EventBus详解

简介

github原文

EventBus...

* simplifies the communication between components
    - decouples event senders and receivers
    - performs well with Activities, Fragments, and background threads
    - avoids complex and error-prone dependencies and life cycle issues
    - makes your code simpler
* is fast
* is tiny (<50k jar)
* is proven in practice by apps with 100,000,000+ installs
* has advanced features like delivery threads, subscriber priorities, etc.

大概意思

EventBus...
* 简化组件之间的通信
    - 发送者和接收者解耦
    - 可以在Activities, Fragments,background threads（服务、子线程等）之间传递数据
    - 避免了依赖和生命周期等容易出错的问题
    - 让你的程序看上去更简洁
* 更快
* 更小（jar包大小小于50k）
* 已经有这么这么多应用用过了EventBus。
* 有一些更高级的功能...

看过简介，应该大概知道EventBus是一个什么东西了吧

Github源码

源码地址：https://github.com/greenrobot/EventBus

API文档地址：https://github.com/greenrobot/EventBus/blob/master/HOWTO.md

个人对EventBus的理解

首先要导入jar包，这个就不用多说了，然后要有一个发送者发送一个消息，最后要有一个接收者接收发送过来的消息

因此，在你要给某个接收者发送消息的时候，一定要保证接收者已经注册了，否者接收者都还没有，谁来接收呢。

为什么提到这个呢，因为我刚用的时候，我是想在一个Activity1里开启另一个Activity2，然后用EventBus传递过去一个数据，于是我发送了一个消息，然后用Intent开启另外一个Activity2，在Activity2里注册了接收者，但是却没有收到消息，之后我又先开启Activity2，等初始化好了以后，再发送消息，Activity2就收到消息了

所以想一下EventBus的应用场景，如果我们有一个服务要在后台一直刷数据，然后在界面上显示，类似这种场景，我们就可以用EventBus来实现

普通的页面跳转需要传递的数据，我们用Intent就完全可以啦。

导入jar包

在Gradle文件里加上jar包

消息发送者

非常简单，就一行代码

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

这里需要传递一个数据对象，就是一个普通的Javabean

public class MessageEvent {
    public final String message;

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

消息接收者

注册接收者

EventBus.getDefault().register(this);

反注册接收者

EventBus.getDefault().unregister(this);

四个接收方法

• PostThread: Subscriber will be called in the same thread, which is posting the event. This is the default. Event delivery implies the least overhead because it avoids thread switching completely. Thus this is the recommended mode for simple tasks that are known to complete is a very short time without requiring the main thread. Event handlers using this mode should return quickly to avoid blocking the posting thread, which may be the main thread. Example:
• 大概意思： 发送者和接收者要在同一个线程，默认是使用这种方式接收，因为他避免了线程的切换，性能开销更小

// Called in the same thread (default)
public void onEvent(MessageEvent event) {
    log(event.message);
}

• MainThread: Subscriber will be called in Android’s main thread (sometimes referred to as UI thread). If the posting thread is the main thread, event handler methods will be called directly. Event handlers using this mode must return quickly to avoid blocking the main thread. Example:
• 大概意思： 接收者在主线程（UI线程），因为是在主线程，所以要避免耗时操作阻塞线程。

// Called in Android UI's main thread
public void onEventMainThread(MessageEvent event) {
    textField.setText(event.message);
}

• BackgroundThread: Subscriber will be called in a background thread. If posting thread is not the main thread, event handler methods will be called directly in the posting thread. If the posting thread is the main thread, EventBus uses a single background thread that will deliver all its events sequentially. Event handlers using this mode should try to return quickly to avoid blocking the background thread.
• 大概意思： 接收者是和发送者在同一个后台线程中接收到消息（它和发送者是在一个线程中，后面有测试），所以如果发送者是在主线程，那么接收者也是在同一个主线程，所以尽量避免耗时的操作阻塞线程。

// Called in the background thread
public void onEventBackgroundThread(MessageEvent event){
    saveToDisk(event.message);
}

