FragmentTransaction in WebViewClient.shouldOverrideUrlLoading throws IllegalStateException

Question

I seem to be fighting with a race condition, the cause of which I can't seem to pin down. When executing the below code, I intermittently get the stack trace below.

Is there some obvious rule of the Fragment lifecycle I am disobeying? I am not clear on what would explicitly forbid me from performing a transaction here to handle the event.

I am using a WebViewClient to detect external URLs clicked within a local .html document - as in, URLs which point to a non-local host. I am using Otto's EventBus to post those actions to an Activity. When the Activity receives those events, I want to show those external URLs in a different Fragment, by calling FragmentTransaction.replace()

---

DefaultWebViewClient.java

@Override
  public boolean shouldOverrideUrlLoading(final WebView view, final String url) {
    boolean shouldOverride;
    if (urlIsLocal(url)) {
      shouldOverride = super.shouldOverrideUrlLoading(view, url);
    } else {
      // trigger an event for the fragment to swap out
      // return true to tell the webview not to load it...
      EventBus.getInstance().post(new LoadExternalUrlEvent(url));
      shouldOverride = true;
    }
    return shouldOverride;
  }

---

FragmentActivity.java

@Subscribe
  public void onLoadExternalUrlEvent(LoadExternalUrlEvent externalLoadEvent) {
    final BrowserFragment browserFragment = new BrowserFragment();
    Bundle args = new Bundle();
    args.putSerializable(BrowserFragment.ARG_LOAD_EXTERNAL_URL_EVENT, externalLoadEvent);
    browserFragment.setArguments(args);
    getSupportFragmentManager().beginTransaction()
        .replace(R.id.fragment_container, browserFragment, BrowserFragment.FRAGMENT_TAG)
        .addToBackStack(null).commit();
  }

---

LoadExternalUrlEvent.java

public class LoadExternalUrlEvent implements Serializable {

  private static final long serialVersionUID = 1L;

  public final String url;

  public LoadExternalUrlEvent(String url) {
    this.url = url;
  }

  @Override
  public String toString() {
    return "LoadExternalUrlEvent [url=" + url + "]";
  }

}

---

EventBus.java

import com.squareup.otto.Bus;

public class EventBus {

  private static Bus _INSTANCE;

  public static synchronized Bus getInstance() {
    if (null == _INSTANCE) {
      _INSTANCE = new Bus();
    }
    return _INSTANCE;
  }

}

---

Stack trace

 java.lang.RuntimeException: Could not dispatch event: class <omitted>.LoadExternalUrlEvent to handler [EventHandler public void <omitted>Activity.onLoadExternalUrlEvent(<omitted>LoadExternalUrlEvent)]: Can not perform this action after onSaveInstanceState
    at com.squareup.otto.Bus.throwRuntimeException(Bus.java:456)
    at com.squareup.otto.Bus.dispatch(Bus.java:386)
    at com.squareup.otto.Bus.dispatchQueuedEvents(Bus.java:367)
    at com.squareup.otto.Bus.post(Bus.java:336)
    at <omitted>DefaultWebViewClient.shouldOverrideUrlLoading(DefaultWebViewClient.java:51)
    at com.android.webview.chromium.WebViewContentsClientAdapter.shouldOverrideUrlLoading(WebViewContentsClientAdapter.java:293)
    at com.android.org.chromium.android_webview.AwContentsClientBridge.shouldOverrideUrlLoading(AwContentsClientBridge.java:96)
    at com.android.org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
    at com.android.org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:27)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:157)
    at android.app.ActivityThread.main(ActivityThread.java:5356)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
    at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
    at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1360)
    at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1378)
    at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:595)
    at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:574)
    at <omitted>Activity.run(<omitted>Activity.java:162)
    at <omitted>Activity.onLoadExternalUrlEvent(<omitted>Activity.java:156)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.squareup.otto.EventHandler.handleEvent(EventHandler.java:89)
    at com.squareup.otto.Bus.dispatch(Bus.java:384)
    ... 15 more
 A/libc(2689): Fatal signal 6 (SIGABRT) at 0x00000a81 (code=-6), thread 2689

Answer 1

I discovered the problem.

Because I was calling EventBus.register() in Activity.onCreate() I was getting multiple instances of the Activity on my backstack which would act as responders to these events.

The solution is to either register your Activity as late as possible with

@Override
  protected void onResume() {
    super.onResume();
    EventBus.getInstance().register(this);
  }

  @Override
  protected void onPause() {
    EventBus.getInstance().unregister(this);
    super.onPause();
  }

or to declare your Activity as a single instance with

android:launchMode="singleTask"