Otto and Android Annotations Compatibility Issue Analysis

Introduction

Otto is a great Android event bus solution developed by SquareUp guys. These guys extract the event bus related classes from Google’s Guava, and optimized it for Android. Event Bus is a great solution to keep your code away from messy anonymous or internal classes only used for event handling! And Otto’s simplicity and performance makes it used to be the most popular event bus solution in Android development.

Android Annotations, as known as AA, is another great library that makes Android developers’ lives a lot easier. Just annotating the code, AA helps developer handles the most boring or error-proning tasks for you, including binding widgets binding, asynchronous tasks, life time management, etc…

Issue

Otto and AA are the libraries focusing on different aspects. Theoretically speaking, there shouldn’t be any compatibility issue between them. But the reality Otto event is never delivered to AA annotated classes. By inspecting the code with step by step debugging, I found the root cause of the issue is Otto failed to located @Produce and @Subscribe annotated methods in AA generated code.

Analysis

After a few study work, I finally understood what is the reason behind:

For performance consideration, Android Annotations actually does it work during the compilation time instead of Runtime. AA will derive the annotated classes, and generate some code according to the annotations applied. During the runtime, the classes got instantiated is actually the derived classes instead of the original one, although in the code is accessed via the original class as interface.

Here is a simple example:

I have the class ServerListAdapter, which is used to provide data for a grid view.

@EBean
public class ServerListAdapter extends AdvBaseAdapter<InetAddress, ServerView> {

    @RootContext
    protected Context context;

    @SystemService
    protected LayoutInflater inflater;

    @Bean
    protected Bus bus;

    public ServerListAdapter() {
        super(new ArrayList<InetAddress>());
    }

    @AfterInject
    protected void afterInject() {
        bus.register(this);
    }

    @Override
    protected ServerView createView(ViewGroup parent) {
        return ServerView_.build(context);
    }

    @Override
    protected ServerView updateView(ServerView itemView, InetAddress item) {
        itemView.update(item);
        return itemView;
    }

    @Subscribe
    public void fetchServerStatus(DiscoveryStatusEvent event) {
        setItems(event.addresses);
    }

    @Subscribe
    public void onServerStatusUpdated(DiscoveryStatusChangedEvent event) {
        switch (event.type) {
            case SERVER_ONLINE:
                getItems().add(event.address);
                break;
            case SERVER_OFFLINE:
                getItems().remove(event.address);
                break;
        }
        notifyDataSetChanged();
    }
}

And this is the derived class generated by AA during the compiling-time:

public final class ServerListAdapter_
    extends ServerListAdapter
{

    private Context context_;

    private ServerListAdapter_(Context context) {
        context_ = context;
        init_();
    }

    public static ServerListAdapter_ getInstance_(Context context) {
        return new ServerListAdapter_(context);
    }

    private void init_() {
        inflater = ((LayoutInflater) context_.getSystemService(Context.LAYOUT_INFLATER_SERVICE));
        context = context_;
        bus = Bus_.getInstance_(context_);
        afterInject();
    }

    public void rebind(Context context) {
        context_ = context;
        init_();
    }

}

And here is how it is consumed:

@EFragment(R.layout.fragment_server_list)
public class ServerListFragment extends Fragment {
    @Bean
    protected DiscoveryService discoveryService;

    @Bean
    protected ServerListAdapter adapter;

    @ViewById(R.id.server_list)
    protected GridView serverGridView;

    @AfterViews
    protected void afterViews() {
        serverGridView.setAdapter(adapter);

        discoveryService.start();
    }

    @Override
    public void onDetach() {
        super.onDetach();
        discoveryService.stop();
    }
}

As you can see, in the ServerListFragment, the ServerListAdapter instance injected into bean is actually the instance of ServerListAdapter_. And due to polymorphic, the instance just behaves like a ServerListAdapter instance.

On the other hand, according to the description on Otto’s Home Page:

In order to receive events, a class instance needs to register with the bus.
…
Registering will only find methods on the immediate class type. Unlike the Guava event bus, Otto will not traverse the class hierarchy and add methods from base classes or interfaces that are annotated. This is an explicit design decision to improve performance of the library as well as keep your code simple and unambiguous.

@OttoOtto Homepage

Otto only search annotations in direct class, which is ServerListAdapter_ in instance, and there isn’t any annotation included. As a consequence, all the @Subscribe annotated methods are ignored by com.squareup.otto.AnnotatedHandlerFinder. So the posted events become dead event due to no subscriber found.

Comments

There is rumor that this issue will be fixed in Otto 2.0. But according to the comment from Jake Wharton, Otto’s developer, Otto 2.0 will take forever to release.

In fact Otto 2.0 Repo has been not touched for 2 years already. Although we could check out the code and build Otto 2.0 by ourselves, but it takes efforts. Especially when some bug is found.

Conclusion

Luckily, although Otto turns into “maintenance mode”, the compatibility issue takes forever to resolve. I found a great Otto alternative, EventBus from GreenRobot. A boring name, but great library.

According to EventBus‘s document, it provides richer feature and better performance than Otto.
The most important EventBus is friendly to AndroidAnnoations. They two works well together.

EventBus and Otto Feature Comparison

	EventBus	Otto
Declare event handling methods	Name conventions	Annotations
Event inheritance	Yes	Yes
Subscriber inheritance	Yes	No
Cache most recent events	Yes, sticky events	No
Event producers (e.g. for coding cached events)	No	Yes
Event delivery in posting thread	Yes (Default)	Yes
Event delivery in main thread	Yes	No
Event delivery in background thread	Yes	No
Asynchronous event delivery	Yes	No

EventBus and Otto Performance Comparison

	EventBus over Otto
Posting 1000 events, Android 2.3 emulator	~70% faster
Posting 1000 events, S3 Android 4.0	~110% faster
Register 1000 subscribers, Android 2.3 emulator	~10% faster
Register 1000 subscribers, S3 Android 4.0	~70% faster
Register subscribers cold start, Android 2.3 emulator	~350% faster
Register subscribers cold start, S3 Android 4.0	About the same