Android中FLAG_ACTIVITY_CLEAR_TASK的作用

FLAG_ACTIVITY_CLEAR_TASK :如果在调用Context.startActivity时传递这个标记，将会导致任何用来放置该activity的已经存在的task里面的已经存在的activity先清空，然后该activity再在该task中启动，也就是说，这个新启动的activity变为了这个空tas的根activity.所有老的activity都结束掉。该标志必须和FLAG_ACTIVITY_NEW_TASK一起使用。

示例：

我们有三个activity:MainActivity、ActivityA、ActivityB. 依次从MainActivity中启动ActivityA，从ActivityA中启动ActivityB.

首先我们所有的都使用缺省的，也即没有设置taskAffinity、launchMode，启动activity时Intent也不带标记，我们看一下启动B之后的堆栈：

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1. ACTIVITY MANAGER ACTIVITIES (dumpsys activity activities)    
2.   Main stack:    
3.     TaskRecord{4199dd68 #56 A com.leaves.ipanel U 0}    
4.     Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.leaves.ipanel/.MainActivity }    
5.       Hist #3: ActivityRecord{4140fdf8 u0 com.leaves.ipanel/.ActivityB}    
6.         Intent { act=com.leaves.ipanel.ActivityB cmp=com.leaves.ipanel/.ActivityB }    
7.         ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    
8.       Hist #2: ActivityRecord{413ff7e8 u0 com.leaves.ipanel/.ActivityA}    
9.         Intent { act=com.leaves.ipanel.ActivityA cmp=com.leaves.ipanel/.ActivityA }    
10.         ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    
11.       Hist #1: ActivityRecord{413f8b28 u0 com.leaves.ipanel/.MainActivity}    
12.         Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.leaves.ipanel/.MainActivity }    
13.         ProcessRecord{41517e80 13350:com.leaves.ipanel/u0a10061}    
14.     TaskRecord{41350f60 #2 A com.android.launcher U 0}    
15.     Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10600000 cmp=com.android.launcher/com.android.launcher2.Launcher }    
16.       Hist #0: ActivityRecord{41616790 u0 com.android.launcher/com.android.launcher2.Launcher}    
17.         Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10000000 cmp=com.android.launcher/com.android.launcher2.Launcher }    
18.         ProcessRecord{41615818 628:com.android.launcher/1000}    

可以看到三者在同一个task中，且都存在。

接下来我们在启动ActivityB时添加FLAG_ACTIVITY_CLEAR_TASK和FLAG_ACTIVITY_NEW_TASK

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1. public void onClick(View v) {    
2.     // TODO Auto-generated method stub    
3.     Log.i(TAG, "--onClick--task id = " + getCurrentTaskId());       
4.         
5.     Intent intent = new Intent("com.leaves.ipanel.ActivityB");      
6.     intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);    
7.     startActivity(intent);     
8. }    

再看下启动ActivityB之后的堆栈：

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1. ACTIVITY MANAGER ACTIVITIES (dumpsys activity activities)    
2.   Main stack:    
3.     TaskRecord{4245bd10 #57 A com.leaves.ipanel U 0}    
4.     Intent { act=com.leaves.ipanel.ActivityB flg=0x10008000 cmp=com.leaves.ipanel/.ActivityB }    
5.       Hist #1: ActivityRecord{413f8b28 u0 com.leaves.ipanel/.ActivityB}    
6.         Intent { act=com.leaves.ipanel.ActivityB flg=0x10008000 cmp=com.leaves.ipanel/.ActivityB }    
7.         ProcessRecord{41413010 13441:com.leaves.ipanel/u0a10061}    
8.     TaskRecord{41350f60 #2 A com.android.launcher U 0}    
9.     Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10600000 cmp=com.android.launcher/com.android.launcher2.Launcher }    
10.       Hist #0: ActivityRecord{41616790 u0 com.android.launcher/com.android.launcher2.Launcher}    
11.         Intent { act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10000000 cmp=com.android.launcher/com.android.launcher2.Launcher }    
12.         ProcessRecord{41615818 628:com.android.launcher/1000}    

可以看到,MainActivity和ActivityA都被结束掉了，原因就是因为该task用来装载ActivityB，而启动时又设置了FLAG_ACTIVITY_CLEAR_TASK和FLAG_ACTIVITY_NEW_TASK