android中的消息队列非常经典，Looper内部循环，从消息队列中取消息，然后再交由对应的handler执行，handler负责发送消息。
今天由c++来实现下这套系统，当然可能会略有不同，比如android中使用了epoll机制
Linux pipe/epoll机制，简单说就是在主线程的MessageQueue没有消息时，便阻塞在loop的queue.next()中的nativePollOnce()方法里，此时主线程会释放CPU资源进入休眠状态，直到下个消息到达或者有事务发生，通过往pipe管道写端写入数据来唤醒主线程工作。这里采用的epoll机制，是一种IO多路复用机制，可以同时监控多个描述符，当某个描述符就绪(读或写就绪)，则立刻通知相应程序进行读或写操作，本质同步I/O，即读写是阻塞的。 所以说，主线程大多数时候都是处于休眠状态，并不会消耗大量CPU资源。
为了简单处理，本例中就使用wait和notify这种简单方式，消费者生产者模型，当消息队列中没有消息时，则阻塞线程，当往消息队列中添加消息时，则唤醒线程。
一：整体设计
首先，整个系统中会有以下几个必备的类
- Message，消息本身，可以分成两类，runnable或根据what值处理的空消息
- MessageQueue，消息队列，负责添加消息，给Looper提供消息
- Handler，负责处理只有what值的空白消息
- Looper，开启循环，从消息队列中取消息，然后处理消息
- Thread，线程封装类，启动线程，并且在线程回调中启动Looper的循环
二：编码
#ifndef FUTURE_MSG_LOOPER_H
#define FUTURE_MSG_LOOPER_H
#include <list>
#include <iostream>
#include <functional>
const int EMPTY_MSG_WHAT = -1;
class Handler;
class Looper;
class BaseMessage;
typedef std::function<void()> Runnable;
using HandlerFunc = std::function<bool(std::shared_ptr<BaseMessage>)>;
using std::cout;
using std::endl;
class BaseMessage : public std::enable_shared_from_this<BaseMessage>{
public:
virtual void run() = 0;
virtual int what() = 0;
};
class Message : public BaseMessage{
public:
explicit Message(int what, std::weak_ptr<Handler> ptr);
void run() override;
int what() override;
private:
int what_;
std::weak_ptr<Handler> weak_handler_;
};
class RunnableMsg : public BaseMessage {
public:
explicit RunnableMsg(Runnable runnable);
void run() override;
int what() override;
private:
Runnable runnable_;
};
class MessageQueue{
public:
void addMsg(std::shared_ptr<BaseMessage> msg);
std::shared_ptr<BaseMessage> getMsg();
void popFront();
bool empty();
int size(){
return msg_list_.size();
}
private:
std::list<std::shared_ptr<BaseMessage>> msg_list_;
};
class Handler : public std::enable_shared_from_this<Handler>{
public:
explicit Handler(HandlerFunc func, std::shared_ptr<Looper> looper);
void handleMsg(std::shared_ptr<BaseMessage> msg);
std::shared_ptr<BaseMessage> obtainMsg(int what);
static std::shared_ptr<Handler> createHandler(HandlerFunc func, std::shared_ptr<Looper> looper);
private:
HandlerFunc handler_func_;
std::shared_ptr<Looper> looper_;
};
class Looper : public std::enable_shared_from_this<Looper>{
public:
Looper();
void loop();
void postMsg(std::shared_ptr<BaseMessage> msg);
void postRunnable(Runnable runnable);
void shutDown();
private:
std::shared_ptr<MessageQueue> msg_queue_;
bool shutdown_;
};
#endif //FUTURE_MSG_LOOPER_H
//-------------------------------------------------
#include "msg_looper.h"
std::mutex msg_mutex;
std::condition_variable msg_cv;
Message::Message(int what, std::weak_ptr<Handler> ptr)
: what_(what), weak_handler_(ptr) {
}
void Message::run() {
auto ptr = weak_handler_.lock();
if (ptr != nullptr) {
ptr->handleMsg(shared_from_this());
}
}
int Message::what() {
return what_;
}
RunnableMsg::RunnableMsg(Runnable runnable): runnable_(runnable) {
}
void RunnableMsg::run() {
if (runnable_ != nullptr) {
runnable_();
}
}
int RunnableMsg::what() {
return EMPTY_MSG_WHAT;
}
void MessageQueue::addMsg(std::shared_ptr<BaseMessage> msg) {
std::unique_lock<std::mutex> lock(msg_mutex);
msg_list_.push_back(msg);
msg_cv.notify_one();
}
std::shared_ptr<BaseMessage> MessageQueue::getMsg() {
if (msg_list_.empty()) {
return nullptr;
} else {
auto msg = msg_list_.front();
return msg;
