引用：http://www.linuxidc.com/Linux/2012-01/51332.htm
当我们从Home点击ShortCut图标启动一个应用程序后，这个应用程序打开了很多个Activity，假设顺序为A,B,C，然后我们按Home键，再次从桌面用图标启动这个应用程序，我们会发现显示的是刚才的C,而不是A。这里我们普遍的想法是按Home键是让程序退到后台，然后让桌面显示出来。那么我们就来看看Home键到底是怎么回事。
在Framework中我们找到源码，我们首先在interceptKeyBeforeDispatching这个方法中找到Home按键代码如下
[java]
- // If the HOME button is currently being held, then we do special
- // chording with it.
- if (mHomePressed) {
- // 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 (keyCode == KeyEvent.KEYCODE_HOME) {
- if (!down) {
- mHomePressed = false;
- if (!canceled) {
- // If an incoming call is ringing, HOME is totally disabled.
- // (The user is already on the InCallScreen at this point,
- // and his ONLY options are to answer or reject the call.)
- boolean incomingRinging = false;
- try {
- ITelephony phoneServ = getPhoneInterface();
- if (phoneServ != null) {
- incomingRinging = phoneServ.isRinging();
- } else {
- Log.w(TAG, "Unable to find ITelephony interface");
- }
- } catch (RemoteException ex) {
- Log.w(TAG, "RemoteException from getPhoneInterface()", ex);
- }
- if (incomingRinging) {
- Log.i(TAG, "Ignoring HOME; there's a ringing incoming call.");
- } else {
- launchHomeFromHotKey();
- }
- } else {
- Log.i(TAG, "Ignoring HOME; event canceled.");
- }
- }
- }
- return true;
- }
这里就是按Home键时执行的方法我们找到
- launchHomeFromHotKey();
- /**
- * A home key -> launch home action was detected. Take the appropriate action
- * given the situation with the keyguard.
- */
- void launchHomeFromHotKey() {
- if (mKeyguardMediator.isShowingAndNotHidden()) {
- // don't launch home if keyguard showing
- } else if (!mHideLockScreen && mKeyguardMediator.isInputRestricted()) {
- // when in keyguard restricted mode, must first verify unlock
- // before launching home
- mKeyguardMediator.verifyUnlock(new OnKeyguardExitResult() {
- public void onKeyguardExitResult(boolean success) {
- if (success) {
- try {
- ActivityManagerNative.getDefault().stopAppSwitches();
- } catch (RemoteException e) {
- }
- sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
- startDockOrHome();
- }
- }
- });
- } else {
- // no keyguard stuff to worry about, just launch home!
- try {
- ActivityManagerNative.getDefault().stopAppSwitches();
- } catch (RemoteException e) {
- }
- sendCloseSystemWindows(SYSTEM_DIALOG_REASON_HOME_KEY);
- startDockOrHome();
- }
- }
再进入startDockOrHome();
- void startDockOrHome() {
- Intent dock = createHomeDockIntent();
- if (dock != null) {
- try {
- mContext.startActivity(dock);
- return;
- } catch (ActivityNotFoundException e) {
- }
- }
- mContext.startActivity(mHomeIntent);
- }
这里我们发现，源码中做了车载模式和普通模式Home的区别，在Android2.3原生系统中有车载模式这个应用程式，打开后按Home键我们返回的是车载模式的桌面，而普通情况下按Home返回正常桌面,再来看看mContext.startActivity(mHomeIntent)这个方法，我们发现，Home键也是打开一个activity，不同的是下面的代码
[java]
- mHomeIntent = new Intent(Intent.ACTION_MAIN, null);
- mHomeIntent.addCategory(Intent.CATEGORY_HOME);
- mHomeIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK
- | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);
这段代码是在init()方法中，对mHomeIntent的初始化，这个Intent就是跳转到Launcher程序中的配置了 <category android:name="android.intent.category.HOME" />这个属性的Launcher Activity,另外我们还看到两个flag
- FLAG_ACTIVITY_NEW_TASK和FLAG_ACTIVITY_RESET_TASK_IF_NEEDED
newTask保证了我们回到之前Launcher所在的栈,reset task if need是设置清理栈标志，保证清除掉该栈除Launcher以外其他activity(在Launcher清单文件中我们还看到android:clearTaskOnLaunch="true"这个属性，后面会描述),这样我们的Launcher当然会在自己单独的Task中,而且android:launchMode="singleTask"这个属性保证不会启动一个新的Launcher。
通过Launcher启动的其他Activity不会跑到Launcher所在的Task中(后面的文章会提到Launcher启动activity的过程)。所以按Home键是打开了Launcher这个activity并且保证他在自己单独的Task中，其他的activity进入stop状态。
另外Launcher中打开一个activity直接进入之前打开的Task 而不是新打开一个activity.
