总是看到Activity的四种启动模式：Standard，SingleTask，SingleTop，SingleInstance，而且都是借助于官方文档理解的。但是最近在写Demo测试这几种启动模式的特点与区别的时候，发现与官方文档有不一致的地方。本篇文章主要讲解的是SingleTask这种LaunchMode。

在Android官方文档中，对“singleTask”模式解释的是：The system creates a new task and instaniates the activity at the root of the new task。

但是在写demo测试的时候，发现这个说法是不准确的。这句话成立的条件是，要启动的Activity的affinity是唯一的，即当前所有的任务中没有与该affinity相同的任务。

Demo中包含两个Activity，Activity的LaunchMode：

MainActivity-Standard
SecondActivity- SingleTask

步骤：

启动MainActivity；
MainActivity中启动SecondActivity
SecondActivity中启动MainActivity
MainActivity中启动SecondActivity

使用命令：“dumpsys activity activities“，dump Activity栈的信息。

2. 详细过程

2.1 启动MainActivity

2.1.1 生命周期

2019-02-25 08:05:36.334 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onCreate:this=com.maureen.testtwoactivity.MainActivity@f6993c1
2019-02-25 08:05:36.528 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onStart:this=com.maureen.testtwoactivity.MainActivity@f6993c1
2019-02-25 08:05:36.531 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onResume:this=com.maureen.testtwoactivity.MainActivity@f6993c1

2.1.2 Activity栈

TaskRecord{832ddac #32 A=com.maureen.testtwoactivity U=0 StackId=1 sz=1}
        Run #0: ActivityRecord{2de94c6 u0 com.maureen.testtwoactivity/.MainActivity t32}

2.2 MainActivity中启动SecondActivity

2.2.1 生命周期

2019-02-25 08:06:22.582 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onPause:this=com.maureen.testtwoactivity.MainActivity@f6993c1
2019-02-25 08:06:22.669 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onCreate:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:06:22.772 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onStart:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:06:22.776 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onResume:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:06:23.520 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onStop:this=com.maureen.testtwoactivity.MainActivity@f6993c1
2019-02-25 08:06:23.522 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onSaveInstanceState:this=com.maureen.testtwoactivity.MainActivity@f6993c1

2.2.2 Activity栈

TaskRecord{832ddac #32 A=com.maureen.testtwoactivity U=0 StackId=1 sz=2}
        Run #1: ActivityRecord{8be2cd6 u0 com.maureen.testtwoactivity/.SecondActivity t32}
        Run #0: ActivityRecord{2de94c6 u0 com.maureen.testtwoactivity/.MainActivity t32}

2.3 SecondActivity中启动MainActivity

2.3.1 生命周期

2019-02-25 08:07:46.098 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onPause:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:07:46.165 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onCreate:this=com.maureen.testtwoactivity.MainActivity@24103c8
2019-02-25 08:07:46.272 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onStart:this=com.maureen.testtwoactivity.MainActivity@24103c8
2019-02-25 08:07:46.277 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onResume:this=com.maureen.testtwoactivity.MainActivity@24103c8
2019-02-25 08:07:47.006 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onStop:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:07:47.008 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onSaveInstanceState:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26

2.3.2 Activity栈

TaskRecord{832ddac #32 A=com.maureen.testtwoactivity U=0 StackId=1 sz=3}
        Run #2: ActivityRecord{3e6e847 u0 com.maureen.testtwoactivity/.MainActivity t32}
        Run #1: ActivityRecord{8be2cd6 u0 com.maureen.testtwoactivity/.SecondActivity t32}
        Run #0: ActivityRecord{2de94c6 u0 com.maureen.testtwoactivity/.MainActivity t32}

2.4 MainActivity中启动SecondActivity

2.4.1 生命周期

2019-02-25 08:08:51.142 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onPause:this=com.maureen.testtwoactivity.MainActivity@24103c8
2019-02-25 08:08:51.174 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onNewIntent:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:08:51.182 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onStart:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:08:51.185 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]SecondActivity: onResume:this=com.maureen.testtwoactivity.SecondActivity@d5d6a26
2019-02-25 08:08:51.898 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onStop:this=com.maureen.testtwoactivity.MainActivity@24103c8
2019-02-25 08:08:51.902 3077-3077/com.maureen.testtwoactivity D/[APP_Maureen]MainActivity: onDestroy:this=com.maureen.testtwoactivity.MainActivity@24103c8

2.4.2 Activity栈

TaskRecord{8ea7a0a #2 A=com.android.car.carlauncher U=0 StackId=0 sz=1}
        Run #0: ActivityRecord{d99cab7 u0 com.android.car.carlauncher/.CarLauncher t2}

3. 总结

“singleTask”模式启动的Activity，在启动时，会先在系统中查找属性值affinity等于它的属性值taskAffinity的任务是否存在：如果存在，则在该任务中启动；如果不存在，则会在新任务中启动。因此，如果想要设置“singleTask”启动模式的Activity在新的任务中启动，就要为它设置一个独立的taskAffinity的值。(这句话更加具体的一个例子是：App1中启动了一个taskAffinity为com.xx.test的ActivityA，App2中要启动一个taskAffinity为com.xx.test的ActivityB，那么ActivityB会在ActivityA所在的任务中启动)。
如果设置了“singleTask”启动模式的Activity不是在新的任务中启动的，它会在已有的任务中查看是否已经存在相应的Activity实例，如果存在，会调用该实例的onNewIntent，并且会把位于这个Activity实例上面的Activit全部结束掉，最终这个Activity实例会位于任务的堆栈顶端。