Android片段popFromBackstack（）仅可运行一次

Question

I've got two fragments that I am initializing, and committing like so....

FragmentA

    getFragmentManager().beginTransaction()
            .add(R.id.filter_fragment, fragmentA)
            .commit();

FragmentB

    getFragmentManager().beginTransaction()
            .replace(R.id.filter_fragment, fragmentB)
            .addToBackStack(null)
            .commit();

FragmentA is created and shown on application start, but FragmentB is not shown until a button is clicked by the user.

The issue that I am having is this...

1. Fragment A loaded at application start
2. User action creates Fragment B; Fragment B shown successfully.
3. User presses device back button, and getFragmentManager.popBackStack() is called. Fragment A shown successfully.
4. User action creates Fragment B again; Fragment B shown successfully.

5. User presses device back button, and getFragmentManager.popBackStack() is called. Application crash with the following error:

    java.lang.IllegalStateException: Fragment already added: FragmentB{3c73b216 #2 id=0x7f0f00c6} 

I've been pulling my hair out for the last day trying to figure out why the transactions and backstack pops work one time, but crash the second time. I've tried playing around with various combinations of add() , replace() , remove() , etc, and it only gets worse. My current crashing implementation is the closest I have gotten.

Answer 1

Set first fragment:

if (null == savedInstanceState) {
    getSupportFragmentManager().beginTransaction().addToBackStack("fragmentA").replace(R.id.container, new FragmentA(), "CURRENT_FRAGMENT_TAG").commit();
}

in OnCreate in your activity

---

Use this function to replace fragments to avoid adding same fragment over and over again to stack:

public void replaceFragment(Fragment fragment, String tag) {
    Fragment currentFragment = getSupportFragmentManager().findFragmentByTag("CURRENT_FRAGMENT_TAG");

    //to avoid adding the same fragment, you can use 'instanceof' instead of comparator
    if (currentFragment.getClass() == fragment.getClass()) {
        return;
    }
    getSupportFragmentManager().popBackStack(tag, FragmentManager.POP_BACK_STACK_INCLUSIVE);
    getSupportFragmentManager()
            .beginTransaction()
            .addToBackStack(tag).replace(R.id.container, fragment, "CURRENT_FRAGMENT_TAG")
            .commit();

}

Example execution:

replaceFragment(new FragmentB(), "fragmentB");

---

Override onBackPressed .

@Override
public void onBackPressed() {
    if (getSupportFragmentManager().getBackStackEntryCount() == 1) {
        finish();
    } else {
        super.onBackPressed();
    }
}

Answer 2

On your activity that calls FragmentA, do this:

getFragmentManager().beginTransaction() 
        .replace(R.id.filter_fragment, fragmentA)
        .commit();

and the R.id.filter_fragment, should be the layout from the activity which will host the fragments' layouts.