EventBus implementation ideas

Define the class:

• MyEventBus Event bus

  Methods:

  • Register to register
  • Unregister cancels registration
  • Post distribution

• The Event Event

• EventHandler EventHandler

The following two diagrams illustrate the implementation principle

Registration:

Distribution:

The most critical aspect of implementing EventBus is maintaining an event/handler registry. Add content to the registry when a registration request is received; When a call request is received, the corresponding handler is fetched from the registry and invoked.

Start implementing an EventBus

Define event – event handler:

public class Event<T> {
    private T entity;
    public Event(a) {}public Event(T entity) {
        this.entity = entity; }}Copy the code

public interface EventHandler {
    void handle(Event event);
}
Copy the code

Define the EventBus:

	// Register the collection
    private Map<Class<? extends Event>, List<EventHandler>> map = new HashMap<Class<? extends Event>, List<EventHandler>>();
    private Executor executor = Executors.newFixedThreadPool(10);

    public void register(Class<? extends Event> event, EventHandler eventHandler) {
        List<EventHandler> list = map.get(event);
        if(list ! =null) {
            list.add(eventHandler);
        } else {
            list = newArrayList<EventHandler>(); list.add(eventHandler); map.put(event,list); }}public void post(final Event event) {
        List<EventHandler> list = map.get(event.getClass());
        if(list ! =null) {
            for (final EventHandler eventHandler : list) {
                executor.execute(new Runnable() {
                    public void run(a) { eventHandler.handle(event); }}); }}}public void unRegister(Class<? extends Event> event, EventHandler eventHandler) {
        List<EventHandler> list = map.get(event);
        if(list ! =null) { list.remove(eventHandler); }}Copy the code

Process invocation:

 		// instantiate and register
        MyEventBus myEventBus = new MyEventBus();
        EventHandler handler = new EventHandler() {
            public void handle(Event event) { User user = (User) event.getEntity(); System.out.println(user.getId()); }}; myEventBus.register(OpenUserEvent.class,handler);// Account opening
        User user = new User();
        user.setId("3");
        user.setName("test");

        // Instantiate the event and send it
        OpenUserEvent openUserEvent = new OpenUserEvent();
        openUserEvent.setEntity(user);
        myEventBus.post(openUserEvent);
Copy the code

We implemented a simple EventBus, but the EventBus had the following problems:

• Thread-safety. Thread-safety issues arise during registration and deregistration
• When registered, the specified event and event handler need to be displayed
• Events in distribution (asynchronous invocation) cannot be sensed by the caller if an exception occurs

In the following section, Google EventBus is used as an example to explain how to solve the above problems