面试官问我：Android EventBus的源码，看我如何用漫画装逼！

一名优秀的Android开发，需要一份完备的 知识体系，在这里，让我们一起成长为自己所想的那样~。

基本使用步骤

1）定义事件Event

public class MyEvent {
    public MyEvent(String id, String message) {
        this.id = id;
        this.message = message;
    }

    public String id;

    public String message;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

2）注册订阅者，并实现回调方法

public class MainActivity extends Activity {

    private TextView message, next;

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

        initView();
        initData();
    }

    /**
     * 初始化布局组件
     */
    private void initView() {
        message = findViewById(R.id.message);
        next = findViewById(R.id.next);

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

    @Subscribe(threadMode = ThreadMode.MAIN)
    public void onMyEvent(MyEvent event) {
        message.setText(event.getMessage());
    }

    /**
     * 初始化数据
     */
    private void initData() {
        EventBus.getDefault().register(this);
    }

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

通过EventBus.getDefault().register(this);方法注册订阅者。
3）发送消息

public class SecondActivity extends Activity {

    private TextView message;

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

        initView();
    }

    /**
     * 初始化布局组件
     */
    private void initView() {
        message = findViewById(R.id.message);
        message.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                EventBus.getDefault().post(new MyEvent("1","这是从SecondActivity发送过来的消息"));
                Toast.makeText(SecondActivity.this, "消息发送成功", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

通过EventBus.getDefault().post(new MyEvent("1","这是从SecondActivity发送过来的消息"));方法发送消息

源码分析

EventBus的使用先注册订阅者，并实现方法，然后再发送post消息，所以我们分析源码也按照这个顺序来。
1）实例化EventBus

EventBus.getDefault().register(this);

该方法首先获取EventBus实例，然后再注册，源码如下所示：

  /** Convenience singleton for apps using a process-wide EventBus instance. */
    public static EventBus getDefault() {
        if (defaultInstance == null) {
            synchronized (EventBus.class) {
                if (defaultInstance == null) {
                    defaultInstance = new EventBus();
                }
            }
        }
        return defaultInstance;
    }

可以看到，实例化EventBus是个双重锁的单例模式。

接下来看EventBus的构造函数，源码如下：

 EventBus(EventBusBuilder builder) {
        logger = builder.getLogger();
        subscriptionsByEventType = new HashMap<>();
        typesBySubscriber = new HashMap<>();
        stickyEvents = new ConcurrentHashMap<>();
        mainThreadSupport = builder.getMainThreadSupport();
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
        backgroundPoster = new BackgroundPoster(this);
        asyncPoster = new AsyncPoster(this);
        indexCount = builder.subscriberInfoIndexes != null ? builder.subscriberInfoIndexes.size() : 0;
        subscriberMethodFinder = new SubscriberMethodFinder(builder.subscriberInfoIndexes,
                builder.strictMethodVerification, builder.ignoreGeneratedIndex);
        logSubscriberExceptions = builder.logSubscriberExceptions;
        logNoSubscriberMessages = builder.logNoSubscriberMessages;
        sendSubscriberExceptionEvent = builder.sendSubscriberExceptionEvent;
        sendNoSubscriberEvent = builder.sendNoSubscriberEvent;
        throwSubscriberException = builder.throwSubscriberException;
        eventInheritance = builder.eventInheritance;
        executorService = builder.executorService;
    }

可以看到，在EventBus的构造函数中，初始化了一大堆变量，这边主要关注前面两个。

subscriptionsByEventType = new HashMap<>();：key:事件类型（如：MyEvent ），value:新的订阅对象的集合，包括订阅者和订阅者包含的方法。具体赋值逻辑在后面会讲，这边先了解即可。
typesBySubscriber = new HashMap<>();：key:事件的订阅者（MainActivity ），value：事件类型的集合。订阅者跟事件类型是一对多的关系，所以一个界面可以支持多个事件类型。

2）注册

 /**
     * Registers the given subscriber to receive events. Subscribers must call {@link #unregister(Object)} once they
     * are no longer interested in receiving events.
     * 

