Subject.hh

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#ifndef SUBJECT_HH
#define SUBJECT_HH
 
#include "Observer.hh"
#include "ranges.hh"
#include "stl.hh"
#include <algorithm>
#include <vector>
#include <cassert>
 
namespace openmsx {
 
template<typename T> class Subject
{
public:
        void attach(Observer<T>& observer);
        void detach(Observer<T>& observer);
        [[nodiscard]] bool anyObservers() const { return !observers.empty(); }
 
protected:
        Subject() = default;
        ~Subject();
        void notify() const;
 
private:
        enum NotifyState {
                IDLE,        // no notify in progress
                IN_PROGRESS, // notify in progress, no detach
                DETACH,      // notify in progress, some observer(s) have been detached
        };
 
        mutable std::vector<Observer<T>*> observers; // unordered
        mutable NotifyState notifyState = IDLE;
};
 
template<typename T> Subject<T>::~Subject()
{
        assert(notifyState == IDLE);
        auto copy = observers;
        for (auto& o : copy) {
                o->subjectDeleted(*static_cast<const T*>(this));
        }
        assert(observers.empty());
}
 
template<typename T> void Subject<T>::attach(Observer<T>& observer)
{
        assert(notifyState == IDLE);
        observers.push_back(&observer);
}
 
template<typename T> void Subject<T>::detach(Observer<T>& observer)
{
        auto it = rfind_unguarded(observers, &observer);
        if (notifyState == IDLE) {
                move_pop_back(observers, it);
        } else {
                *it = nullptr; // mark for removal
                notifyState = DETACH; // schedule actual removal pass
        }
}
 
template<typename T> void Subject<T>::notify() const
{
        assert(notifyState == IDLE);
        notifyState = IN_PROGRESS;
 
        for (auto& o : observers) {
                try {
                        o->update(*static_cast<const T*>(this));
                } catch (...) {
                        assert(false && "Observer::update() shouldn't throw");
                }
        }
 
        if (notifyState == DETACH) {
                observers.erase(ranges::remove(observers, nullptr), observers.end());
        }
        notifyState = IDLE;
}
 
} // namespace openmsx
 
#endif