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> ¬ify) {
sp<AMessage> msg = new AMessage(kWhatStop, this);
msg->setMessage("notify", notify);
msg->post();
}
void MediaPuller::stopAsync(const sp<AMessage> ¬ify) {
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", ¬ify));
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
