们介绍了android系统的截屏事件，由于截屏事件是一种系统全局处理事件，所以事件的处理逻辑不是在App中执行，而是在PhoneWindowManager中执行。而本文我们现在主要讲解android系统中HOME按键的事件处理，和截屏事件类似，这里的HOME按键也是系统级别的按键事件监听，所以其处理事件的逻辑也应该和截屏事件处理流程类似，从上一篇文章的分析过冲中我们不难发现，系统级别的按键处理逻辑其实都是在PhoneWindowManager中，所以HOME按键的处理逻辑也是在PhoneWindowManager的dispatchUnhandledKey方法中执行，那么我们就从dispatchUnhandleKey方法开始分析HOME按键的处理流程。
好吧我们看一下PhoneWindowManager的dispatchUnhandleKey方法的实现：
@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) {
if (DEBUG_INPUT) {
Slog.d(TAG, "Fallback: keyCode=" + fallbackAction.keyCode
+ " metaState=" + Integer.toHexString(fallbackAction.metaState));
}
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();
}
}
}
if (DEBUG_INPUT) {
if (fallbackEvent == null) {
Slog.d(TAG, "No fallback.");
} else {
Slog.d(TAG, "Performing fallback: " + fallbackEvent);
}
}
return fallbackEvent;
}通过查看源码，我们重点看一下dispatchUnhandledKey方法中调用的interceptFallback方法，关于HOME按键的处理逻辑也是在这个方法体中的，所以继续看一下interceptFallback方法的实现：
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;
}通过分析源码我们知道关于HOME按键的处理逻辑主要是在interceptKeyBeforeDispatching方法的实现的，既然这样，我们看一下interceptKeyBeforeDispatching方法的实现：
@Override
public long interceptKeyBeforeDispatching(WindowState win, KeyEvent event, int policyFlags) {
...
// 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.
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;
if (mHomeConsumed) {
mHomeConsumed = false;
return -1;
}
if (canceled) {
Log.i(TAG, "Ignoring HOME; event canceled.");
return -1;
}
// If an incoming call is ringing, HOME is totally disabled.
// (The user is already on the InCallUI at this point,
// and his ONLY options are to answer or reject the call.)
TelecomManager telecomManager = getTelecommService();
if (telecomManager != null && telecomManager.isRinging()) {
Log.i(TAG, "Ignoring HOME; there's a ringing incoming call.");
return -1;
}
// Delay handling home if a double-tap is possible.
if (mDoubleTapOnHomeBehavior != DOUBLE_TAP_HOME_NOTHING) {
mHandler.removeCallbacks(mHomeDoubleTapTimeoutRunnable); // just in case
mHomeDoubleTapPending = true;
mHandler.postDelayed(mHomeDoubleTapTimeoutRunnable,
ViewConfiguration.getDoubleTapTimeout());
return -1;
}
handleShortPressOnHome();
return -1;
}
// If a system window has focus, then it doesn't make sense
// right now to interact with applications.
WindowManager.LayoutParams attrs = win != null ? win.getAttrs() : null;
if (attrs != null) {
final int type = attrs.type;
if (type == WindowManager.LayoutParams.TYPE_KEYGUARD_SCRIM
|| type == WindowManager.LayoutParams.TYPE_KEYGUARD_DIALOG
|| (attrs.privateFlags & PRIVATE_FLAG_KEYGUARD) != 0) {
// the "app" is keyguard, so give it the key
return 0;
}
final int typeCount = WINDOW_TYPES_WHERE_HOME_DOESNT_WORK.length;
for (int i=0; i<typeCount; i++) {
if (type == WINDOW_TYPES_WHERE_HOME_DOESNT_WORK[i]) {
// don't do anything, but also don't pass it to the app
return -1;
}
}
}
// Remember that home is pressed and handle special actions.
