在上一篇文章《C++实现简化版Qt的QObject（4）：增加简单实用的事件机制》中，我们实现了普通线程的事件机制。
但是事件机制往往需要和操作系统主线程消息循环一起工作。
因此，今天，我们在之前的CEventLoop的实现之上，通过IEventLoopHost扩展，实现一个windows系统的主线程消息循环扩展，使得我们可以在主线程post task。
CEventLoop 代码
首先，昨天我们写的CEventLoop代码如下：
class CEventLoop {public:using Clock = std::chrono::steady_clock;using TimePoint = Clock::time_point;using Duration = Clock::duration;using Handler = std::function<void()>;struct TimedHandler {TimePoint time;Handler handler;bool operator<(const TimedHandler& other) const {return time > other.time;}};class IEventLoopHost {public:virtual void onPostTask() = 0;virtual void onWaitForTask(std::condition_variable& cond, std::unique_lock<std::mutex>& locker) = 0;virtual void onEvent(TimedHandler& event) = 0;virtual void onWaitForRun(std::condition_variable& cond, std::unique_lock<std::mutex>& locker, const TimePoint& timePoint) = 0;};private:IEventLoopHost* host = nullptr;std::priority_queue<TimedHandler> tasks_;std::mutex mutex_;std::condition_variable cond_;std::atomic<bool> running_{ true };public:void setHost(IEventLoopHost* host) {this->host = host;}void post(Handler handler, Duration delay = Duration::zero()) {std::unique_lock<std::mutex> lock(mutex_);tasks_.push({ Clock::now() + delay, std::move(handler) });cond_.notify_one();if (host) {host->onPostTask();}}void run() {while (running_) {std::unique_lock<std::mutex> lock(mutex_);if (tasks_.empty()) {if (host) {host->onWaitForTask(cond_, lock);}else {cond_.wait(lock, [this] { return !tasks_.empty() || !running_; });}}while (!tasks_.empty() && tasks_.top().time <= Clock::now()) {auto task = tasks_.top();tasks_.pop();lock.unlock();if (host) {host->onEvent(task);}else {task.handler();}lock.lock();}if (!tasks_.empty()) {if (host) {host->onWaitForRun(cond_, lock, tasks_.top().time);}else {cond_.wait_until(lock, tasks_.top().time);}}}}void stop() {running_ = false;cond_.notify_all();}};实现扩展
在Windows API中，消息循环通常涉及到调用GetMessage、TranslateMessage和DispatchMessage函数。以下是一个IEventLoopHost的实现，它将Windows消息循环集成到CEventLoop类中：
#include <Windows.h>class WindowsEventLoopHost : public CEventLoop::IEventLoopHost {
public:// 当任务被投递到事件循环时调用void onPostTask() override {// 可以使用Windows的PostMessage函数发送一个自定义的消息来唤醒消息循环PostThreadMessage(GetCurrentThreadId(), WM_USER, 0, 0);}// 当事件循环需要等待任务时调用void onWaitForTask(std::condition_variable& cond, std::unique_lock<std::mutex>& locker) override {locker.unlock();MSG msg;// 使用PeekMessage而不是GetMessage来避免阻塞，以便定时器事件可以被处理while (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE)) {TranslateMessage(&msg);DispatchMessage(&msg);}locker.lock();}// 当事件循环处理事件时调用void onEvent(CEventLoop::TimedHandler& event) override {// 这里简单地调用事件处理器event.handler();}// 当事件循环需要等待直到特定时间点运行时调用void onWaitForRun(std::condition_variable& cond, std::unique_lock<std::mutex>& locker, const CEventLoop::TimePoint& timePoint) override {locker.unlock();// 计算需要等待的时间auto now = CEventLoop::Clock::now();auto delay_ms = std::chrono::duration_cast<std::chrono::milliseconds>(timePoint - now).count();if (delay_ms > 0) {// 设置Windows计时器SetTimer(NULL, 0, (UINT)delay_ms, NULL);MSG msg;// 等待计时器或其他消息while (GetMessage(&msg, NULL, 0, 0)) {if (msg.message == WM_TIMER) {break; // 计时器消息，继续事件循环}TranslateMessage(&msg);DispatchMessage(&msg);}KillTimer(NULL, 0);}locker.lock();}
