从 backstack 获取片段后按回崩溃应用程序，“Android IllegalStateException Fragment already added”

Question

So I have an activity that holds a MapsFragment and first initializes it without storing it in backstack (if I store it in backstack, pressing back button pops off fragment and leaves empty container, which is not what I want). this is the code:

transaction.replace(mFragmentContainerRes, 
                   MapsFragment.newInstance(), MapsFragment.getClass().getName()).commit();

I then replace it with a ChatRoomFragment like so:

transaction.replace(mFragmentContainerRes, 
                   ChatRoomFragment .newInstance(), ChatRoomFragment .getClass().getName())
                .addToBackStack(ChatRoomFragment .getClass().getName());
                transaction.commit();

So then I have a Navigation Drawer in my main activity, and when the home button is pressed, I retrieve the MapsFragment like so from the backstack:

Fragment fragment = manager.findFragmentByTag(MapFragment.getClass().getName());

 transaction.replace(mFragmentContainerRes, fragment, backStateName).commit();

Now if I press the back button while in the retrieved MapsFragment, the app crashes with this error thrown:

java.lang.IllegalStateException: 
Fragment already added: MapsFragment{e34dc65 #0 id=0x7f0f00b4 com.MapsFragment}
                                                                                  at android.support.v4.app.FragmentManagerImpl.addFragment(FragmentManager.java:1882)
                                                                                  at android.support.v4.app.BackStackRecord.executePopOps(BackStackRecord.java:825)
                                                                                  at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2577)
                                                                                  at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
                                                                                  at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
                                                                                  at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:851)
                                                                                  at android.support.v4.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:794)
                                                                                  at android.support.v4.app.FragmentActivity.onBackPressed(FragmentActivity.java:174)
                                                                                  at com.main.MainActivity.onBackPressed(MainActivity.java:111)

For some reason, the Activity is trying to add the MapsFragment again when I press the back button. Why is this? How can I fix this?

According to my research, this usually happens if you .add instead of .replace, but I am calling .replace everywhere on my Fragment transactions.

Answer 1

Solved this issue by popping my entire backstack when wanting to return to the home fragment instead of replacing the current fragment with the home fragment:

            getSupportFragmentManager().popBackStack(null,
                    FragmentManager.POP_BACK_STACK_INCLUSIVE);

Answer 2

The fragment backstack just stores FragmentTransactions, not actual fragments. And when it's popped when you click back, it simply reverses this transaction as it got it. So in your case you load ChatRoomFragment with replace, which is the same as removing all fragments and adding ChatRoomFragment. So the FragmentTransaction going forward is [1 - Remove MapsFragment, 2 - Add ChatRoomFragment] . This is the FragmentTransaction and is executed and now you're on ChatRoomFragment. If you popBackStack now, it works like it should. The transaction is reversed and ChatRoomFragment is removed and MapsFragment is added.

When you click the Home button on the NavDrawer, you replace MapsFragment, which is the same as removing ChatRoomFragment and adding MapsFragment. Note this transaction isn't added to the backstack so the current backstack is still [1 - Add MapsFragment, 2 - Remove ChatRoomFragment] , the reverse of the initial FragmentTransaction in the previous paragraph. The stupid backstack doesn't know you replaced MapsFragment and it's the current fragment so when you click the back button now, it just executes the current backstack and tries to add the MapsFragment again and you get that Fragment already added error.

Your selected solution works because you're simply popping the backstack when a user clicks the Home button on the NavDrawer and the backstack is clear now.