In Fragment on back button pressed Activity is bla

问题:

I have an Activity and many fragments inflated in same FrameLayout

<FrameLayout
    android:id="@+id/content_frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

example: mainActivity > any fragment (press back button) > activity is blank.

In onCreate:

layout = (FrameLayout)findViewById(R.id.content_frame);
layout.setVisibility(View.GONE);

When I start a fragment:

FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.content_frame, profileFragment);
ft.addToBackStack(null);
ft.commit();
layout.setVisibility(View.VISIBLE);

I suppose I need to make the frameLayout's visibility GONE again on back pressed, but how do I do this?

---

I tried onBackPressed and set layout.setVisibility(View.GONE); but I cannot go back through fragments, as I go directly to main page.

回答1:

If you have more than one fragment been used in the activity or even if you have only one fragment then the first fragment should not have addToBackStack defined. Since this allows back navigation and prior to this fragment the empty activity layout will be displayed.

 // fragmentTransaction.addToBackStack() // dont include this for your first fragment.

But for the other fragment you need to have this defined otherwise the back will not navigate to earlier screen (fragment) instead the application might shutdown.

回答2:

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    }
    else {
        int fragments = getSupportFragmentManager().getBackStackEntryCount();
        if (fragments == 1) {
            finish();
        } else {
            if (getFragmentManager().getBackStackEntryCount() > 1) {
                getFragmentManager().popBackStack();
            } else {
                super.onBackPressed();
            }
        }
    }
}

To add a fragment

 getSupportFragmentManager().beginTransaction()
                .replace(R.id.layout_main, dashboardFragment, getString(R.string.title_dashboard))
                .addToBackStack(getString(R.string.title_dashboard))
                .commit();

回答3:

Sorry for the late response.

You don't have to add ft.addToBackStack(null); while adding first fragment.

FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.content_frame, profileFragment);
// ft.addToBackStack(null); --remove this line.
ft.commit();
// ... rest of code

回答4:

You can override onBackPressed and check to see if there is anything on the backstack.

@Override
public void onBackPressed() {
    int fragments = getFragmentManager().getBackStackEntryCount();
    if (fragments == 1) { 
        // make layout invisible since last fragment will be removed
    }
    super.onBackPressed();
}

回答5:

If you want to track by the fragments you should override the onBackPressed method, like this

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

回答6:

On a recent personal project, I solved this by not calling addToBackStack if the stack is empty.

    // don't add the first fragment to the backstack
    // otherwise, pressing back on that fragment will result in a blank screen
    if (fragmentManager.getFragments() != null) {
        transaction.addToBackStack(tag);
    }

Here's my full implementation:

    String tag = String.valueOf(mCurrentSectionId);
    FragmentManager fragmentManager = mActivity.getSupportFragmentManager();
    Fragment fragment = fragmentManager.findFragmentByTag(tag);

    if (fragment != null) {
        // if the fragment exists then no need to create it, just pop back to it so
        // that repeatedly toggling between fragments doesn't create a giant stack
        fragmentManager.popBackStackImmediate(tag, 0);
    } else {
        // at this point, popping back to that fragment didn't happen
        // So create a new one and then show it
        fragment = createFragmentForSection(mCurrentSectionId);

        FragmentTransaction transaction = fragmentManager.beginTransaction()
                .setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN)
                .replace(R.id.main_content, fragment, tag);

        // don't add the first fragment to the backstack
        // otherwise, pressing back on that fragment will result in a blank screen
        if (fragmentManager.getFragments() != null) {
            transaction.addToBackStack(tag);
        }

        transaction.commit();
    }

回答7:

irscomp's solution works if you want to end activity when back button is pressed on first fragment. But if you want to track all fragments, and go back from one to another in back order, you add all fragments to stack with:

ft.addToBackStack(null);

and then, add this to the end of onCreate() to avoid blank screen in last back pressed; you can use getSupportFragmentManager() or getFragmentManager() depending on your API:

FragmentManager fm = getSupportFragmentManager();
    fm.addOnBackStackChangedListener(new OnBackStackChangedListener() {
        @Override
        public void onBackStackChanged() {
            if(getSupportFragmentManager().getBackStackEntryCount() == 0) finish();             
        }
});

Final words: I don't suggest you to use this solution, because if you go from fragment1 to fragment 2 and vice versa 10 times, when you press back button 10 times it will do it in back order which users will not want it.

回答8:

I still could not fix the issue through getBackStackEntryCount() and i solved my issue by making the main page a fragment too, so in the end i have an activity with a FrameLayout only; and all other fragments including the main page i inflate into that layout. This solved my issue.

回答9:

Almost same as Goodlife's answer, but in Xamarin.Android way:

Load fragment (I wrote helper method for that, but it's not necessary):

public void LoadFragment(Activity activity, Fragment fragment, string fragmentTitle = "")
{
    var fragmentManager = activity.FragmentManager;
    var fragmentTransaction = fragmentManager.BeginTransaction();

    fragmentTransaction.Replace(Resource.Id.mainContainer, fragment);
    fragmentTransaction.AddToBackStack(fragmentTitle);

    fragmentTransaction.Commit();
}

Back button (in MainActivity):

public override void OnBackPressed()
{
    if (isNavDrawerOpen()) drawerLayout.CloseDrawers();
    else
    {
        var backStackEntryCount = FragmentManager.BackStackEntryCount;

        if (backStackEntryCount == 1) Finish();
        else if (backStackEntryCount > 1) FragmentManager.PopBackStack();
        else base.OnBackPressed();
    }
}

And isNavDrawerOpen method:

bool isNavDrawerOpen()
{
    return drawerLayout != null && drawerLayout.IsDrawerOpen(Android.Support.V4.View.GravityCompat.Start);
}