1. Overview

1.概述

Guava library provides the EventBus which allows publish-subscribe communication between components. In this tutorial, we will look at how to use some of the features of the EventBus.

Guava库提供了EventBus，允许组件之间进行发布-订阅通信。在本教程中，我们将了解如何使用EventBus的一些功能。

2. Setup

2.设置

To start we add the Google Guava library dependency in the pom.xml:

首先，我们在pom.xml中添加Google Guava库的依赖性：。

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>31.0.1-jre</version>
</dependency>

The latest version can be found here.

3. Using the EventBus

3.使用 EventBus

Let’s start by using a simple example.

让我们先用一个简单的例子来说明。

3.1. Setup

3.1.设置

We start by looking at the EventBus object. It can register listeners and post events. Using it is as simple as instantiating the class:

我们先看一下EventBus对象。它可以注册监听器和发布事件。使用它就像实例化该类一样简单。

EventBus eventBus = new EventBus();

Guava library gives you the freedom of using the EventBus in any way that best suits your development needs.

Guava库让你可以以任何最适合你的开发需求的方式自由使用EventBus。

3.2. Creating Listeners

3.2.创建监听器

We create a listener class that has handler methods to receive specific events. We annotate the handler methods with @Subscribe. The method accepts as an argument an object of the same type as the event being posted:

public class EventListener {

    private static int eventsHandled;

    @Subscribe
    public void stringEvent(String event) {
        eventsHandled++;
    }
}

3.3. Registering Listeners

3.3.注册监听器

We can subscribe to an event by registering our EventListener class on the EventBus:

EventListener listener = new EventListener();
eventBus.register(listener);

3.4. Unregistering Listeners

3.4.取消注册监听器

If for any reason we want to unregister a class from the EventBus, that can also be easily done:

如果出于任何原因，我们想从EventBus中取消一个类的注册，这也可以很容易做到。

eventBus.unregister(listener);

3.5. Posting Events

3.5.发布事件

We can post events as well with the EventBus:

@Test
public void givenStringEvent_whenEventHandled_thenSuccess() {
    eventBus.post("String Event");
    assertEquals(1, listener.getEventsHandled());
}

3.6. Posting Custom Events

3.6.发布自定义事件

We can also specify a custom event class and post that event. We start by creating a custom event:

public class CustomEvent {
    private String action;

    // standard getters/setters and constructors
}

Adding a handler method in the EventListener class for that event:

在EventListener类中为该事件添加一个处理方法。

@Subscribe
public void someCustomEvent(CustomEvent customEvent) {
    eventsHandled++;
}

We can now post our custom event:

我们现在可以发布我们的自定义事件。

@Test
public void givenCustomEvent_whenEventHandled_thenSuccess() {
    CustomEvent customEvent = new CustomEvent("Custom Event");
    eventBus.post(customEvent);

    assertEquals(1, listener.getEventsHandled());
}

**3.7. Handling an Unsubscribed Event**

**3.7.处理一个未订阅的事件**

We are provided with a DeadEvent class that allows us to handle any events that have no listeners. We can add a method to handle the DeadEvent class:

我们获得了一个DeadEvent类，它允许我们处理任何没有监听器的事件。我们可以添加一个方法来处理DeadEvent类。

@Subscribe
public void handleDeadEvent(DeadEvent deadEvent) {
    eventsHandled++;
}

4. Conclusion

4.结论

In this tutorial, we used a simple example as a guide on how to use the Guava EventBus.

在本教程中，我们用一个简单的例子来指导如何使用Guava EventBus。

You can find the complete source code and all code snippets for this article over on GitHub.

你可以在GitHub上找到本文的完整源代码和所有代码片段。