difference between popBackStackImmediate vs popBackStack

Question

Could some one please tell me the difference between popBackStackImmediate and popBackStack ? I really don't get it and additionally what is the "flag" 0(zero) means in 2nd of popBackStack?

Thank you very much for your helps guys...

Answer 1

popBackStackImmediate() will perform the popping commands immediately in the call. The results of which can be verified immediately after the call. It is somewhat slower since all the popping actions are performed within the call.

popBackStack() will perform the popping commands within the next event loop cycle (i.e. next draw phase). So it's asynchronous to the rest of the code. That means the FragmentTransaction will not be removed from the backstack after this is executed. In most cases you don't need the FragmentTransaction immediately popped, so it waits until everything else is finished before it actually happens. All this happens so fast that the user wouldn't recognize it.

The flag at the end is unrelated. It can currently only be set to POP_BACK_STACK_INCLUSIVE. The FragmentManager allows you to set an ID on the backstack. If you set the flag, then it will pop the FragmentTransaction that match the ID specified until there is one that does not match the ID or the bottom is reached. If the flag is not set, then all FragmentTransactions that don't match the ID are popped until one is reached that does match the ID or the bottom is reached.

Answer 2

popBackStack() will pop the back stack, but it won't perform the pop until slightly later - it posts a message to do it so you don't have to wait for a heavyweight operation to occur.

popBackStackImmediate() does it right now, before the function returns. Its slower and can cause perf issues. Use the non-immediate version when possible.

0 as the second parameter means to use the default behavior (remove the top element in the backstack). You can also pass it a series of boolean ORed flags. The only flag currently supported is POP_BACK_STACK_INCLUSIVE, which changes it to take out multiple fragments.