android home键

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.