I had a look at the source code for the ActivityManager. The flag Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED does indeed do some magic that Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP does not do: It triggers task reparenting.
Here’s an (albeit lame) example:
In App A we have the root Activity RootA and we have another Activity ReparentableA:
<application
android:label="@string/app_name">
<activity android:name=".RootA">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<activity android:name=".ReparentableA"
android:allowTaskReparenting="true"/>
</application>
App A has the package name “com.app.a” so the default taskAffinity of its components is “com.app.a”.
In App B we have the root Activity RootB:
<application
android:label="@string/app_name">
<activity android:name="RootB">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
</application>
App B has the package name “com.app.b” so the default taskAffinity of its components is “com.app.b”.
Now we launch App B from the HOME screen. This starts a new task and creates a new instance of Activity RootB as the root Activity in that task. Activity RootB now launches Activity ReparentableA in the standard way, without any special flags. An instance of ReparentableA is created and put on top of RootB in the current task.
Press HOME.
Now we launch App A from the HOME screen. This starts a new task and creates a new instance of Activity RootA as the root Activity in that task. NOTE: When Android launches a “launcher” Intent, it automatically sets the flags Intent.FLAG_ACTIVITY_NEW_TASK and Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED. Because of this, launching RootA now triggers task reparenting. Android looks to see if there are any activities in any other tasks that have an affinity for this new task (and are reparentable). It finds ReparentableA (which has the same task affinity as RootA) in the App B task and moves it to the new App A task. When launching App A we do not see RootA, we actually see ReparentableA, as it is moved to the top of the new task.
If we return to App B, we can see that ReparentableA is gone from the task stack and that task now consists of only one Activity: RootB.
Notes about using Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP
The important thing to remember about using these flags to “reset a task” is that it only works if there is already an instance of the target Activity at the root of the task. If your root Activity ever finishes, you cannot clear your task by starting the root Activity with Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP. Android will just create a new instance of the target (root) Activity and put it on top of the existing activities in the task, which is probably not at all what you want.
Difference between Intent.FLAG_ACTIVITY_CLEAR_TASK and Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP:
As noted above, using CLEAR_TOP | SINGLE_TOP only works if there is already an instance of the target Activity in the task. CLEAR_TASK, however, removes all activities from the task, regardless of whether or not there was an instance of the target Activity in the task. Also, using CLEAR_TASK ensures that the target Activity becomes the root Activity of the task, without you needing to know what Activity was the root Activity before you cleared the task.
Difference between Intent.FLAG_ACTIVITY_CLEAR_TASK and Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED:
As indicated above, using CLEAR_TASK will always remove all activities from the task and launch a new instance of the target activity. In contrast, RESET_TASK_IF_NEEDED will only reset the task in certain situations (the “IF_NEEDED” part). The task is only “reset” if Android is either:
- Creating a new task (in which case the “reset” functionality involves the task reparenting explained above), or
- If Android is bringing a background task to the foreground (in which case the task is only cleared of any activities that were launched with
Intent.FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESETand any activities that are on top of those activities). NOTE: The root Activity is never cleared in this case.
IMPORTANT NOTE: When you are testing, please note that there is a difference in the way Android behaves when launching apps from the HOME screen (or from the list of available applications) and when selecting tasks from the recent task list.
In the first case (launching an app by selecting it from the list of available applications or from a shortcut on the HOME screen), a launcher Intent with Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED is created. This is used regardless of whether or not the app is already running. The Intent is launched and then the ActivityManager figures out what to do.
In the second case (selecting a task from the list of recent tasks), if the task still exists, it is just brought the front. The task “reset” is NOT performed if the task is just brought to the front using the recent task list. It isn’t obvious to me how this is managed and I’ve not had a chance to look through the source code to figure out why that is.
I hope this answers your questions. Looking forward to your feedback and test results.