c++ - 在 C++ 中进行事件处理的正确方法是什么？

Question

我有一个应用程序需要以下列方式响应某些事件:

void someMethodWithinSomeClass() {
    while (true) {
        wait for event;
        if (event == SomeEvent) {
            doSomething();
            continue;
        }
        if (event == SomeOtherEvent) {
            doSomethingElse();
            continue;
        }
    } 
}

这将是一些线程。在其他一些线程中，操作将创建并触发事件。

如何让这些事件到达上述方法/类？在 C++ 中实现事件处理的正确策略或架构是什么？

Answer 1

通常，事件队列被实现为 command design pattern :

In object-oriented programming, the command pattern is a design pattern in which an object is used to represent and encapsulate all the information needed to call a method at a later time. This information includes the method name, the object that owns the method and values for the method parameters.

在 C++ 中，拥有方法和方法参数值的对象是一个空仿函数(即不带参数的仿函数)。它可以使用 boost::bind() 或 C++11 lambdas 创建。并包装到 boost::function 中。

这是一个极简示例，如何在多个生产者和多个消费者线程之间实现事件队列。用法:

void consumer_thread_function(EventQueue::Ptr event_queue)
try {
    for(;;) {
        EventQueue::Event event(event_queue->consume()); // get a new event 
        event(); // and invoke it
    }
}
catch(EventQueue::Stopped&) {
}

void some_work(int n) {
    std::cout << "thread " << boost::this_thread::get_id() << " : " << n << '\n';
    boost::this_thread::sleep(boost::get_system_time() + boost::posix_time::milliseconds(500));
}

int main()
{
    some_work(1);

    // create an event queue that can be shared between multiple produces and multiple consumers
    EventQueue::Ptr queue(new EventQueue);

    // create two worker thread and pass them a pointer to queue
    boost::thread worker_thread_1(consumer_thread_function, queue);
    boost::thread worker_thread_2(consumer_thread_function, queue);

    // tell the worker threads to do something
    queue->produce(boost::bind(some_work, 2));
    queue->produce(boost::bind(some_work, 3));
    queue->produce(boost::bind(some_work, 4));

    // tell the queue to stop
    queue->stop(true);

    // wait till the workers thread stopped
    worker_thread_2.join();
    worker_thread_1.join();

    some_work(5);
}

输出:

./test
thread 0xa08030 : 1
thread 0xa08d40 : 2
thread 0xa08fc0 : 3
thread 0xa08d40 : 4
thread 0xa08030 : 5

实现:

#include <boost/function.hpp>
#include <boost/thread/thread.hpp>
#include <boost/thread/condition.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/smart_ptr/intrusive_ptr.hpp>
#include <boost/smart_ptr/detail/atomic_count.hpp>
#include <iostream>

class EventQueue
{
public:
    typedef boost::intrusive_ptr<EventQueue> Ptr;
    typedef boost::function<void()> Event; // nullary functor
    struct Stopped {};

    EventQueue()
        : state_(STATE_READY)
        , ref_count_(0)
    {}

    void produce(Event event) {
        boost::mutex::scoped_lock lock(mtx_);
        assert(STATE_READY == state_);
        q_.push_back(event);
        cnd_.notify_one();
    }

    Event consume() {
        boost::mutex::scoped_lock lock(mtx_);
        while(STATE_READY == state_ && q_.empty())
            cnd_.wait(lock);
        if(!q_.empty()) {
            Event event(q_.front());
            q_.pop_front();
            return event;
        }
        // The queue has been stopped. Notify the waiting thread blocked in
        // EventQueue::stop(true) (if any) that the queue is empty now.
        cnd_.notify_all();
        throw Stopped();
    }

    void stop(bool wait_completion) {
        boost::mutex::scoped_lock lock(mtx_);
        state_ = STATE_STOPPED;
        cnd_.notify_all();
        if(wait_completion) {
            // Wait till all events have been consumed.
            while(!q_.empty())
                cnd_.wait(lock);
        }
        else {
            // Cancel all pending events.
            q_.clear();
        }
    }

private:
    // Disable construction on the stack. Because the event queue can be shared between multiple
    // producers and multiple consumers it must not be destroyed before the last reference to it
    // is released. This is best done through using a thread-safe smart pointer with shared
    // ownership semantics. Hence EventQueue must be allocated on the heap and held through
    // smart pointer EventQueue::Ptr.
    ~EventQueue() {
        this->stop(false);
    }

    friend void intrusive_ptr_add_ref(EventQueue* p) {
        ++p->ref_count_;
    }

    friend void intrusive_ptr_release(EventQueue* p) {
        if(!--p->ref_count_)
            delete p;
    }

    enum State {
        STATE_READY,
        STATE_STOPPED,
    };

    typedef std::list<Event> Queue;
    boost::mutex mtx_;
    boost::condition_variable cnd_;
    Queue q_;
    State state_;
    boost::detail::atomic_count ref_count_;
};