Android EventBus使用（不含源码解析）

官方文档：https://github.com/greenrobot/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.
这句话大概是说：
简化组件之间的通信
解耦事件发送者和接收者
对活动、片段和后台线程进行良好的操作
而且非常快
jar包小至50k
已经有超过了一亿用户安装
而且还可以定义优先级

不看了，反正对于开发者来说就一句话：好用！

不废话了，下面开始说使用教程：
1、加入EventBus3.0依赖

implementation 'org.greenrobot:eventbus:3.0.0'

2、既然说了EventBus是用来传值用的，那么先定义这个值吧。
创建一个实体类，MyStudent

public class MyStudent extends Observable {

    private String name;
    private int sex;
    private int old;

    public String getName() {
        return name == null ? "" : name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getSex() {
        return sex;
    }

    public void setSex(int sex) {
        this.sex = sex;
    }

    public int getOld() {
        return old;
    }

    public void setOld(int old) {
        this.old = old;
    }

    @Override
    public String toString() {
        return "MyStudent{" +
                "name='" + name + '\'' +
                ", sex=" + sex +
                ", old=" + old +
                '}';
    }

3、值有了，那么这个值有入口和出口的吧
建立两个Activity，我这里就建两个，一个MainActivity，一个Main2Activity，（这里创建流程就不写了，只写Activity中的核心代码）

public class MainActivity extends AppCompatActivity {
    private Button button;
    private MyStudent myStudent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        EventBus.getDefault().register(this);//注册eventbus
        button = findViewById(R.id.main_btn);

        button .setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this, Main2Activity.class));
            }
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        EventBus.getDefault().unregister(MainActivity.this);
    }
//接收事件，EventBus3.0之后采用注解的方式
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void Event(MyStudent myStudent) {
        Log.e("MainActivity", myStudent.toString());
    }
}

下面看看Main2

public class Main2Activity extends AppCompatActivity {
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        button = findViewById(R.id.main2_btn);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyStudent myStudent = new MyStudent();
                myStudent.setName("eventbus");
                myStudent.setOld(2);
                myStudent.setSex(2);
                EventBus.getDefault().post(myStudent);
               finish()；
            }
        });
    }

这里Log的打印结果是：（我不说，打印结果希望看博客的同学可以自己动手操作一波，这样你的记忆力才深刻。）

4、其实最基本的使用到这里就完了，有一些需要注意的地方在这里说一下：
我们可以看到，在接收参数的方法上面会有一个注解：

 @Subscribe(threadMode = ThreadMode.MAIN) 

在接收参数的方法上一定要带这个注解，不然参数会接收不到。
注解中的：
threadMode = ThreadMode.MAIN，指的是在什么线程下操作。我们点进去源码看看

public enum ThreadMode {
    /**
     * 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 must return quickly to avoid blocking the posting thread, which may be the main thread.
     */
    POSTING,

    /**
     * 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.
     */
    MAIN,

    /**
     * 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.
     */
    BACKGROUND,

    /**
     * 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.
     */
    ASYNC
}

哦，是个枚举类型。
POSTING：意思大概是，为了避免线程切换，在什么线程发的你接受默认就是什么线程

MAIN：主线程，也就是ui线程，不要做耗时操作哟

BACKGROUND：顾名思义，就是子线程啦。

ASYNC：异步，我感觉EventBus很贴心，异步都提供了。

5、EventBus还有一种使用，那就是EventBus的粘性事件（这里仅仅简单举个例子，我目前并没有在实际场景中用到）
依旧是这两个Activity

public class MainActivity extends AppCompatActivity {
    private Button button;
    private MyStudent myStudent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        EventBus.getDefault().register(this);//注册eventbus
        button = findViewById(R.id.main_btn);

        button .setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this, Main2Activity.class));
            }
        });
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        EventBus.getDefault().unregister(MainActivity.this);
    }
//接收事件，EventBus3.0之后采用注解的方式
    @Subscribe(threadMode = ThreadMode.MAIN , sticky = true)//sticky是为了声明是粘性事件
    public void Event(MyStudent myStudent) {
        Log.e("MainActivity", myStudent.toString());
    }
}

看看Main2

public class Main2Activity extends AppCompatActivity {
    Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        button = findViewById(R.id.main2_btn);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyStudent myStudent = new MyStudent();
                myStudent.setName("eventbus");
                myStudent.setOld(2);
                myStudent.setSex(2);
                EventBus.getDefault().postSticky(myStudent);
               finish()；
            }
        });
    }

为什么叫粘性事件呢？
先举个小例子，比如说：你定报纸，本来按理说你必须提前订阅了，在发报纸的时候才能收到。 而粘性事件是： 你别管他什么时候发的，就算他先发了报纸，那么你订阅的时候你也能收到这个报纸。（我觉得这个例子已经很形象了）
那么EventBus的粘性事件也是这样，如果他先发消息，发的时候你还没注册，不要紧，你什么时候注册什么时候接收，处理下面的事情。

学习的同学可以多打印log看看。多看多试。
这节课就到这里，下节课再见。