ObservableValue.hpp

//------------------------------------------------------------------------------
/*
    This file is part of clio: https://github.com/XRPLF/clio
    Copyright (c) 2025, the clio developers.
 
    Permission to use, copy, modify, and distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================
 
#pragma once
 
#include <boost/signals2/connection.hpp>
#include <boost/signals2/signal.hpp>
#include <boost/signals2/variadic_signal.hpp>
 
#include <atomic>
#include <concepts>
#include <type_traits>
 
namespace util {
 
template <typename T>
concept SomeAtomic =
    std::same_as<std::remove_cvref_t<T>, std::atomic<std::remove_cvref_t<typename T::value_type>>>;

template <typename T>
concept Observable =
    std::equality_comparable<T> && std::copy_constructible<T> && std::move_constructible<T>;
 
namespace impl {

template <Observable T>
class ObservableValueBase {
protected:
    boost::signals2::signal<void(T const&)> onUpdate_;
 
public:
    virtual ~ObservableValueBase() = default;

    boost::signals2::connection
    observe(std::invocable<T const&> auto&& fn)
    {
        return onUpdate_.connect(std::forward<decltype(fn)>(fn));
    }

    [[nodiscard]] bool
    hasObservers() const
    {
        return not onUpdate_.empty();
    }

    virtual void
    forceNotify() = 0;
 
protected:
    void
    notifyObservers(T const& value)
    {
        onUpdate_(value);
    }
};
 
}  // namespace impl
 
// Forward declaration
template <typename T>
class ObservableValue;

template <Observable T>
    requires(not SomeAtomic<T>)
class ObservableValue<T> : public impl::ObservableValueBase<T> {
    T value_;

    struct ObservableGuard {
        T const oldValue;         
        ObservableValue<T>& ref;  

        ObservableGuard(ObservableValue<T>& observable) : oldValue(observable), ref(observable)
        {
        }

        ~ObservableGuard()
        {
            if (oldValue != ref.value_)
                ref.notifyObservers(ref.value_);
        }

        [[nodiscard]]
        operator T&()
        {
            return ref.value_;
        }
    };
 
public:
    ObservableValue(std::convertible_to<T> auto&& value)
        : value_{std::forward<decltype(value)>(value)}
    {
    }

    ObservableValue()
        requires std::default_initializable<T>
        : value_{}
    {
    }
 
    ObservableValue(ObservableValue const&) = delete;
    ObservableValue(ObservableValue&&) = default;
    ObservableValue&
    operator=(ObservableValue const&) = delete;
    ObservableValue&
    operator=(ObservableValue&&) = default;

    ObservableValue&
    operator=(std::convertible_to<T> auto&& val)
    {
        set(val);
        return *this;
    }

    [[nodiscard]] ObservableGuard
    operator->()
    {
        return {*this};
    }

    [[nodiscard]]
    operator T const&() const
    {
        return value_;
    }

    [[nodiscard]] T const&
    get() const
    {
        return value_;
    }

    void
    set(std::convertible_to<T> auto&& val)
    {
        if (value_ != val) {
            value_ = std::forward<decltype(val)>(val);
            this->notifyObservers(value_);
        }
    }

    void
    forceNotify() override
    {
        this->notifyObservers(value_);
    }
};

template <Observable T>
class ObservableValue<std::atomic<T>> : public impl::ObservableValueBase<T> {
    std::atomic<T> value_;
 
public:
    ObservableValue(std::convertible_to<T> auto&& value)
        : value_{std::forward<decltype(value)>(value)}
    {
    }

    ObservableValue()
        requires std::default_initializable<T>
        : value_{}
    {
    }
 
    ObservableValue(ObservableValue const&) = delete;
    ObservableValue(ObservableValue&&) = default;
    ObservableValue&
    operator=(ObservableValue const&) = delete;
    ObservableValue&
    operator=(ObservableValue&&) = default;

    ObservableValue&
    operator=(std::convertible_to<T> auto&& val)
    {
        set(std::forward<decltype(val)>(val));
        return *this;
    }

    [[nodiscard]] T
    get() const
    {
        return value_.load();
    }

    [[nodiscard]]
    operator T() const
    {
        return get();
    }

    void
    set(std::convertible_to<T> auto&& val)
    {
        T newValue = std::forward<decltype(val)>(val);
        T oldValue = value_.load();
 
        // Use compare-and-swap to atomically update
        while (!value_.compare_exchange_weak(oldValue, newValue)) {
            // compare_exchange_weak updates oldValue with current value on failure
            // Continue until we succeed
        }
 
        // Notify observers if we actually changed the value
        // Note: oldValue now contains the actual previous value that was replaced
        if (oldValue != newValue) {
            this->notifyObservers(newValue);
        }
    }

    void
    forceNotify() override
    {
        this->notifyObservers(value_.load());
    }
};
 
}  // namespace util