• Async: Event handler methods are called in a separate thread. This is always independent from the posting thread and the main thread. Posting events never wait for event handler methods using this mode. Event handler methods should use this mode if their execution might take some time, e.g. for network access. Avoid triggering a large number of long running asynchronous handler methods at the same time to limit the number of concurrent threads. EventBus uses a thread pool to efficiently reuse threads from completed asynchronous event handler notifications.
• 大概意思： 异步的形式，发送者和接收者不是同一个线程， 如果有一些耗时的操作，例如联网获取一些数据等情况，可以用异步，但是为了避免同时有太多的并行事件，要尽量限制并发线程的数量

// Called in a separate thread
public void onEventAsync(MessageEvent event){
    backend.send(event.message);
}

案例

后台一个Service刷数据，页面显示数据

Demo源码

下载地址（Android Studio工程）：http://download.csdn.net/detail/q4878802/9057545

效果图

Activity

package com.kongqw.kqweventbusdemo;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TextView;

import com.kongqw.kqweventbusdemo.bean.MessageEvent;
import com.kongqw.kqweventbusdemo.service.KqwService;

import de.greenrobot.event.EventBus;


public class MainActivity extends Activity {

    private TextView mTvShow;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 显示信息的TextView
        mTvShow = (TextView) findViewById(R.id.tv_show);
        // 开启服务 刷数据
        Intent service = new Intent(this, KqwService.class);
        startService(service);
    }

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

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

    // Called in Android UI's main thread
    public void onEventMainThread(MessageEvent event) {
        mTvShow.setText("onEventMainThread : \n" + event.message);
    }
}

Service

package com.kongqw.kqweventbusdemo.service;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.util.Log;

import com.kongqw.kqweventbusdemo.bean.MessageEvent;

import de.greenrobot.event.EventBus;

public class KqwService extends Service {
    public KqwService() {
    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        throw new UnsupportedOperationException("Not yet implemented");
    }

    @Override
    public void onCreate() {
        super.onCreate();

        EventBus.getDefault().post(new MessageEvent("Service 传递过来的数据"));
        /*
         * 模拟网络刷新数据
         */
        new Thread() {
            @Override
            public void run() {

                for (int x = 0; x < Integer.MAX_VALUE; x++) {
                    try {
                        sleep(1000);
                        EventBus.getDefault().post(new MessageEvent("Service 传递过来的数据 : " + x));
                        long id = Thread.currentThread().getId();
                        Log.d("KqwService", "Service发送了数据：" + x + "线程ID ： " + id);
                    } catch (Exception e) {
                        // TODO
                        Log.d("KqwService", "error ：" + e);
                    }
                }
            }
        }.start();
    }
}

XML页面

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_show"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="显示数据"
        android:textSize="25dp" />

</RelativeLayout>

测试onEvent

• 测试发送者和接收者在同一个线程

// This method will be called when a MessageEvent is posted
public void onEvent(MessageEvent event) {
    long id = Thread.currentThread().getId();
    Log.d("onEventMainThread", "Thread id = " + id);
    mTvShow.setText("onEvent : " + event.message);
}

• 结果，线程ID相同
  

测试onEventMainThread

• 测试接收者是在主线程

// Called in Android UI's main thread
public void onEventMainThread(MessageEvent event) {
    long id = Thread.currentThread().getId();
    Log.d("onEventMainThread", "Thread id = " + id);
    mTvShow.setText("onEventMainThread : \n" + event.message);
}

• 结果，接收者在主线程
  

测试onEventBackgroundThread

• 测试发送者和接收者是在同一个线程

// Called in the background thread
public void onEventBackgroundThread(final MessageEvent event){
    long id = Thread.currentThread().getId();
    Log.d("onEventBackgroundThread", "Thread id = " + id);
    runOnUiThread(new Runnable() {
        @Override
        public void run() {
            mTvShow.setText("onEventBackgroundThread : " + event.message);
        }
    });
}

• 结果，发送者和接收者是在同一个线程
  

测试onEventAsync

• 测试发送者和接收者不是在同一个线程

// Called in a separate thread
public void onEventAsync(final MessageEvent event) {
    long id = Thread.currentThread().getId();
    Log.d("onEventAsync", "Thread id = " + id);
    runOnUiThread(new Runnable() {
        @Override
        public void run() {
            mTvShow.setText("onEventAsync : " + event.message);
        }
    });
}

• 结果，发送者和接收者不是在同一个线程