Auto Unsubscriber

#pragma once

#include <utility>

namespace tme
{

template<typename Observer, typename Observable>
class DoUnsubscribe final
{
public:

    void operator()(Observable& observable, const Observer&) const
    {
        observable.unsubscribe();
    }

};

template <typename Observer, typename Observable, typename Unsubscribe = DoUnsubscribe<Observer, Observable>>
class AutoUnsubscribe final
{
public:

    AutoUnsubscribe() = default;

    explicit AutoUnsubscribe(const Observer& observer)
        : observer_(observer)
    {
    }

    explicit AutoUnsibescribe(Observer&& observer)
        : observer_(std::move(observer))
    {
    }

    AutoUnsubscribe& plug(const Observer& observer)
    {
        unsubscribe();
        observer_ = observer;
        return *this;
    }

    AutoUnsubscribe& plug(Observer&& observer)
    {
        unsubscribe();
        observer_ = std::move(observer);
        return *this;
    }

    Observer subscribe(Observable& host)
    {
        UNI_ASSERT(nullptr == observable_);
        observable_ = &host;
        return observer_;
    }

    ~AutoUnsubscribe()
    {
        unsubscribe();
    }

private:

    void unsubscribe()
    {
        if (nullptr != observable_)
        {
            Unsubscribe{}(*observable_, observer_);
        }
    }

    Observer observer_;

    Observable* observable_ = nullptr;

};

} // namespace tme

/*
usage
*/
class SinkMessagesBuffer final : public UniLog::ILogMessageSink
{
public:

    using AutoUnsubscribe = tme::AutoUnsubscribe<std::function<void()>, SinkMessagesBuffer>;

    void subscribe_on_messages_deliver(AutoUnsubscribe& listener)
    {
        on_new_message_ = listener.subscribe(*this);
    }

    void unsubscribe()
    {
        on_new_message_ = {};
    }

// some other stuff
private:

    std::function<void()> on_new_message_;

// some other stuff
};

SinkMessagesBuffer log_buffer;

SinkMessagesBuffer::AutoUnsubscribe log_buffer_connection;

log_buffer->subscribe_on_messages_deliver(log_buffer_connection.plug([this]() { cool_stuff_on_event(); }));