NullPointerException calling Flow.get(this) in the onCreate method.

[epool]

I am getting the following error when I call Flow.get(this) inside the onCreate() method.

Caused by: java.lang.NullPointerException: Attempt to read from field 'flow.Flow flow.InternalLifecycleIntegration.flow' on a null object reference
                                                     at flow.InternalContextWrapper.getSystemService(InternalContextWrapper.java:54)
                                                     at android.view.ContextThemeWrapper.getSystemService(ContextThemeWrapper.java:123)
                                                     at android.app.Activity.getSystemService(Activity.java:5263)
                                                     at flow.InternalContextWrapper.getFlow(InternalContextWrapper.java:31)
                                                     at flow.Flow.get(Flow.java:46)
                                                     at flow.sample.basic.BasicSampleActivity.onCreate(BasicSampleActivity.java:30)
                                                     at android.app.Activity.performCreate(Activity.java:6237)
                                                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                     at android.os.Looper.loop(Looper.java:148) 
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                     at java.lang.reflect.Method.invoke(Native Method)

The code:

public class BasicSampleActivity extends Activity {

  @Override protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.basic_activity_frame);

    Flow flow = Flow.get(this); // <============== ERROR!!! NPE
    System.out.println(flow);
  }

  @Override protected void attachBaseContext(Context baseContext) {
    baseContext = Flow.configure(baseContext, this) //
        .dispatcher(new BasicDispatcher(this)) //
        .defaultKey(new WelcomeScreen()) //
        .keyParceler(new BasicKeyParceler()) //
        .install();
    super.attachBaseContext(baseContext);
  }

  @Override public void onBackPressed() {
    if (!Flow.get(this).goBack()) {
      super.onBackPressed();
    }
  }
}

Somebody can explain me why this is happening in the onCreate() method? If I do the same in onStart() or onResume() everything works well.

I am not sure if this is a bug or not, but I think a wiki/documentation is required for this awesome library.

[Zhuinden]

It would work with Flow.get(getBaseContext())

[epool]

@Zhuinden It did not work for me =/ still crashing with NPE Caused by: java.lang.NullPointerException: Attempt to read from field 'flow.Flow flow.InternalLifecycleIntegration.flow' on a null object reference.

[Zhuinden]

@epool you might be correct, Flow is initialized in onActivityCreated() method of InternalLifecycleIntegration (retained fragment), so you might be better off if you move that logic to onPostCreate()

[epool]

@Zhuinden That worked, Thanks!

[rjrjr]

I don't know if the behavior can change, but we should be able to do better than throwing an NPE. Plan is to document that it is unsafe to call Flow#get(Context) before the first call to Activity#onResume().

[rjrjr]

Also, very curious to know why people are trying to get their hands on the Flow instance before that point.

[Zhuinden]

Go to login or go to feed in onCreate?

[rjrjr]

Why does that need to happen in onCreate rather than onResume? Or why not calculate the default key to be login or feed before kicking things off?

Actually, I think the best place to handle that kind of ACL concern is in the dispatcher itself. On each dispatch, if auth has been lost, redirect to the login screen instead of honoring the dispatch. Something like:

@Override public void dispatch(Traversal traversal, TraversalCallback callback) {
  if (!authorizer.readyFreddy()) {
    getFlow().setHistory(History.single(LoginScreen.INSTANCE));
    callback.onTraversalCompleted();
    return;
  }

  // etc.
}

[loganj]

+1 to handling this in the dispatcher.