Android Activity启动模式和任务栈

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系统提供了两种方式来设置一个Activity的启动模式。一种是在Androidmenifest.xml声明启动模式，另一种是通过Intent的Flag来设置启动模式。

在Androidmenifest.xml中设置启动模式：

standard:
这是默认模式，每次都会创建一个实例，因此onCreate、onStart、onResume都会重复调用。

在standard模式下，堆栈中可能有多个实例。

singletop：
如果该实例已经在堆栈的栈顶上则不会重新创建，onCreate、onStart不会被调用，而是会调用onNewIntent;

如果不在栈顶上，则会重新创建一个实例，onCreate、onStart、onResume都会调用。

在singletop模式下，堆栈中可能有多个实例。

singletask：
如果实例已经在栈顶，则复用，调用onNewIntent；

如果实例不在栈顶，则会destroy其上的所有任务，调用onNewIntent。

总之在singletask模式下，栈中只会有一个实例。

singleinstanse：
只有一个实例，自己单独放在了一个任务栈里面，所以这样的模式至少有两个以上的任务栈。

通过Intent设置启动模式：

    /**
     * If set, the new activity is not kept in the history stack.  As soon as
     * the user navigates away from it, the activity is finished.  This may also
     * be set with the {@link android.R.styleable#AndroidManifestActivity_noHistory
     * noHistory} attribute.
     */
    public static final int FLAG_ACTIVITY_NO_HISTORY = 0x40000000;

使用FLAG_ACTIVITY_NO_HISTORY模式启动的Activity，当该Activity启动其他Activity后，该Activity就消失了，不会保留在Activity栈中

    /**
     * If set, the activity will not be launched if it is already running
     * at the top of the history stack.
     */
    public static final int FLAG_ACTIVITY_SINGLE_TOP = 0x20000000;

使用FLAG_ACTIVITY_SINGLE_TOP模式启动的Activity，效果与配置android:launchMode="singleTop"相同

    /**
     * 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;

使用FLAG_ACTIVITY_NEW_TASK模式启动的Activity，会使用一个新的Task来启动一个Activity，且启动的每个Activity都在一个新的Task中

    /**
     * This flag is used to create a new task and launch an activity into it.
     * This flag is always paired with either {@link #FLAG_ACTIVITY_NEW_DOCUMENT}
     * or {@link #FLAG_ACTIVITY_NEW_TASK}. In both cases these flags alone would
     * search through existing tasks for ones matching this Intent. Only if no such
     * task is found would a new task be created. When paired with
     * FLAG_ACTIVITY_MULTIPLE_TASK both of these behaviors are modified to skip
     * the search for a matching task and unconditionally start a new task.
     *
     * <strong>When used with {@link #FLAG_ACTIVITY_NEW_TASK} do not use this
     * flag unless you are implementing your own
     * top-level application launcher.</strong>  Used in conjunction with
     * {@link #FLAG_ACTIVITY_NEW_TASK} to disable the
     * behavior of bringing an existing task to the foreground.  When set,
     * a new task is <em>always</em> started to host the Activity for the
     * Intent, regardless of whether there is already an existing task running
     * the same thing.
     *
     * <p><strong>Because the default system does not include graphical task management,
     * you should not use this flag unless you provide some way for a user to
     * return back to the tasks you have launched.</strong>
     *
     * See {@link #FLAG_ACTIVITY_NEW_DOCUMENT} for details of this flag's use for
     * creating new document tasks.
     *
     * <p>This flag is ignored if one of {@link #FLAG_ACTIVITY_NEW_TASK} or
     * {@link #FLAG_ACTIVITY_NEW_DOCUMENT} is not also set.
     *
     * <p>See
     * <a href="{@docRoot}guide/topics/fundamentals/tasks-and-back-stack.html">Tasks and Back
     * Stack</a> for more information about tasks.
     *
     * @see #FLAG_ACTIVITY_NEW_DOCUMENT
     * @see #FLAG_ACTIVITY_NEW_TASK
     */
    public static final int FLAG_ACTIVITY_MULTIPLE_TASK = 0x08000000;

使用FLAG_ACTIVITY_MULTIPLE_TASK模式启动的Activity，会无条件的创建新的Task

    /**
     * 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;

使用FLAG_ACTIVITY_CLEAR_TOP模式启动的Activity，效果与配置android:launchMode=:"singleTask"相同