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◆ main openmsx / src / utils / Subject.hh
2.0 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
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#ifndef SUBJECT_HH
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#define SUBJECT_HH
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#include "Observer.hh"
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#include "stl.hh"
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#include <cassert>
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#include <cstdint>
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#include <vector>
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namespace openmsx {
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/**
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 * Generic Gang-of-Four Subject class of the Observer pattern, templatized
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 * edition.
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 */
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template<typename T> class Subject
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{
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public:
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	Subject(const Subject&) = delete;
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	Subject(Subject&&) = delete;
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	Subject& operator=(const Subject&) = delete;
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	Subject& operator=(Subject&&) = delete;
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	void attach(Observer<T>& observer);
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	void detach(Observer<T>& observer);
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	[[nodiscard]] bool anyObservers() const { return !observers.empty(); }
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protected:
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	Subject() = default;
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	~Subject();
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	void notify() const;
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private:
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	enum NotifyState : uint8_t {
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		IDLE,        // no notify in progress
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		IN_PROGRESS, // notify in progress, no detach
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		DETACH,      // notify in progress, some observer(s) have been detached
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	};
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	mutable std::vector<Observer<T>*> observers; // unordered
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	mutable NotifyState notifyState = IDLE;
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};
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template<typename T> Subject<T>::~Subject()
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{
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	assert(notifyState == IDLE);
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	for (auto copy = observers; auto& o : copy) {
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		o->subjectDeleted(*static_cast<const T*>(this));
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	}
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	assert(observers.empty());
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}
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template<typename T> void Subject<T>::attach(Observer<T>& observer)
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{
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	assert(notifyState == IDLE);
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	observers.push_back(&observer);
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}
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template<typename T> void Subject<T>::detach(Observer<T>& observer)
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{
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	auto it = rfind_unguarded(observers, &observer);
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	if (notifyState == IDLE) {
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		move_pop_back(observers, it);
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	} else {
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		*it = nullptr; // mark for removal
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		notifyState = DETACH; // schedule actual removal pass
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	}
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}
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template<typename T> void Subject<T>::notify() const
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{
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	assert(notifyState == IDLE);
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	notifyState = IN_PROGRESS;
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	for (auto& o : observers) {
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		try {
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			o->update(*static_cast<const T*>(this));
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		} catch (...) {
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			assert(false && "Observer::update() shouldn't throw");
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		}
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	}
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	if (notifyState == DETACH) {
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		std::erase(observers, nullptr);
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	}
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	notifyState = IDLE;
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}
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} // namespace openmsx
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#endif