Activity Flag

参考：Activity 启动模式及常见启动Flag总结_ws6013480777777的博客-优快云博客

其中，常用的Flags 定义在Intent

    /**
     * If set, this activity will become the start of a new task on this
     * history stack.  A task (from the activity that started it to the
     * next task activity) defines an atomic group of activities that the
     * user can move to.  Tasks can be moved to the foreground and background;
     * all of the activities inside of a particular task always remain in
     * the same order.  See
     * <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and Back
     * Stack</a> for more information about tasks.
     *
     * <p>This flag is generally used by activities that want
     * to present a "launcher" style behavior: they give the user a list of
     * separate things that can be done, which otherwise run completely
     * independently of the activity launching them.
     *
     * <p>When using this flag, if a task is already running for the activity
     * you are now starting, then a new activity will not be started; instead,
     * the current task will simply be brought to the front of the screen with
     * the state it was last in.  See {@link #FLAG_ACTIVITY_MULTIPLE_TASK} for a flag
     * to disable this behavior.
     *
     * <p>This flag can not be used when the caller is requesting a result from
     * the activity being launched.
     */
    public static final int FLAG_ACTIVITY_NEW_TASK = 0x10000000;

    /**
     * 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.
     */
    public static final int FLAG_ACTIVITY_CLEAR_TOP = 0x04000000;