My app is designed to run as a single instance and the Back button does not allow you to exit the app and return to the Start screen because it is used internally to navigate a hierarchy of screens where each screen can be an activity.
However, an external app can launch one of the app's internal activities. When the user is done with whatever the activity is designed for, the user's intuitive action is to hit the Back button to return back to the calling client. But because I prevent the Back button from exiting, the user cannot return.
I can add code to override this when the code detects that the activity is being launched by a client. The problem however is that if the app closes to return, the user might return to the app from where they left off. But since I closed the app to return to the calling client, the user cannot return back to the app as it was last opened. My app needs to remain as a single instance, so the activity that gets launched cannot be created more than once. Any suggestions on how to return back to the calling client but also keep the app running if it was running when the calling client used one of its activities?
In general, you can programmatically control the back navigation trail. Take a look at TaskStackBuilder and the documentation for handling notifications Responding to Notifications. It seems that what you're trying to do is control the so-called "back stack". If you use TaskStackBuilder, the behavior will match the platform version you're on.
In pre 3.0 platforms, the Back button went all the way through the back stack to the first task the user did since the phone was turned on. Post 3.0, back does not traverse task boundaries; to get to other tasks, the user clicks the "Recent Items" icon. There's also the "up" icon in an app to navigate to the "beginning" of a task within an app. TaskStackBuilder will "do the right thing" for all versions.
In the current platform version, not allowing Back to exit your app is OK, because Back should only go to the first Activity in the current task. In versions previous to 3.0, not allowing Back to exit the app is more problematic, and I personally wouldn't do it that way, but it's up to you.
What happens when the user clicks Back after an Intent starts your app should be clear from the documentation I've cited. Basically, what you want to do is go back to the previous task.