Android Handler.post()

前面分析过handler相关的类handler源码分析，post方法实际上也是发送message，下面看看源码实现。

Handler.post()

public final boolean post(Runnable r){
        return sendMessageDelayed(getPostMessage(r), 0);
    }

public final boolean postDelayed(Runnable r, long delayMillis){
        return sendMessageDelayed(getPostMessage(r), delayMillis);
    }
//将传过来的Runnable赋值给message.callback，
//实际上就是发送一个带有Runnable的消息。
private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;
    }

两个方法都调用sendMessageDelayed()

public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

继续跟

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }

接下来就是MessageQueue.enqueueMessage()，前面分析过发送message流程，这里不细说。最终Looper获取到消息调用Handler.dispatchMessage()完成回调。

Handler.dispatchMessage()

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

msg.callback不为空，会走handleCallback(msg)

private static void handleCallback(Message message) {
        message.callback.run();
    }

handler接收到消息后回调Runnable.run()。也就是说通过主线程初始化的handler，调用post()发送消息，Runnable.run()最终会回调在主线程。

子线程中经常会用runOnUiThread()更新ui，Activity.runOnUiThread()。

    final Handler mHandler = new Handler();
    ...
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

mHandler是activity的成员变量，初始化在ui线程。runOnUiThread()也是调用Handler.post()发送带有Runnable的消息切换到ui线程。