更换片段时如何防止“java.lang.IllegalStateException：Fragment already added”？

Question

Despite my efforts to prevent fragments from being added more than once, I continue to encounter java.lang.IllegalStateException: Fragment already added: VideoFragment .

I have an activity where VideoFragment is instantiated in onCreate only. In the only place I attempt to display the VideoFragment, I first check whether this fragment has been added already.

private VideoFragment videoFragment;

public void onCreate(Bundle savedInstanceState) {
    ...
    videoFragment = new VideoFragment();
    ...
}

private void showVideoFragment() {
    if (!videoFragment.isAdded()) {
        FragmentTransaction transaction = getFragmentManager().beginTransaction();
        transaction.replace(R.id.fragment_container, videoFragment, "video").commit();
    }
}

I have not been able to consistently reproduce this problem to examine in the debugger, but my runtime error reporting continues to report the exception java.lang.IllegalStateException: Fragment already added: VideoFragment for users, with stack traces composed of Android classes.

/FragmentManager.java:1133→ android.app.FragmentManagerImpl.addFragment
/BackStackRecord.java:648→ android.app.BackStackRecord.run
/FragmentManager.java:1453→ android.app.FragmentManagerImpl.execPendingActions
/FragmentManager.java:443→ android.app.FragmentManagerImpl$1.run
/Handler.java:733→ android.os.Handler.handleCallback
/Handler.java:95→ android.os.Handler.dispatchMessage
/Looper.java:146→ android.os.Looper.loop
/ActivityThread.java:5487→ android.app.ActivityThread.main
/Method.java:-2→ java.lang.reflect.Method.invokeNative
/Method.java:515→ java.lang.reflect.Method.invoke
/ZygoteInit.java:1283→ com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run
/ZygoteInit.java:1099→ com.android.internal.os.ZygoteInit.main
/NativeStart.java:-2→ dalvik.system.NativeStart.main

Does the definition of added in isAdded() not match the one used to check fragment transactions?

Or is there some way the videoFragment reference in the activity is not the same? Is this something I need to explicitly handle when saving state http://developer.android.com/guide/components/activities.html#SavingActivityState ?

Or is there a reliable alternative way of checking whether the fragment has already been added?

---

UPDATE

I have figured out how to semi-reliably produce the problem.

1. Start application
2. Navigate away from application, and run other programs for awhile. On my Galaxy Nexus (which is pretty slow these days), using Chrome to read a few news articles seems sufficient. When returning to the home screen, if it takes a few seconds to render then the application will likely throw the fragment exception.
3. Restart application and trigger fragment change

If I kill and simply run the application, everything appears to be fine. Or if I navigate away from the application and come back immediately, it works okay. It's only if the application is left in the background for a bit (enough to remove from memory?), that the fragment issue appears.

I also tried, to no effect, in onCreate

View v = findViewById(R.id.fragment_container);
if(v != null){
    Log.d(TAG, "disabling save for fragment_container");
    v.setSaveEnabled(false);
    v.setSaveFromParentEnabled(false);
}

I also tried checking Fragment prior = getFragmentManager().findFragmentByTag("video"); and Fragment prior2 = getFragmentManager().findFragmentById(R.id.fragment_container); before running the replace fragment transaction, but these come up null .

My problem in fact looks very similar to https://code.google.com/p/android/issues/detail?id=61247 though the time appears less an issue than memory/cache effects. It is completely unclear to me why that issue was closed.

I will try to produce a simple application that replicates this issue. My current one uses webrtc, and the logcat output is completely cluttered with webrtc messages.

Answer 1

I see a few things here:

1. Your problem occours propably when Activity is recreated by the system. You can simply simulate it by changing device orientation.
2. isAdded() returns false , because Activity was recreated so this method is called for new instatnce of VideoFragment which hasn't knowledge about previous add.
3. showVideoFragment() actually add fragment to Activity instead of just show it. I suggest you rename that method to somethink like "addVideoFragment" and move it to onCreate() method. If you do that you resolve the issue.

4. If you really want to show or hide fragment use methods from FragmentTransaction eg:

     FragmentManager fm = getFragmentManager(); fm.beginTransaction() .setCustomAnimations(android.R.animator.fade_in, android.R.animator.fade_out) .show(somefrag) // or hide .commit(); 

Hint:
When you a priori know that you fragment is always the VideoFragment you can simply use:

<fragment 
     android:name="com.example.VideoFragment"
     android:id="@+id/video_fragment"
     android:layout_width="match_parent"
     android:layout_height="match_parent" />

find it:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_layout);
    VideoFragment fragment = (VideoFragment) getFragmentManager().findFragmentById(R.id.video_fragmen);
}

and make whatever you want with the instance.

Answer 2

I think I have successfully fixed this error, by trying to reproduce this in a simpler example: https://stackoverflow.com/a/30672516/4107809

I was making a mistake where multiple instances of a fragment (not the VideoFragment) were added in successive calls to onCreate caused by recreation of the Activity. This fragment addition did not trigger the java.lang.IllegalStateException: Fragment already added because apparently this happens only if you try to add the same fragment instance more than once, not multiple instances of the same fragment.

Upon calling the fragment replace method, the java.lang.IllegalStateException: Fragment already added is generated for the new VideoFragment, even though the VideoFragment is only added once using replace.

By ensuring the different fragment was added only once, the replace by the VideoFragment no longer generates java.lang.IllegalStateException: Fragment already added: VideoFragment , at least for the steps for reproducing I outlined above. The IllegalStateException appears to have nothing to do with adding/replacing the VideoFragment, but with the state of the fragments being replaced.

I am displeased by this resolution for two reasons:

1. The error message is misleading. It says the VideoFragment has already been added, and I have resolved this by making sure that a different fragment is not added more than once, which did not generate an exception.

2. The replace documentation is very misleading. Based on my reading, it should not matter what the state of the fragment container is prior to calling to replace; the end state should be determined solely by the fragment that is added from the replace argument. I think this discrepancy is most clear in the linked question, though the answerer in that question disagrees.

Replace an existing fragment that was added to a container. This is essentially the same as calling remove(Fragment) for all currently added fragments that were added with the same containerViewId and then add(int, Fragment, String) with the same arguments given here.