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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
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