cpp-utils/Event.h

//
// An event/listener system.
//

#ifndef EVENT_H
#define EVENT_H

#include <functional>
#include <unordered_map>

class Event;

class CallbackHandle {
  public:
    CallbackHandle(Event *evt, int id) : m_evt(evt), m_id(id) {
    }
    ~CallbackHandle();

  private:
    Event *m_evt;
    int m_id;
};

class Event {
  public:
    using Callback = std::function<void()>;

    CallbackHandle addListener(Callback cb) {
        int id = m_nextId++;
        m_callbacks[id] = std::move(cb);
        return CallbackHandle(this, id);
    }

    void remove(int id) {
        m_callbacks.erase(id);
    }

    void fire() {
        for (auto &kv : m_callbacks)
            kv.second();
    }

  private:
    friend class CallbackHandle;
    std::unordered_map<int, Callback> m_callbacks;
    int m_nextId = 0;
};

CallbackHandle::~CallbackHandle() {
    if (m_evt)
        m_evt->remove(m_id);
}

#endif // EVENT_H