if (repeatCount == 0) {
mHomePressed = true;
if (mHomeDoubleTapPending) {
mHomeDoubleTapPending = false;
mHandler.removeCallbacks(mHomeDoubleTapTimeoutRunnable);
handleDoubleTapOnHome();
} else if (mLongPressOnHomeBehavior == LONG_PRESS_HOME_RECENT_SYSTEM_UI
|| mDoubleTapOnHomeBehavior == DOUBLE_TAP_HOME_RECENT_SYSTEM_UI) {
preloadRecentApps();
}
} else if ((event.getFlags() & KeyEvent.FLAG_LONG_PRESS) != 0) {
if (!keyguardOn) {
handleLongPressOnHome(event.getDeviceId());
}
}
return -1;
}
// Let the application handle the key.
return 0;
}这里我们主要看一下对android系统HOME按键的处理逻辑，通过分析源码我们知道HOME按键进入launcher界面的主要逻辑是在handleShortPressOnHome();方法中执行的，所以我们继续看一下handleShortPressOnHome方法的实现。
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();
}可以看到在handleShortPressOnHome方法中调用了launchHomeFromHotKey方法，该方法的注释用于go home，所以继续看一下该方法的实现：
void launchHomeFromHotKey() {
launchHomeFromHotKey(true /* awakenFromDreams */, true /*respectKeyguard*/);
}可以看到在launchHomeFromHotKey方法中我们又调用了launchHomeFromHotkey的重构方法，这样我们看一下这个重构方法的实现。
void launchHomeFromHotKey(final boolean awakenFromDreams, final boolean respectKeyguard) {
if (respectKeyguard) {
if (isKeyguardShowingAndNotOccluded()) {
// don't launch home if keyguard showing
return;
}
if (!mHideLockScreen && mKeyguardDelegate.isInputRestricted()) {
// when in keyguard restricted mode, must first verify unlock
// before launching home
mKeyguardDelegate.verifyUnlock(new OnKeyguardExitResult() {
@Override
public void onKeyguardExitResult(boolean success) {
if (success) {
try {
ActivityManagerNative.getDefault().stopAppSwitches();
} catch (RemoteException e) {
}
sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
startDockOrHome(true /*fromHomeKey*/, awakenFromDreams);
}
}
});
return;
}
}
// no keyguard stuff to worry about, just launch home!
try {
ActivityManagerNative.getDefault().stopAppSwitches();
} catch (RemoteException e) {
}
if (mRecentsVisible) {
// Hide Recents and notify it to launch Home
if (awakenFromDreams) {
awakenDreams();
}
sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
hideRecentApps(false, true);
} else {
// Otherwise, just launch Home
sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
startDockOrHome(true /*fromHomeKey*/, awakenFromDreams);
}
}可以发现在方法中我们首先调用了ActivityManagerNative.getDefault().stopAppSwitches();该方法主要用于暂停后台的打开Activity的操作，避免打扰用户的操作。比如这时候我们在后台打开一个新的App，那么这时候由于要回到home页面，所以需要先延时打开。方法执行这个方法之后然后执行了sendCloseSystemWindows方法，该方法主要实现了对当前系统App页面的关闭操作，下面我们先看一下ActivityManagerNative.getDefault().stopAppSwitches();方法的实现，这里的ActivityManagerNative.getDefault我们在前面已经多次说过了其是一个Binder对象，是应用进程Binder客户端用于与ActivityManagerService之间通讯，所以这里最终调用的是ActivityManagerService的stopAppsSwitches方法，这样我们就继续看一下ActivityManagerService的stopAppsSwitches方法的实现。
@Override
public void stopAppSwitches() {
if (checkCallingPermission(android.Manifest.permission.STOP_APP_SWITCHES)
!= PackageManager.PERMISSION_GRANTED) {
throw new SecurityException("Requires permission "
+ android.Manifest.permission.STOP_APP_SWITCHES);
}
synchronized(this) {
mAppSwitchesAllowedTime = SystemClock.uptimeMillis()
+ APP_SWITCH_DELAY_TIME;
mDidAppSwitch = false;
mHandler.removeMessages(DO_PENDING_ACTIVITY_LAUNCHES_MSG);
Message msg = mHandler.obtainMessage(DO_PENDING_ACTIVITY_LAUNCHES_MSG);
mHandler.sendMessageDelayed(msg, APP_SWITCH_DELAY_TIME);
}
}可以发现这里主要是发送了一个异步消息，并且msg.what为DO_PENDING_ACTIVITY_LAUNCHES_MSG，即跳转Activity，然后我们继续我们看一下mHandler的handleMessage方法当msg.what为DO_PENDING_ACTIVITY_LAUNCHES_MSG时的操作。而且我们可以发现这里的异步消息是一个延时的异步消息，延时的时间为APP_SWITCH_DELAY_TIME，我们可以看一下该变量的定义：
// Amount of time after a call to stopAppSwitches() during which we will
// prevent further untrusted switches from happening.
