singleTask app doesn't reuse the Task already present in recent apps when started with startActivityForResult method

Question

I have two android apps, lets call them app A and app B.
Inside app A I create a share intent as follows:
shareIntent=Intent.createChooser(sendIntent, null); // sendIntent contains data to share.
this will be used to pass data to app B.
App B's MainActivity (its only Activity) is configured to have launchMode="singleTask".

Behavior 1:
If from app A I use startActivity(shareIntent) and app B was in background, it comes to foreground and handles the intent with onNewIntent method. If app B wasn't in background it starts a new task for app B with its initial intent set to sendIntent that was passed from app A.

Behavior 2:
If from app A I use startActivityForResult(shareIntent, SUCCESS_CODE) it doesn't matter if app B was in background or not, It is started afresh, moreover the MainActivity of app B is started and pushed to the backStack of app A, it doesn't start in a separate task, after this I have 2 MainActivities of app B, one which I kept in background, and one launched inside app A.

Why does startActivityForResult causes Behavior 2?
What can be done as a developer of app B who has no control over how app A starts it to always show Behavior 1?

Answer 1

You cannot start an Activity using startActivityForResult() if that Activity has launch mode of singleTask or singleInstance. The reason is that startActivityForResult() requires the target Activity (i: the one being launched) to run in the same task as the Activity that expects the result.

Normally, if you try to launch an Activity and that Activity is declared with launch mode of singleTask or singleInstance, Android automatically adds Intent.FLAG_ACTIVITY_NEW_TASK to the Intent and ensures that the target Activity is launched in a separate task.

If a developer expects that his Activity will be launched using startActivityForResult() and he codes the Activity to actually return a result, it makes absolutely no sense to declare this Activity with launch mode of singleTask or singleInstance.

This behaviour is not what you say you are seeing, but it is clear that there is a conflict between the contract for startActivityForResult() and launch mode of singleTask.