【EventBus】EventBus 源码解析 ( 事件发送 | 发布线程为 子线程 切换到 主线程 执行订阅方法的过程分析 )

// 假如发布线程是子线程 , 则将事件加入队列 , 通过 Handler 切换线程执行 
                    mainThreadPoster.enqueue(subscription, event);

public class EventBus {
    // @Nullable
    private final MainThreadSupport mainThreadSupport;
    // @Nullable
    private final Poster mainThreadPoster;
    EventBus(EventBusBuilder builder) {
        mainThreadSupport = builder.getMainThreadSupport();
        // 如果当前线程是主线程 , 才会创建 mainThreadPoster , 否则为空
        mainThreadPoster = mainThreadSupport != null ? mainThreadSupport.createPoster(this) : null;
    }
    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
      // 获取该 订阅方法 的线程模式 
        switch (subscription.subscriberMethod.threadMode) {
            case MAIN:
                if (isMainThread) {
                  // 假如在主线程中 , 直接调用 
                    invokeSubscriber(subscription, event);
                } else {
                  // 假如发布线程是子线程 , 则将事件加入队列 , 通过 Handler 切换线程执行 
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
  }
  }
}

package org.greenrobot.eventbus;
import android.os.Looper;
/**
 * 接口到“主”线程，可以是您喜欢的任何线程。通常在Android上使用Android的主线程。
 */
public interface MainThreadSupport {
    boolean isMainThread();
    Poster createPoster(EventBus eventBus);
    class AndroidHandlerMainThreadSupport implements MainThreadSupport {
        private final Looper looper;
        public AndroidHandlerMainThreadSupport(Looper looper) {
            this.looper = looper;
        }
        @Override
        public boolean isMainThread() {
            return looper == Looper.myLooper();
        }
        @Override
        public Poster createPoster(EventBus eventBus) {
            return new HandlerPoster(eventBus, looper, 10);
        }
    }
}

public class HandlerPoster extends Handler implements Poster {
    private final PendingPostQueue queue;
    private final int maxMillisInsideHandleMessage;
    private final EventBus eventBus;
    private boolean handlerActive;
    protected HandlerPoster(EventBus eventBus, Looper looper, int maxMillisInsideHandleMessage) {
        super(looper);
        this.eventBus = eventBus;
        this.maxMillisInsideHandleMessage = maxMillisInsideHandleMessage;
        queue = new PendingPostQueue();
    }
  // 将订阅者和订阅方法 , 事件对象 传入该方法 
    public void enqueue(Subscription subscription, Object event) {
      // PendingPost 是一个链表数据结构 , 将所有的 事件对象 , 订阅者 都封装在了该链表中 ; 
      //  将 订阅者 和 事件对象 加入到 PendingPost 链表中 
        PendingPost pendingPost = PendingPost.obtainPendingPost(subscription, event);
        synchronized (this) {
            queue.enqueue(pendingPost);
            if (!handlerActive) {
                handlerActive = true;
                // 向 Handler 发送消息 , 执行 handleMessage 方法
                if (!sendMessage(obtainMessage())) {
                    throw new EventBusException("Could not send handler message");
                }
            }
        }
    }
  // 不断取出 PendingPost 链表中的数据 , 执行订阅方法 
    @Override
    public void handleMessage(Message msg) {
        boolean rescheduled = false;
        try {
            long started = SystemClock.uptimeMillis();
            while (true) {
              // 取出链表元素 , 每个元素中封装了 订阅者 和 事件对象
                PendingPost pendingPost = queue.poll();
                if (pendingPost == null) {
                    synchronized (this) {
                        // Check again, this time in synchronized
                        pendingPost = queue.poll();
                        if (pendingPost == null) {
                            handlerActive = false;
                            return;
                        }
                    }
                }
                // 通过反射执行订阅方法 
                eventBus.invokeSubscriber(pendingPost);
                long timeInMethod = SystemClock.uptimeMillis() - started;
                if (timeInMethod >= maxMillisInsideHandleMessage) {
                    if (!sendMessage(obtainMessage())) {
                        throw new EventBusException("Could not send handler message");
                    }
                    rescheduled = true;
                    return;
                }
            }
        } finally {
            handlerActive = rescheduled;
        }
    }
}

final class PendingPost {
    private final static List<PendingPost> pendingPostPool = new ArrayList<PendingPost>();
    Object event;
    Subscription subscription;
    PendingPost next;
}