EventBus 3.0 的三大要素

官网关系图

EventBus is an open-source library for Android and Java using the publisher/subscriber pattern for loose coupling. EventBus enables central communication to decoupled classes with just a few lines of code – simplifying the code, removing dependencies, and speeding up app development.

EventBus 3.0 三步走

1. Define Events

public class MessageEvent {
 
    public final String message;
 
    public MessageEvent(String message) {
        this.message = message;
    }
}

2.Prepare subscribers

Firstly, we need to register/unregister in the lifecycle of the activity or fragment. Such as:

@Override
public void onStart() {
     super.onStart();
     EventBus.getDefault().register(this);
}

@Override
protected void onStop() {
     super.onStop();
     EventBus.getDefault().unregister(this);
    }

onResume() / onPause();

Subscribers implement event handling methods (also called “subscriber methods”) that will be called when an event is posted. These are defined with the @Subscribe annotation.
Note that with EventBus 3 the method name can be chosen freely

// This method will be called when a MessageEvent is posted (in the UI thread for Toast)
@Subscribe(threadMode = ThreadMode.MAIN)
public void onMessageEvent(MessageEvent event) {
    Toast.makeText(getActivity(), event.message, Toast.LENGTH_SHORT).show();
}
 
// This method will be called when a SomeOtherEvent is posted
@Subscribe
public void handleSomethingElse(SomeOtherEvent event) {
    doSomethingWith(event);
}

3.Post events

EventBus.getDefault().post(new MessageEvent("Hello everyone!"));

When we have 2 activities, activity A and activity B, we hope that activity B post() the value to the activity A after clicking button. At this time, we can't put the post() in the onClick().

Activity B:

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        ......
        Button mButton2 = findViewById(R.id.button2);
        mButton2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //EventBus.getDefault().post(new MessageEvent("EventBus送我过来的！"));
                Intent intent = new Intent(SecondActivity.this, MainActivity.class);
                startActivity(intent);
            }
        });
    }

We could put the post() in the onStop(), because before posting we need to register() in the target activity at first.

    @Override
    protected void onStop() {
        super.onStop();
        EventBus.getDefault().post(new MessageEvent("EventBus送我过来的！"));
    }

Activity A：

public class MainActivity extends AppCompatActivity {
    private String TAG = "MainActivity";
    public TextView mTextView;
    public Button mButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getSupportActionBar().setTitle("MainActivity");
        mTextView = findViewById(R.id.tv_mainActivity);
        mButton = findViewById(R.id.button);

        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, SecondActivity.class);
                startActivity(intent);
            }
        });

    @Override
    public void onStart() {
        super.onStart();
        EventBus.getDefault().register(this);
    }

    @Override
    public void onStop() {
        super.onStop();
        EventBus.getDefault().unregister(this);
    }

    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onMessageEvent(MessageEvent event) {
        mTextView.setText(event.message);
    }
}

In Activity B, when we click Button2 we will open the Activity A, then the Activity A will run onCreate(), onStart(). At last the Activity B will run onStop().