};
在这个实现中，我们通过IEventLoopHost的四个虚函数，完成与Windows消息循环协同工作：
onPostTask使用PostThreadMessage来发送一个自定义的消息，以唤醒可能正在等待的消息循环。onWaitForTask使用PeekMessage来处理消息而不阻塞，这样定时器消息可以在适当的时间被处理。onEvent简单地调用传入的事件处理器，这里我们没有对Windows消息做特别的处理。onWaitForRun使用SetTimer来创建一个Windows计时器，它会在指定的毫秒数后发送一个WM_TIMER消息。我们使用GetMessage等待这个消息或者其他消息的到来。如果计时器消息到来，我们就终止循环，返回到事件循环，以便继续处理其他任务。
继续优化
在自测过程中进一步的优化代码结构，优化性能，处理WM_QUIT消息，处理一些其他细节等。
最终代码如下：
#include <Windows.h>
class CWindowsEventLoopHost : public base::CEventLoop::IEventLoopHost {static constexpr UINT WM_WAKEUP = WM_USER + 15151515;UINT_PTR timerID_ = 0;public:~CWindowsEventLoopHost() {if (timerID_) {KillTimer(NULL, timerID_);}}void onPostTask() override {PostThreadMessage(GetCurrentThreadId(), WM_WAKEUP, 0, 0);}void onWaitForTask(std::condition_variable& cond, std::unique_lock<std::mutex>& locker) override {MSG msg;while (GetMessage(&msg, NULL, 0, 0)) {if (handleWindowsMessage(msg)) {break;}if (cond.wait_for(locker, std::chrono::milliseconds(0), [] { return true; })) {break;}}}void onEvent(base::CEventLoop::TimedHandler& event) override {event.handler();}void onWaitForRun(std::condition_variable& cond, std::unique_lock<std::mutex>& locker, const std::chrono::steady_clock::time_point& timePoint) override {auto now = std::chrono::steady_clock::now();if (now < timePoint) {auto delay_ms = std::chrono::duration_cast<std::chrono::milliseconds>(timePoint - now).count();timerID_ = SetTimer(NULL, 0, (UINT)delay_ms, NULL);//通过timer事件唤醒MSG msg;while (GetMessage(&msg, NULL, 0, 0)) {if (handleWindowsMessage(msg)) {break;}}}}bool handleWindowsMessage(MSG& msg) {if (msg.message == WM_QUIT) {if (this->eventLoop) {this->eventLoop->stop();//退出消息循环}return true;}if (msg.message == WM_TIMER && msg.wParam == timerID_) {KillTimer(NULL, timerID_);timerID_ = 0;return true; // Timer event occurred}TranslateMessage(&msg);DispatchMessage(&msg);return false;}
};
使用示例
由于setHost函数里面没有加锁，而且可能出现在运行过程中被修改host出现不可预期情况。因此，后来去掉setHost函数，改为在CEventLoop的构造函数传入host，避免误用。
使用示例如下：
// 使用示例
int main() {CWindowsEventLoopHost host;base::CEventLoop loop(&host);loop.post([]() {std::cout << "Immediate task\n";});//马上执行loop.post([]() {std::cout << "Delayed task\n";}, std::chrono::seconds(1));//延时一秒loop.post([]() {std::cout << "Delayed task in 3 second\n";}, std::chrono::seconds(3));//延时3秒loop.post([&]() {std::cout << "stop msg loop in 6 second\n";loop.stop();}, std::chrono::seconds(6));//延时6秒if (true) {loop.run();//主线程直接run}else {// 或者起一个其他线程run:std::thread loop_thread([&loop]() {loop.run();});std::this_thread::sleep_for(std::chrono::seconds(10));loop.stop();//停止消息循环loop_thread.join();}
}
完整代码在：https://github.com/kevinyangli/simple_qt_qobject.git
