从 Backstack 添加 Fragment 时出现 IllegalStateException

Question

From time to time I get the following crash report:

java.lang.IllegalStateException: 
  at androidx.fragment.app.FragmentManagerImpl.addFragment (FragmentManagerImpl.java:1916)
  at androidx.fragment.app.BackStackRecord.executePopOps (BackStackRecord.java:828)
  at androidx.fragment.app.FragmentManagerImpl.executeOps (FragmentManagerImpl.java:2622)
  at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether (FragmentManagerImpl.java:2411)
  at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute (FragmentManagerImpl.java:2366)
  at androidx.fragment.app.FragmentManagerImpl.execPendingActions (FragmentManagerImpl.java:2273)
  at androidx.fragment.app.FragmentManagerImpl$1.run (FragmentManagerImpl.java:733)
  at android.os.Handler.handleCallback (Handler.java:808)
  at android.os.Handler.dispatchMessage (Handler.java:101)
  at android.os.Looper.loop (Looper.java:166)
  at android.app.ActivityThread.main (ActivityThread.java:7529)
  at java.lang.reflect.Method.invoke (Method.java)
  at com.android.internal.os.Zygote$MethodAndArgsCaller.run (Zygote.java:245)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:921)

In line 1916 of FragmentManagerImpl I can find:

throw new IllegalStateException("Fragment already added: " + fragment);

So it says that some fragment is already added. Unfortunately Google does not show the message (which fragment was already added) in the Google Play Console anymore. As far as I understand the Stacktrace this exception occurs when adding a fragment from the backstack right?

I have one FrameLayout in which I add/remove Fragments. I always add them with following code:

public void addFragment(FragmentActivity activity, Fragment fragment) {
    FragmentTransaction transaction = activity.getSupportFragmentManager().beginTransaction();
    String tag = fragment.getClass().getCanonicalName();
    Fragment prev = activity.getSupportFragmentManager().findFragmentByTag(tag);
    if (fragment.isAdded()) {
        return;
    }
    if (prev != null) {
        transaction.remove(prev);
    }
    transaction.replace(R.id.fragment_container, fragment, tag);
    transaction.addToBackStack(null);
    transaction.commit();

And I add DialogFragments with following method:

public void openFragmentDialog(FragmentActivity activity, DialogFragment dialogFragment, String tag) {
    FragmentTransaction transaction = activity.getSupportFragmentManager().beginTransaction();
    Fragment prev = activity.getSupportFragmentManager().findFragmentByTag(tag);
    if (prev != null) {
        transaction.remove(prev);
    }
    transaction.addToBackStack(null);
    dialogFragment.show(transaction, tag);
}

In which situation could it happen that the IllegalStateException occurs? Am I adding/replacing the Fragments/DialogFragments the wrong way? I could never reproduce that error. But I get reports from Android 4.4 - Android 9 and all types of devices and I have no idea where it could happen.

Could it be something with animations or slow devices? Because it only happens occasionally.

Answer 1

tag need to be unique. If possible replace it with null , otherwise with something unique.

public void addFragment(FragmentActivity activity, Fragment fragment) {
    FragmentTransaction transaction = activity.getSupportFragmentManager().beginTransaction();
    transaction.replace(R.id.fragment_container, fragment);
    transaction.addToBackStack(null);
    transaction.commit();
}

Answer 2

Note, you are adding and removing two different fragments with the same tag -- the tag should then also be different. If the prev fragment is an instance of a different class, why are you using fragment 's class name as the tag to find it?

You should add an if statement to check if prev and fragment are not sometimes the same instance (because of same tag).

If the two fragments are the same instance and you still want to remove and then add the same fragment -- you might have to do it in two different transactions to prevent "Fragment already added" exception.

Answer 3

Here's what I've done to check for and prevent duplicate fragments:

MyFragment myFragment = (MyFragment) fragmentManager.findFragmentByTag(MY_FRAGMENT_TAG);

if(myFragment != null && myFragment.isVisible()){
    // Return early if the fragment already exists & is visible
    return;

}else if(myFragment == null){
    // Create a new instance of the fragment if none already exist
    myFragment = new MyFragment();
}

// Perform the fragment transaction
fragmentManager.beginTransaction()
    .replace(R.id.content_container, myFragment, MY_FRAGMENT_TAG)
    .addToBackStack(MY_FRAGMENT_TAG) // Pass in the tag if you want to add to the back stack
    .commit();

For Dialog Fragments I just create a new instance and call .show() on it. The dialog should be dismissed through interaction with it:

MyDialog myDialog = new MyDialog();

// I call getChildFragmentManager here b/c I was using this from within a fragment
// but you can get whichever Fragment Manager is appropriate
myDialog.show(getChildFragmentManager(), MY_DIALOG_TAG);