Activity的四种启动模式和相关的Intent flag

本文主要是介绍Activity的四种启动模式和相关的Intent flag，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

一、启动模式

1、 standard模式。

        默认模式，可以不用写配置。在这个模式下，都会默认创建一个新的实例。因此，在这种模式下，可以有多个相同的实例，也允许多个相同Activity叠加。应用场景：绝大多数Activity。

2、singleTop模式

        栈顶复用模式，如果要开启的activity在任务栈的顶部已经存在，就不会创建新的实例，而是调用 onNewIntent() 方法来将 intent 转送给该实例，避免栈顶的activity被重复的创建。

• singleTop模式，只在当前任务栈中生效。

• 如果通过 startActivityForResult 启动一个设置了 singleTop 的 activity，singleTop 模式将无效。

3、singleTask模式

        栈内复用模式，activity只会在任务栈里面存在一个实例。如果要激活的activity，在任务栈里面已经存在，就不会创建新的activity，而是复用这个已经存在的activity，调用onNewIntent() 方法，并且清空这个activity任务栈上面所有的activity, 如果实例不存在，就创建实例并放入栈中。    

4、singleInstance模式

        与 "singleTask" 相似，唯一不同的是系统不会将任何其他 Activity 启动到包含该实例的任务中。该 Activity 始终是其任务唯一的成员；由该 Activity 启动的任何 Activity 都会在其他的任务中打开。

onNewIntent() 使用注意

方法体中需手动调用 setIntent(intent)，否则之后的 getIntent() 获取的都是旧的 intent 对象。

二、常用的 Intent Flag

1、 FLAG_ACTIVITY_NEW_TASK

    /*** 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;

     如果设置了，此活动将成为此历史堆栈上新任务的开始。当使用这个 flag 时,如果 task 中已经有了你要启动的 Activity ，就不再启动一个新的 Activity；当前 task 会被带到前台。可以用 FLAG_ACTIVITY_MULTIPLE_TASK 使这种行为无效。当调用者 startActivityForResult() 时，不能使用此标志。

     比如栈中情况是 A,B,C，在 C 中启动 D，如果在 Manifest.xml 文件中给 D 添加了 Affinity(默认是包名) 的值和 C 所在的 Task 中的不一样，则会在新标记的 Affinity 所存在的 Task 中看 D 是否已经启动，如果已经启动直接将 D 所在的 task 带入到前台，否则直接将 activity 启动；如果是默认的或者指定的 Affinity 和 Task 一样，就和标准模式一样启动一个新的 Activity。此 flag 与启动模式 singleTask 效果不太一样，对于非 Activity 启动的 Activity（比如Service或者通知中启动的Activity）需要显示的设置 Intent.FLAG_ACTIVITY_NEW_TASK。

2、 FLAG_ACTIVITY_CLEAR_TOP

    /*** 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;

     如果申明了 launch mode 是 "multiple" (默认情况就是)且没有在同一个 Intent 中设置 FLAG_ACTIVITY_SINGLE_TOP，Activity 将会结束且重新创建（回调 onCreate 生命周期方法)；对于其他所有的 launch modes 或者设置了 FLAG_ACTIVITY_SINGLE_TOP，Intent 将被分发给实例的 onNewIntent() 方法。

     比如栈中情况是 A,B,C,D，在 D 中启动 B(加入该flag)， 栈中的情况将为 A,B，B 会执行 onCreate() ...。如果希望与 launch mode 中 singleTask 效果相同执行 onNewIntent()，可以同时加上 FLAG_ACTIVITY_SINGLE_TOP。  

3、 FLAG_ACTIVITY_SINGLE_TOP 

    /*** 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;

     如果设置，如果此 activity 已经在历史堆栈的顶部将不会被启动。

     相当于 launch mode 的 singleTop，比如栈中情况是 A,B,C,D，在 D 中启动D(加入该flag)，栈中的情况还是 A,B,C,D。

4、 FLAG_ACTIVITY_CLEAR_TASK

    /*** If set in an Intent passed to {@link Context#startActivity Context.startActivity()},* this flag will cause any existing task that would be associated with the* activity to be cleared before the activity is started.  That is, the activity* becomes the new root of an otherwise empty task, and any old activities* are finished.  This can only be used in conjunction with {@link #FLAG_ACTIVITY_NEW_TASK}.*/public static final int FLAG_ACTIVITY_CLEAR_TASK = 0X00008000;

     如果在一个 Intent 中设置，会导致在此 activity 开启之前，任何与该 activity 相关的 task 都会被清除。此 activity 将会是一个空 task 的最底部的 activity，之前所有的 activities 将被结束，此 flag 只能与 FLAG_ACTIVITY_NEW_TASK 配合使用。