}
}
void MessageQueue::popFront() {
if (msg_list_.size() > 0) {
msg_list_.pop_front();
}
}
bool MessageQueue::empty() {
return msg_list_.empty();
}
Handler::Handler(HandlerFunc func, std::shared_ptr<Looper> looper) : handler_func_(func), looper_(looper) {
}
std::shared_ptr<Handler> Handler::createHandler(HandlerFunc func, std::shared_ptr<Looper> looper) {
std::shared_ptr<Handler> handler = std::make_shared<Handler>(func, looper);
return handler;
}
std::shared_ptr<BaseMessage> Handler::obtainMsg(int what) {
auto msg = std::make_shared<Message>(what, shared_from_this());
return msg;
}
void Handler::handleMsg(std::shared_ptr<BaseMessage> msg) {
handler_func_(msg);
}
Looper::Looper() : shutdown_(false) {
msg_queue_ = std::make_shared<MessageQueue>();
postRunnable([](){
cout << "the first run msg" << endl;
});
}
void Looper::shutDown() {
shutdown_ = true;
}
void Looper::loop() {
cout << "loop start" << endl;
for (;;) {
std::unique_lock<std::mutex> lock(msg_mutex);
cout << "msg queue.size = " << msg_queue_->size() << endl;
msg_cv.wait(lock, [this](){
return !(this->msg_queue_->empty()) || shutdown_;
});
if (shutdown_) {
cout << "looper shut down" << endl;
break;
}
auto msg = msg_queue_->getMsg();
cout << "what = " << msg->what() << endl;
msg->run();
msg_queue_->popFront();
}
}
void Looper::postMsg(std::shared_ptr<BaseMessage> msg) {
this->msg_queue_->addMsg(msg);
}
void Looper::postRunnable(Runnable runnable) {
std::shared_ptr<RunnableMsg> msg = std::make_shared<RunnableMsg>(runnable);
this->msg_queue_->addMsg(msg);
}
//--------------------------------------
#ifndef FUTURE_LOOPERTHREAD_H
#define FUTURE_LOOPERTHREAD_H
#include <thread>
#include "msg_looper.h"
#include "promise.h"
class LooperThread{
public:
LooperThread(std::string name);
void createThread();
std::shared_ptr<Looper> getLooper();
private:
std::shared_ptr<Looper> looper_;
std::string name_;
};
//----------------------------------------------
#include "LooperThread.h"
LooperThread::LooperThread(std::string name): name_(name){
this->looper_ = std::make_shared<Looper>();
}
void LooperThread::createThread() {
std::thread thread([this](){
this->looper_->loop();
});
thread.detach();
}
std::shared_ptr<Looper> LooperThread::getLooper() {
if (looper_ == nullptr){
std::cout << "thread looper is null" << endl;
} else {
std::cout << "thread looper is not null" << endl;
}
return looper_;
}
#endif //FUTURE_LOOPERTHREAD_H
//-----------------------------------------------
void test_looper_thread(){
LooperThread looperThread("ok");
looperThread.createThread();
std::string str = "test ok";
cout << "-----" << endl;
auto looper = looperThread.getLooper();
looper->postRunnable([&str](){
test_printf_in_looper(str);
});
auto handler = Handler::createHandler([](std::shared_ptr<BaseMessage> msg)->bool{
if (msg->what() != EMPTY_MSG_WHAT) {
cout << "msg what = " << msg->what() << endl;
}
return true;
}, looper);
auto message = handler->obtainMsg(10);
looper->postMsg(message);
getchar();
}
三、总结
c++的线程回调方法比较奇怪，是不能直接用类的成员方法的，因为类的成员方法中会被默认添加一个隐藏参数，this，而且是第一个参数。
*** Func(this, ...)
类的成员方法就类似上面的Func方法，它的第一个参数是this，所以如果用它作为线程的回调函数，会出现参数不匹配的情况，无法编译通过。
但c++有无赖的lambda，lambda可以捕捉当前可见的局部变量，包括this，所以可以在lambda中调用相应的成员方法。
仔细思考下例中的Runnable，它其实是一个无参但也无返回的方法，但我们往线程中调用的方法不可能一直无参也无返回值的，参数可以通过lambda的捕捉添加进来，返回值当然是通过future带出去。所以即使Runnable是一个无参无返回的方法，也是有广泛适应性的
