Intent-- FLAG_ACTIVITY_CLEAR_TOP

今天在项目中接触到了这么一个关键字，记录下来以备以后查看。

首先来看代码:

Intent intent = new Intent(FirstActivity.this, SecondActivity.class);
it.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);

intent的源码,官方的解释如下:

/**
     * If set, and the activity being launched is already running in the
     * current task, then instead of launching a new instance of that activity,
     * all of the other activities on top of it will be closed and this Intent
     * will be delivered to the (now on top) old activity as a new Intent.
     *
     * <p>For example, consider a task consisting of the activities: A, B, C, D.
     * If D calls startActivity() with an Intent that resolves to the component
     * of activity B, then C and D will be finished and B receive the given
     * Intent, resulting in the stack now being: A, B.
     *
     * <p>The currently running instance of activity B in the above example will
     * either receive the new intent you are starting here in its
     * onNewIntent() method, or be itself finished and restarted with the
     * new intent.  If it has declared its launch mode to be "multiple" (the
     * default) and you have not set {@link #FLAG_ACTIVITY_SINGLE_TOP} in
     * the same intent, then it will be finished and re-created; for all other
     * launch modes or if {@link #FLAG_ACTIVITY_SINGLE_TOP} is set then this
     * Intent will be delivered to the current instance's onNewIntent().
     *
     * <p>This launch mode can also be used to good effect in conjunction with
     * {@link #FLAG_ACTIVITY_NEW_TASK}: if used to start the root activity
     * of a task, it will bring any currently running instance of that task
     * to the foreground, and then clear it to its root state.  This is
     * especially useful, for example, when launching an activity from the
     * notification manager.
     *
     * <p>See
     * <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and Back
     * Stack</a> for more information about tasks.
     */

解释很简单，如果intent设置了这个FLAG，假设需要新启动的Activity为B，并且任务栈中已经存在B的实例，那么会将任务栈中B之上的所有Activity（如果存在）结束。此时，任务栈栈顶的Activity为一个接收到了intent信息的B。

举例说明，假设任务栈中有四个Activity，分别为A-B-C-D（栈顶为D）。此时D调用startActivity() 方法去生成B，此时任务栈中的C和D将会被结束，栈顶为接收到了Intent的B。任务栈中剩余的Activity为A-B。

最后上例中B接收intent的方式取决于它的启动模式，当他的启动模式为标准模式时，B会结束自身在重新创建新的实例来接收intent。而其他启动模式，则不会创建新的实例，而是在onNewIntent()方法中接收intent信息。