Question

In my small Android app, I have a ViewPager with 3 pages, each page contains one fragment, that’s all working fine with a FragmentStatePagerAdapter (see earlier question).

Now for larger screens, I would like to keep a ViewPager, but I want the first two fragments to be on the same first page.

For this, I implement my own PagerAdapter and this is mostly working, the only problem I am trying to resolve is when I am rotating the screen, I get an error:

12-11 16:34:13.526: ERROR/AndroidRuntime(9221): FATAL EXCEPTION: main
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.matthieu/com.matthieu.TestActivity}: java.lang.IllegalArgumentException: No view found for id 0x7f030002 (com.matthieu:id/top_fragment) for fragment TestFragment{481a3a98 #0 id=0x7f030002}
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3815)
        at android.app.ActivityThread.access$2400(ActivityThread.java:125)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2037)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:123)
        at android.app.ActivityThread.main(ActivityThread.java:4627)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:521)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:871)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
        at dalvik.system.NativeStart.main(Native Method)
        Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f030002 (com.matthieu:id/top_fragment) for fragment TestFragment{481a3a98 #0 id=0x7f030002}
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:903)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1070)
        at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:1861)
        at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:547)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1129)
        at android.app.Activity.performStart(Activity.java:3781)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2636)
        ... 12 more

I put a sample code simulating what I am trying to do (and showing this error message) on GitHub.

I guess one problem I have is because of the Fragments I have in the real application, I cannot really have a static newInstance constructor, but instead the Fragments are created ahead of time and have ‘setRetainInstance(true);’ set.

I understand the id of the viewGroup that will contain the fragment should be part of the hierarchy, and eventually, I guess, it is…

If it makes any difference, I am using the support library and the SherlockActionBar to maintain backwards compatibility with older devices.

Any idea on how to go about this?

EDIT: I have found that if I remove the Fragments from the FragmentManager in onSaveInstanceState I am not getting any crash anymore. All the fragments are recreated from scratch which is not what I want, but I guess I might have to do this and somehow recover those fragments later….

Answer 1

All right, even though I moved to another way of doing things, I don’t like to leave things incomplete…

Going the way of the nested Fragments as suggested by Mark. The link posted in the comments goes to 404, but I found your example code about nested fragments.

A few things I’ve learned on the way:

• Nested fragments cannot have setRetainInstance(true), or it will get:

  java.lang.IllegalStateException: Can’t retain fragements that are nested in other fragments

• It was a mistake to create a FrameLayout in the page of the ViewPager to hold the fragment. Looking in the Android code (and specifically the FragmentPagerAdapter), I should just use container.getId() to find where to add the Fragment. Looks like you cannot do anything more sophisticated at that level or it will break.

• From my understanding, if the fragments hold some information (like I do) that takes a long time to load, the different ways to handle this are:

  • Use onSaveInstanceState (preferred way ?), but not possible if the Fragment is nested. The (empty) constructor will get called when needed by the framework.
  • Use setRetainInstance(true), in that case savedInstanceBundle is always null, but that Fragment (as Java object) will not be destroyed and it will still go through onAttach / onCreateView.
  • Find some other way to save and restore what you need…

Anyway, I pushed my solution using nested fragments on github