android中的singleTask launchMode无法正常工作

Question

So, I have 4 activities Activity Activity1 , Activity2 , Activity3 and Activity4 . I start from Activity1 then on some event I start Activity2 then on some event on Activity2 I start Activity3 as a new task as

public void onClick(View v) {
    Intent intent = new Intent(Activity2.this, Activity3.class);
    intent.setFlags(Intent.FLAG_ACTIVITY_MULTIPLE_TASK|Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(intent);
}

Here Activity2 launchMode is decalared as singleTask.

Then I navigate from Activity3 to Activity4

When I start Activity2 from Activity4 I believe it should have backstack like

Task A

|Activity2|

|Activity1|

Task B

|Activity4|

|Activity3|

as shown in image below

but instead new instance of Activity2 is added to the current task as

Task B

|Activity2|

|Activity4|

|Activity3|

Task A

|Activity2|

|Activity1|

can someone please help me understand this?

Answer 1

Android will not create a new task unless the taskAffinity of the Activity you are trying to launch is different from the taskAffinity of the current task. taskAffinity takes precendence over anything else.

If you really want to start a separate task, you'll need to adjust the taskAffinity of the Activity3 like this:

<activity android:name=".Activity3"
          android:taskAffinity=""/>

Also, even though Activity4 is declared with launchMode="singleTask" , Android will not create a new task for it unless if has a different taskAffinity from the other activities.

Please be aware that creating multiple tasks for the same application has a lot of side-effects. You should provide a uniqyue icon and a unique label for the root Activity in each task, otherwise the user will be confused when he wants to return to your app (he won't be able to determine which of the various tasks is the correct one). Also, taskReparenting can occur which will unexpectedly move activities from one task to another. This is all very complex stuff and most developers don't completely understand it and therefore get it wrong.

Also, the use of FLAG_ACTIVITY_MULTIPLE_TASK is certainly wrong. The only type of application that needs to use this is a launcher. If you launch multiple tasks with the same Activity in it, you will never be able to return to a specific one programatically, and will totally confuse your user. This is definitely not what you want.

In most cases you don't need multiple tasks, it usually causes more problems than it solves. Please explain why you think you need multiple tasks.