Back button closing app even when using FragmentTransaction.addToBackStack()

问题

None of the other questions I have read on stackoverflow have been able to help with my problem. As far as I can tell, I am doing everything correctly.

I have a master/detail flow with fragments.

Upon creation of the main activity, the master fragment is loaded with the following code:

    Fragment frag;
    frag = new MainListFragment();//<-- **the master fragment**

    FragmentManager fm = getFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    transaction.replace(R.id.fragment_container, frag);
    Log.d("My Debug Bitches", "stack:" + fm.getBackStackEntryCount());
    transaction.commit();

The master fragment has a ListView; clicking on a list item brings up the details fragment like so:

@Override
public void onListItemClick(ListView listView, View view, int position, long id) {
    super.onListItemClick(listView, view, position, id);


    FragmentManager fm = getFragmentManager();
    FragmentTransaction transaction = fm.beginTransaction();
    SubListFragment frag = new SubListFragment();//<-- **the detail fragment**

    transaction.replace(R.id.fragment_container, frag);
    transaction.addToBackStack(null);
    transaction.commit();
    fm.executePendingTransactions();
    Log.d("My Debug Bitches", "stack:" + fm.getBackStackEntryCount());

}

Now, according to LogCat, the BackStackEntryCount changes from 0 to 1 after I navigate from master fragment to detail fragment:

So why is it that, when I click the back button while in the details fragment, that the app closes instead of returning to the master fragment??????????

回答1:

You have to add the popBackStack() call to the onBackPressed() method of the activity.

Ex:

@Override
public void onBackPressed() {
    if (fragmentManager.getBackStackEntryCount() > 0) {
        fragmentManager.popBackStack();
    } else {
        super.onBackPressed();
    }
}

回答2:

@Bobbake4's answer is awesome, but there is one little problem. Let's say I have three fragments A, B and C. A is the main Fragment (the fragment that shows when I launch my app), B and C are fragments I can navigate to from the navigation drawer or from A. Now, when I use the back button from B or C, I go back to the previous fragment (A) alright, but the title of the previous fragment (fragment B or C) now shows in the actionBar title of Fragment A. I have to press the back button again to "truly" complete the back navigation (to display the view and correct title for the fragment and returning to)

This is how I solved this problem. Declare these variables.

 public static boolean IS_FRAG_A_SHOWN = false;
 public static boolean IS_FRAG_B_SHOWN = false;
 public static boolean IS_FRAG_C_SHOWN = false;

In the MainActivity of my app where am handling navigation drawer methods, I have a method displayView(position) which handles switching of my fragments.

private void displayView(int position) {
    IS_FRAG_A_SHOWN = false;
    IS_FRAG_B_SHOWN = false;
    IS_FRAG_C_SHOWN = false;
    // update the main content by replacing fragments
    Fragment fragment = null;
    switch (position) {
        case 0:
            fragment = new FragmentA();
            IS_FRAG_A_SHOWN = true;
            break;
        case 1:
            fragment = new FragmentB();
            IS_FRAG_B_SHOWN = true;
            break;
        case 2:
            fragment = new FragmentC();
            IS_FRAG_C_SHOWN = true;
            break;

        default:
            break;
    }

finally, in my onBackPressed method, I do this:

public void onBackPressed() {
    if(fragmentManager.getBackStackEntryCount() != 0) {
        fragmentManager.popBackStack();
        if (IS_FRAG_A_SHOWN) { //If we are in fragment A when we press the back button, finish is called to exit
            finish();
        } else  {
            displayView(0); //else, switch to fragment A
        }

    } else {
        super.onBackPressed();
    }
}

来源：https://stackoverflow.com/questions/23728216/back-button-closing-app-even-when-using-fragmenttransaction-addtobackstack