Android Home键之后启动Activity延迟5秒

按home键之后，所有activity的启动请求都延迟了5秒，下面我们分析一个系统是怎么做的

    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) {
        }
......
}

PhoneWindowManager处理Home键之后调用stopAppSwitches()

    @Override
    public void stopAppSwitches() {
        if (checkCallingPermission(android.Manifest.permission.STOP_APP_SWITCHES)
                != PackageManager.PERMISSION_GRANTED) {
            throw new SecurityException("viewquires 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);
        }
    }

ActivityManagerService收到stopAppSwitches请求后，取消DO_PENDING_ACTIVITY_LAUNCHES_MSG,发送APP_SWITCH_DELAY_TIME,在Stub端将所有的activity启动请求延迟了5秒，这个是Google为了提升用户体验有意为之。