轻巧如燕的AMessage

Android通过AMessage进行消息传递，充满了极其轻巧灵活的使用方式，让人不得不称赞

1）自发自用，发完不用管，最简单场景

```

void MediaPuller::pause() {

    (new AMessage(kWhatPause, this))->post();

}

void MediaPuller::onMessageReceived(const sp<AMessage> &msg) {

    switch (msg->what()) {

          case kWhatPause:

        {

            mPaused = true;

            break;

        }

···

2）发送后需要等待响应

···

status_t MediaPuller::start() {

    return postSynchronouslyAndReturnError(new AMessage(kWhatStart, this));

}

调用msg->postAndAwaitResponse（）该函数里面实际是个condition_wait(),发送方卡在这边

status_t MediaPuller::postSynchronouslyAndReturnError(

        const sp<AMessage> &msg) {

    sp<AMessage> response;

    status_t err = msg->postAndAwaitResponse(&response);

    if (err != OK) {

        return err;

    }

    if (!response->findInt32("err", &err)) {

        err = OK;

    }

    return err;

}

接收方

void MediaPuller::onMessageReceived(const sp<AMessage> &msg) {

    switch (msg->what()) {

        case kWhatStart:

        {

           .......

            sp<AMessage> response = new AMessage;

            response->setInt32("err", err);

            sp<AReplyToken> replyID;

            CHECK(msg->senderAwaitsResponse(&replyID)); //查看msg是否设置过replyToken,设置过才需要回复

            response->postReply(replyID); //looper->postReply(replyToken, this);  实际是将发送方设置的tokern里reply message sp指向response； 然后broadcast signal 通知发送方就可以使用response消息了

            break;

···

3）消息里面带消息，然后等事件处理完成后再通过消息触发下一步操作

···

void WifiDisplaySource::PlaybackSession::Track::stopAsync() {

    sp<AMessage> msg = new AMessage(kWhatMediaPullerStopped, this);

             mMediaPuller->stopAsync(msg);

void MediaPuller::stopAsync(const sp<AMessage> &notify) {

    sp<AMessage> msg = new AMessage(kWhatStop, this);

    msg->setMessage("notify", notify);

    msg->post();

}

void MediaPuller::stopAsync(const sp<AMessage> &notify) {

    sp<AMessage> msg = new AMessage(kWhatStop, this);

    msg->setMessage("notify", notify);

    msg->post();

}

void MediaPuller::onMessageReceived(const sp<AMessage> &msg) {

    switch (msg->what()) {   

        case kWhatStop:

        {//执行完响应后回复消息

            sp<AMessage> notify;

            CHECK(msg->findMessage("notify", &notify));

            notify->post();

            break;

         }

···

暂时先总结这3种， android的关于message的使用场景非常丰富，等看到后面奇妙的地方再分享