如何处理片段回栈

Question

I'm having problem with fragment. Lets try to understand my issue, I have two fragment A and B . When app start with main activity,i start fragment A as you can see:

 getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,new MusicFragment())
                .commit();

When i click on a button, it starts fragment B

 getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,new BarFragment())
                .addToBackStack(null)
                .commit();

Main problem is after starting fragment B ,when i pressed back to go back to fragment A , Fragment A Recreated with new state. I don' want to recreate fragment A . I only want to start fragment from old state where i left. How to fix it?

Answer 1

Instead of calling the replace method you should be calling the add method with a subsequent call to addToBackStack with an argument of null . The add method adds the fragement to the stack with a null tag and calling the addToBackStack with an argument of null then the current fragment is stopped upon commit. If the method is not called then the current fragment is destroyed and recreated when coming back.

getSupportFragmentManager().beginTransaction()
            .add(R.id.fragment_container,new BarFragment())
            .addToBackStack(null)
            .commit();

You can clearly find it in the documentation quote saying this:

If you don't call addToBackStack() when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call addToBackStack() when removing a fragment, then the fragment is stopped and is later resumed if the user navigates back.