QXmppTask.h

// SPDX-FileCopyrightText: 2022 Linus Jahn <lnj@kaidan.im>
// SPDX-FileCopyrightText: 2022 Jonah Brüchert <jbb@kaidan.im>
//
// SPDX-License-Identifier: LGPL-2.1-or-later
 
#ifndef QXMPPTASK_H
#define QXMPPTASK_H
 
#include "qxmpp_export.h"
 
#include <functional>
#include <memory>
#include <optional>
 
#include <QFuture>
#include <QPointer>
 
template<typename T>
class QXmppPromise;
 
namespace QXmpp::Private {
 
template<typename T>
struct TaskData {
    std::function<void(TaskData &)> continuation;
    std::conditional_t<std::is_void_v<T>, std::monostate, std::optional<T>> result;
    bool finished = false;
};
 
}  // namespace QXmpp::Private
 
template<typename T>
class QXmppTask
{
public:
    ~QXmppTask() = default;
 
#ifndef QXMPP_DOC
    template<typename Continuation>
#endif
    void then(const QObject *context, Continuation continuation)
    {
        using namespace QXmpp::Private;
        if constexpr (std::is_void_v<T>) {
            static_assert(std::is_invocable_v<Continuation>, "Function needs to be invocable without arguments.");
        } else {
            static_assert(std::is_invocable_v<Continuation, T &&>, "Function needs to be invocable with T &&.");
        }
 
        if (d->finished) {
            if constexpr (std::is_void_v<T>) {
                continuation();
            } else {
                if (hasResult()) {
                    auto value = std::move(*d->result);
                    d->result.reset();
                    continuation(std::move(value));
                }
            }
        } else {
            d->continuation = [context = QPointer(context),
                               continuation = std::forward<Continuation>(continuation)](TaskData<T> &d) mutable {
                if (context) {
                    if constexpr (std::is_void_v<T>) {
                        continuation();
                    } else {
                        // move out value (only one continuation allowed)
                        auto value = std::move(*d.result);
                        d.result.reset();
                        continuation(std::move(value));
                    }
                }
            };
        }
    }
 
    [[nodiscard]]
    bool isFinished() const
    {
        return d->finished;
    }
 
#ifndef QXMPP_DOC
    template<typename U = T, std::enable_if_t<(!std::is_void_v<U>)> * = nullptr>
#endif
    [[nodiscard]]
    bool hasResult() const
    {
        return d->result.has_value();
    }
 
#ifdef QXMPP_DOC
    [[nodiscard]]
    const T &result() const
#else
    template<typename U = T, std::enable_if_t<(!std::is_void_v<U>)> * = nullptr>
    [[nodiscard]]
    const U &result() const
#endif
    {
        Q_ASSERT(isFinished());
        Q_ASSERT(hasResult());
        return d->result.value();
    }
 
#ifdef QXMPP_DOC
    [[nodiscard]]
    T takeResult()
#else
    template<typename U = T, std::enable_if_t<(!std::is_void_v<U>)> * = nullptr>
    [[nodiscard]]
    U takeResult()
#endif
    {
        Q_ASSERT(isFinished());
        Q_ASSERT(hasResult());
        auto value = std::move(*d->result);
        d->result.reset();
        return std::move(value);
    }
 
    QFuture<T> toFuture(const QObject *context)
    {
        QFutureInterface<T> interface;
 
        if constexpr (std::is_same_v<T, void>) {
            then(context, [interface]() mutable {
                interface.reportFinished();
            });
        } else {
            then(context, [interface](T &&val) mutable {
                interface.reportResult(val);
                interface.reportFinished();
            });
        }
 
        return interface.future();
    }
 
private:
    friend class QXmppPromise<T>;
 
    explicit QXmppTask(std::shared_ptr<QXmpp::Private::TaskData<T>> data)
        : d(std::move(data))
    {
    }
 
    std::shared_ptr<QXmpp::Private::TaskData<T>> d;
};
 
#endif  // QXMPPTASK_H