Intro to EventBus android library

I started to use EventBus library of greenrobot and I wanted to share it with you. EventBus is a publish/subscribe event bus optimized for Android according to the description in the readme file on their repo. In other word it's a Bus for events to transfer the data between your application classes.
I am using MVP as a design pattern for my apps but I will just show how to work with the EventBus library on an empty Android Project so every time I'll mention the Activity It will be your Presenter if you're using MVP like me.
Also I created a github repo with an example project and you will find its link on the end of this article.

Create a new Android project

create a new Android project and add the following line to the app gradle file :

compile 'org.greenrobot:eventbus:3.0.0'

That's how you tell Android Studio to download the EventBus library and include it to your project.

Set the UI of the MainActivity

Open the activity_main.xml file and add the following lines in it.

<TextView
    android:id="@+id/mainactivity_textview"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:layout_marginTop="20dp"
    android:textSize="25sp"
    android:text="The message will be here" />

<Button
    android:id="@+id/mainactivity_button"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:text="Go To Next Activity"/>

Then your edit the MainActivity class to be like this
public class MainActivity extends AppCompatActivity {

    TextView mainActivityTextView;
    Button mainActivityButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    // init the views
    mainActivityTextView = (TextView) findViewById(R.id.mainactivity_textview);
    mainActivityButton = (Button) findViewById(R.id.mainactivity_button);
}
Create the ChildActivity

Create a new activity called ChildActivity.

XML :

<EditText
    android:id="@+id/childactivity_edittext"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center"
    android:layout_marginTop="20dp"
    android:textSize="25sp"
    />

<Button
    android:id="@+id/childactivity_button"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:text="Send To Main Activity"/>

Java :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_child);
    // init the views
    childActivityEditText = (EditText) findViewById(R.id.childactivity_edittext);
    childActivityButton = (Button) findViewById(R.id.childactivity_button);
}
Create the Message class

We have to create a class for the message which will be transfered between our activities and you have the freedom to call it whatever you like and to have any method, In this case we will call it MessageEvent and we will put a String var called message and set a getter&setter for it.

public class MessageEvent {
    private String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}
Register your MainActivity to the EventBus

You have to register your activity to the EventBus you have to put this like in the onCreate method :

EventBus.getDefault().register(this);

and to understand it, the EventBus class is a Singleton class and the getDefault() is a static method which returns the instance of this class.
also the register() is the method which register the views/class to the EventBus and we pass the mainactivity object to it.

Now your MainActivity is registered but how to pick up the call when the EventBus publish the event?
just create a onMessageEvent method and use the @Subscribe annotation above it.

@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent messageEvent){
    mainActivityTextView.setText(messageEvent.getMessage());
}

And now every time the EventBus will publish the MessageEvent you activity will know about it.

Post the message to the EventBus

On your ChildActivity put this code

    childActivityButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            MessageEvent messageEvent = new MessageEvent();
            messageEvent.setMessage(childActivityEditText.getText().toString());

            EventBus.getDefault().post(messageEvent);
        }
    });

which is just creating a new object of MessageEvent and uses the post() method of the EventBus to tell it to publish the new MessageEvent object to whoever cares about it.

Why to use EventBus?

It will help you to apply the publisher/subscriber pattern without getting your hand dirty with the details, and of course they already took care of the performance and the optimizations for you, so don't reinvent the wheel.

Example

I create a Github repo to be an example for you : https://github.com/Morxander/TestingEventBus

I welcome any suggestion or advices about what I wrote here on this article :)