     * Subscribers have event handling methods that must be annotated by {@link Subscribe}.
     * The {@link Subscribe} annotation also allows configuration like {@link
     * ThreadMode} and priority.
     */
    public void register(Object subscriber) {
        //1.拿到订阅对象的类型；
        Class subscriberClass = subscriber.getClass();
        //2.通过findSubscriberMethods方法获取该订阅者中的所有订阅方法，因为可能包含多个订阅方法，所以返回集合。
        List subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
               //3.通过subscribe方法为每个订阅方法进行订阅。 
                subscribe(subscriber, subscriberMethod);
            }
        }
    }

如何获取该订阅者中的所有订阅方法，这边主要看第二件事情的处理过程，及findSubscriberMethods方法的源码

List findSubscriberMethods(Class subscriberClass) {
        //先拿订阅对象在本地缓存中查找，提高性能。
        List subscriberMethods = METHOD_CACHE.get(subscriberClass);
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        //默认情况下ignoreGeneratedIndex值是false的
        if (ignoreGeneratedIndex) {
           //使用反射方法拿到订阅者中的订阅方法  
            subscriberMethods = findUsingReflection(subscriberClass);
        } else {
            //使用编译期间生成的SubscriberInfo
            subscriberMethods = findUsingInfo(subscriberClass);
        }
        //如果没有订阅方法，则抛出异常
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass
                    + " and its super classes have no public methods with the @Subscribe annotation");
        } else {
            //本地存储订阅方法，方便下次获取，提高了性能
            METHOD_CACHE.put(subscriberClass, subscriberMethods);
            return subscriberMethods;
        }
    }

可以看到上面方法做了三件事：
1.先拿订阅对象在本地缓存中查找订阅方法
2.如果本地获取不到，则根据ignoreGeneratedIndex的值决定获取订阅方法的方式
3.本地保存订阅方法。
可以看到获取订阅的关键方法为：findUsingInfo()，那么赶紧看下他的源码，如下：

private List findUsingInfo(Class subscriberClass) {
        FindState findState = prepareFindState();
        findState.initForSubscriber(subscriberClass);
        while (findState.clazz != null) {
            // 获取订阅者信息，没有配置MyEventBusIndex返回null
            findState.subscriberInfo = getSubscriberInfo(findState);
            if (findState.subscriberInfo != null) {
                SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();
                for (SubscriberMethod subscriberMethod : array) {
                    if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {
                        findState.subscriberMethods.add(subscriberMethod);
                    }
                }
            } else {
                // 通过反射来查找订阅方法，所以为了提高性能，我们还是要用索引的形式使用EventBus
                findUsingReflectionInSingleClass(findState);
            }
            findState.moveToSuperclass();
        }
        return getMethodsAndRelease(findState);
    }

获取到订阅方法后，我们看真正的注册方法subscribe(subscriber, subscriberMethod);

    // Must be called in synchronized block
    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        //获取到事件类型（如：MyEvent ）
        Class eventType = subscriberMethod.eventType;
        //把订阅者和订阅者方法重新封装成新的对象
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        //事件类型为key，新的订阅者对象列表为value，存储进subscriptionsByEventType
        //该集合很重要，当post消息的时候，就是从该集合中查找订阅者对象列表
        CopyOnWriteArrayList subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions == null) {
            //如果订阅者对象列表为空，则初始化出来，并加到subscriptionsByEventType中
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }

        int size = subscriptions.size();
        for (int i = 0; i <= size; i++) {
            // 根据订阅方法的优先级，添加到订阅列表
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }
        //订阅者为key，事件类型列表为value，存储进typesBySubscriber
        List> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);

        if (subscriberMethod.sticky) {
            if (eventInheritance) {
                // Existing sticky events of all subclasses of eventType have to be considered.
                // Note: Iterating over all events may be inefficient with lots of sticky events,
                // thus data structure should be changed to allow a more efficient lookup
                // (e.g. an additional map storing sub classes of super classes: Class -> List).
                Set, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry, Object> entry : entries) {
                    Class candidateEventType = entry.getKey();
                    if (eventType.isAssignableFrom(candidateEventType)) {
                        Object stickyEvent = entry.getValue();
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }

可以看到上面方法做了两件事：
1.根据订阅方法的优先级，添加到订阅列表，事件类型为key，订阅列表为value，存储进subscriptionsByEventType
2.订阅者为key，事件类型列表为value，存储进typesBySubscriber

3）发送消息

    /** Posts the given event to the event bus. */
    public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List