static final long APP_SWITCH_DELAY_TIME = 5*1000;然后我们可以看一下mHander的handleMessage方法的具体实现：
case DO_PENDING_ACTIVITY_LAUNCHES_MSG: {
synchronized (ActivityManagerService.this) {
mStackSupervisor.doPendingActivityLaunchesLocked(true);
}
} break;可以发现这里直接调用了mStackSupervisor.doPendingActivityLaunchesLocked方法，好吧，继续看一下doPendingActivityLaunchesLocked方法的实现。
final void doPendingActivityLaunchesLocked(boolean doResume) {
while (!mPendingActivityLaunches.isEmpty()) {
PendingActivityLaunch pal = mPendingActivityLaunches.remove(0);
startActivityUncheckedLocked(pal.r, pal.sourceRecord, null, null, pal.startFlags,
doResume && mPendingActivityLaunches.isEmpty(), null, null);
}
}可以发现这里就是调用了startActivity的操作了，看过Activity启动流程的同学应该知道：android源码解析之（十四）-->Activity启动流程 这里就是开始启动Activity了，所以当我们按下HOME按键的时候，后台的startActivity都会延时5秒钟执行...
然后回到我们的launchHomeFromHotKey方法，看一下launchHomeFromHotKey方法的实现。
void sendCloseSystemWindows(String reason) {
PhoneWindow.sendCloseSystemWindows(mContext, reason);
}可以发现这里调用了PhoneWindow的静态方法sendCloseSystemWindow,继续看一下该方法的实现逻辑。
public static void sendCloseSystemWindows(Context context, String reason) {
if (ActivityManagerNative.isSystemReady()) {
try {
ActivityManagerNative.getDefault().closeSystemDialogs(reason);
} catch (RemoteException e) {
}
}
}看到这里，很明显了又是调用了Binder的进程间通讯，最终ActivityManagerService的closeSystemDialogs方法会被执行，所以我们继续看一下ActivityManagerService的closeSystemDialogs方法的实现。
@Override
public void closeSystemDialogs(String reason) {
enforceNotIsolatedCaller("closeSystemDialogs");
final int pid = Binder.getCallingPid();
final int uid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
try {
synchronized (this) {
// Only allow this from foreground processes, so that background
// applications can't abuse it to prevent system UI from being shown.