5、 FLAG_ACTIVITY_REORDER_TO_FRONT

    /*** If set in an Intent passed to {@link Context#startActivity Context.startActivity()},* this flag will cause the launched activity to be brought to the front of its* task's history stack if it is already running.** <p>For example, consider a task consisting of four activities: A, B, C, D.* If D calls startActivity() with an Intent that resolves to the component* of activity B, then B will be brought to the front of the history stack,* with this resulting order:  A, C, D, B.** This flag will be ignored if {@link #FLAG_ACTIVITY_CLEAR_TOP} is also* specified.*/public static final int FLAG_ACTIVITY_REORDER_TO_FRONT = 0X00020000;

     如果在 Intent 中设置 ，如果待启动的 activity 已经开启在运行了，此 flag 将使其位于任务历史堆栈的前面。例如栈中情况是 A,B,C,D，如果 D 启动 B，栈中将变成A,C,D,B （B 将回调 onNewIntent() ）。如果 FLAG_ACTIVITY_CLEAR_TOP 也被指定，此标志将被忽略。       

6、 FLAG_ACTIVITY_FORWARD_RESULT  

    /*** If set and this intent is being used to launch a new activity from an* existing one, then the reply target of the existing activity will be* transfered to the new activity.  This way the new activity can call* {@link android.app.Activity#setResult} and have that result sent back to* the reply target of the original activity.*/public static final int FLAG_ACTIVITY_FORWARD_RESULT = 0x02000000;

     如果在 Intent 中设置此 flag 从现有的 activity 去开启一个新的 activity ，现有的 activty 将会把回复的目标转移给新 activity. 新 activity 可以调用 setResult() 将结果发送给现有 activity 的回复目标。

     例如：A 通过 startActivityForResult 启动 B，B 启动 C，但 B 为过渡页可以 finish 了，A 在期望 C 把结果返回。这种情况，B 可以在启动 C 的时候加入该flag。 

7、 FLAG_ACTIVITY_PREVIOUS_IS_TOP

    /*** If set and this intent is being used to launch a new activity from an* existing one, the current activity will not be counted as the top* activity for deciding whether the new intent should be delivered to* the top instead of starting a new one.  The previous activity will* be used as the top, with the assumption being that the current activity* will finish itself immediately.*/public static final int FLAG_ACTIVITY_PREVIOUS_IS_TOP = 0x01000000;

     如果当前的 activity 在开启新 activity 的 intent 设置此 flag， 当前 activity 之前的 activity 将被当视为 top，当前的 activity 将立即结束。

     例如：栈中情况 A,B,C，C 启动 D 时使用此标志，在启动时 C 不会被当成栈顶 Activity，而是 B 作为栈顶启动 D，然后 C 会 finish()。经常与 FLAG_ACTIVITY_FORWARD_RESULT 一起配合使用。

8、 FLAG_ACTIVITY_NO_HISTORY

    /*** 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.** <p>If set, {@link android.app.Activity#onActivityResult onActivityResult()}* is never invoked when the current activity starts a new activity which* sets a result and finishes.*/public static final int FLAG_ACTIVITY_NO_HISTORY = 0x40000000;

     如果设置了此 flag，开启的 activity 将不会存在历史堆栈中。一旦用户离开它，activity 就结束了。也可以用{@link android.R来设置。styleable # AndroidManifestActivity_noHistory noHistory}属性。如果设置了此 flag，activity 将不会回调 onActivityResult()。

9、 FLAG_ACTIVITY_TASK_ON_HOME

    /*** If set in an Intent passed to {@link Context#startActivity Context.startActivity()},* this flag will cause a newly launching task to be placed on top of the current* home activity task (if there is one).  That is, pressing back from the task* will always return the user to home even if that was not the last activity they* saw.   This can only be used in conjunction with {@link #FLAG_ACTIVITY_NEW_TASK}.*/public static final int FLAG_ACTIVITY_TASK_ON_HOME = 0X00004000;

     如果 intent 中设置此 flag 将使新启动的 task 置于当前 home activity 任务之上(如果有的话)。也就是说，从任务中返回总是会将用户返回到home，即使这不是他们看到的最后一个 activity。此 flag 只能与 FLAG_ACTIVITY_NEW_TASK 配合使用。

10、FLAG_EXCLUDE_STOPPED_PACKAGES   

* If set, this intent will not match any components in packages that
* are currently stopped.  If this is not set, then the default behavior
* is to include such applications in the result.

     如果设置，intent 将与当前停止的包中的任何组件不匹配。如果未设置此flag，则默认行为是在结果中包含此类应用程序。

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