What is Android Task Affinity used for?
An android application has Activities that form a stack like a deck of cards. If you start an android application, and start five activities A,B,C,D,E. They will form a stack
E - chat view
D - weather screen
C - map view
B - weather screen
A - login screen
E was the last Activity to be started and it is showing. If you close E, D will be shown. If you close D, C will be shown. etc.
Notice that Activities B and D are the same activity. What if the user were to make some modifications to the D weather screen, and then decided to close the activity, then close the C Map view?
Then the user would be back at the weather screen and the user would be unhappy because the changes made at level D weather screen were not saved in level B weather screen. Although it's the same activity, it's a different STATE of that activity.
The user had a 5 layer stack of activities, and two of them were the same activity. Popping all 5 off the stack will create the phenomenon where you will be interacting with two different versions of the same activity and can be quite confusing.
Users don't usually think in terms of a rigid stack of activities. They think: "ooh the weather view I'll make a change there" and then they want to go back to the Map view. Then back up again because they want to go back to the Login screen. Why is the B weather app showing and why didn't it save the settings from level D?
The programmer might be able to alleviate some confusion if Activities B and D were linked in state. That way changes to one changes the other. Each time the user opens up a new weather screen, it secretly opens the single instance of the weather screen.
In these circumstances, changing the taskAffinity of the Activity might be desirable. The user would change level D. Then back up to level B. And see the changes in B that were made to D.
The program keeps a stack you can backup through, which is nice, and when the user opens up X instances of the same activity in random places, they are all one.
Slideshow with more explanation:
http://www.slideshare.net/RanNachmany/manipulating-android-tasks-and-back-stack