if (uid >= Process.FIRST_APPLICATION_UID) {
ProcessRecord proc;
synchronized (mPidsSelfLocked) {
proc = mPidsSelfLocked.get(pid);
}
if (proc.curRawAdj > ProcessList.PERCEPTIBLE_APP_ADJ) {
Slog.w(TAG, "Ignoring closeSystemDialogs " + reason
+ " from background process " + proc);
return;
}
}
closeSystemDialogsLocked(reason);
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}可以发现其实在方法体中将关闭窗口的逻辑下发到了closeSystemDialogsLocked中，所以我们继续看一下closeSystemDialogsLocked方法的实现。
void closeSystemDialogsLocked(String reason) {
Intent intent = new Intent(Intent.ACTION_CLOSE_SYSTEM_DIALOGS);
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY
| Intent.FLAG_RECEIVER_FOREGROUND);
if (reason != null) {
intent.putExtra("reason", reason);
}
mWindowManager.closeSystemDialogs(reason);
mStackSupervisor.closeSystemDialogsLocked();
broadcastIntentLocked(null, null, intent, null, null, 0, null, null, null,
AppOpsManager.OP_NONE, null, false, false,
-1, Process.SYSTEM_UID, UserHandle.USER_ALL);
}可以发现在方法体中首先调用了mWindowManager.closeSystemDialogs方法，该方法就是关闭当前页面中存在的系统窗口，比如输入法，壁纸等：
@Override
public void closeSystemDialogs(String reason) {
synchronized(mWindowMap) {
final int numDisplays = mDisplayContents.size();
for (int displayNdx = 0; displayNdx < numDisplays; ++displayNdx) {
final WindowList windows = mDisplayContents.valueAt(displayNdx).getWindowList();
final int numWindows = windows.size();
for (int winNdx = 0; winNdx < numWindows; ++winNdx) {
final WindowState w = windows.get(winNdx);
if (w.mHasSurface) {
try {
w.mClient.closeSystemDialogs(reason);
} catch (RemoteException e) {
}
}
}
}
}
}讲过这样一层操作之后，我们就关闭了当前中存在的系统窗口。然后还是回到我们的launchHomeFromHotKey方法，我们发现在方法体的最后我们调用了startDockOrHome方法，这个方法就是实际的跳转HOME页面的方法了，我们可以具体看一下该方法的实现。
void startDockOrHome(boolean fromHomeKey, boolean awakenFromDreams) {
if (awakenFromDreams) {
awakenDreams();
}
Intent dock = createHomeDockIntent();
if (dock != null) {
try {
if (fromHomeKey) {
dock.putExtra(WindowManagerPolicy.EXTRA_FROM_HOME_KEY, fromHomeKey);
}
startActivityAsUser(dock, UserHandle.CURRENT);
return;
} catch (ActivityNotFoundException e) {
}
}
Intent intent;
if (fromHomeKey) {
intent = new Intent(mHomeIntent);
intent.putExtra(WindowManagerPolicy.EXTRA_FROM_HOME_KEY, fromHomeKey);
} else {
intent = mHomeIntent;
}
startActivityAsUser(intent, UserHandle.CURRENT);
}可以发现我们在方法体中调用了createHomeDockIntent，这个方法的作用就是创建到达HOME页面的Intent对象，然后我们调用了startActivityAsUser方法，这样经过一系列的调用之后就调起了home页面的Activity，所以这时候系统就返回到了HOME页面。
总结：
系统也是在PhoneWindowManager中监听HOME按键的点击并进行处理；
系统监听到HOME按键之后会首先关闭相应的系统弹窗；
另外对android源码解析方法感兴趣的可参考我的：
android源码解析之（一）-->android项目构建过程
android源码解析之（二）-->异步消息机制
android源码解析之（三）-->异步任务AsyncTask
android源码解析之（四）-->HandlerThread
android源码解析之（五）-->IntentService
android源码解析之（六）-->Log
android源码解析之（七）-->LruCache
android源码解析之（八）-->Zygote进程启动流程
android源码解析之（九）-->SystemServer进程启动流程
android源码解析之（十）-->Launcher启动流程
android源码解析之（十一）-->应用进程启动流程
android源码解析之（十二）-->系统启动并解析Manifest的流程
android源码解析之（十三）-->apk安装流程
android源码解析之（十四）-->Activity启动流程
android源码解析之（十五）-->Activity销毁流程
android源码解析（十六）-->应用进程Context创建流程
android源码解析（十七）-->Activity布局加载流程
android源码解析（十八）-->Activity布局绘制流程
android源码解析（十九）-->Dialog加载绘制流程
android源码解析（二十）-->Dialog取消绘制流程
android源码解析（二十一）-->PopupWindow加载绘制流程
android源码解析（二十二）-->Toast加载绘制流程
android源码解析（二十三）-->Android异常处理流程
android源码解析（二十四）-->onSaveInstanceState执行时机
android源码解析（二十五）-->onLowMemory执行流程
android源码解析（二十六）-->截屏事件流程
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