libcruft-util/signal.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2011-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "types/traits.hpp"
#include "debug/assert.hpp"

#include <functional>


namespace cruft {
    /// A signal object whose clients can listen for invocations.
    ///
    /// The arguments supplied to the invocation of the parent are supplied
    /// to the callback of each child.
    ///
    /// Every client must store the cookie from `connect`. When this goes out
    /// of scope it will disconnect the callback from the signal object.
    /// It is permissible to release a cookie while it is being invoked, but
    /// no other cookie for the duration.
    ///
    /// All cookies _should_ be destroyed or released before the signal is
    /// destroyed.
    ///
    /// \tparam FunctionT   The type of the callback
    template <
        typename FunctionT
    >
    class signal {
        public:
            using function_type = FunctionT;

            ///////////////////////////////////////////////////////////////////////
            /// A single node in a doubly linked list of callbacks to invoke when
            /// a signal is invoked.
            ///
            /// They perform no direct processing by themselves. Instead they are
            /// entirely focused on managing lifetimes of, and pointers to, the
            /// callbacks that will be invoked.
            struct cookie {
                cookie (FunctionT &&_callback)
                    : callback (std::move (_callback))
                    , next (nullptr)
                    , prev (nullptr)
                { ; }

                cookie (cookie &&rhs) noexcept
                    : cookie (std::move (rhs.callback))
                {
                    replace (rhs);
                }

                cookie& operator= (cookie &&rhs) noexcept
                {
                    replace (rhs);
                    callback = std::move (rhs.callback);
                    return *this;
                }

                cookie (cookie const&) = delete;
                cookie& operator= (cookie const&) = delete;

                ~cookie ()
                {
                    release ();
                }

                /// Take the position of `rhs` in the linked list.
                void replace (cookie &rhs)
                {
                    // It could be more efficient to special case these
                    // operations but this shouldn't be an operation that
                    // occurs in a hot loop anyway.
                    release ();
                    rhs.append (*this);
                    rhs.release ();
                }

                /// Link the provided cookie into the linked list as our
                /// successor.
                ///
                /// The provided cookie must not currently be part of any
                /// linked list.
                void append (cookie &rhs)
                {
                    CHECK (rhs.next == nullptr);
                    CHECK (rhs.prev == nullptr);

                    if (next) {
                        CHECK_EQ (next->prev, this);
                        next->prev = &rhs;
                    }

                    rhs.next = next;
                    rhs.prev = this;

                    next = &rhs;
                }

                /// If this cookie is part of a linked list then remove it
                /// from the linked list. Otherwise this is a noop.
                void release (void)
                {
                    if (next) next->prev = prev;
                    if (prev) prev->next = next;

                    next = nullptr;
                    prev = nullptr;
                }

                FunctionT callback;

                /// The next node in the linked list. Or nullptr if this is the end.
                cookie *next;
                /// The previous node in the linked list. Or nullptr if this is the start.
                cookie *prev;
            };

        public:
            signal ()
                : m_head (FunctionT{})
            { ; }

            signal (signal const&) = delete;
            signal& operator= (signal const&) = delete;

            signal (signal &&rhs) noexcept = default;
            signal& operator= (signal &&rhs) noexcept = default;

            ~signal ()
            {
                CHECK (empty ());
            }

            /// Add a callback to list.
            cookie
            connect [[nodiscard]] (FunctionT &&_callback)
            {
                cookie res (std::move (_callback));
                m_head.append (res);
                return res;
            }


            /// Disconnect all callbacks
            void clear (void);


            /// Returns the number of callbacks connected.
            std::size_t size (void) const
            {
                std::size_t accum = 0;
                for (auto cursor = m_head.next; cursor; cursor = cursor->next)
                    ++accum;
                return accum;
            }


            bool empty (void) const
            {
                return m_head.next == nullptr;
            }


            /// Execute all callbacks
            template <typename ...ArgsT>
            decltype(auto)
            operator() (ArgsT&&... tail)
            {
                // We need to cache the cursor and advance _before_ we invoke
                // the cookie so that we have saved a pointer to the next
                // child in case it gets released on us during the call.
                cookie *cursor = m_head.next;

                while (cursor) {
                    auto now = cursor;
                    cursor = cursor->next;
                    std::invoke (now->callback, tail...);
                }
            }


        private:
            /// The first node in the linked list of callbacks. We
            /// unconditionally store this node to simplify anchoring the
            /// linked list to the signal object and removing nodes via their
            /// destructors.
            cookie m_head;
    };


    //-------------------------------------------------------------------------
    // Function references are _actually_ used as if they are std::function
    // objects in our libraries.
    //
    // TODO: avoid this implicit conversion without need a massive rewrite.
    template <typename ReturnT, typename ...ArgsT>
    class signal<ReturnT(ArgsT...)> :
        public signal<std::function<ReturnT(ArgsT...)>>
    { };


    ///////////////////////////////////////////////////////////////////////////
    // wrap a value in a signal and trigger on assignment
    //template <typename T, template <typename> class ReductionT>
    template <typename ValueT, typename FunctionT = std::function<void(ValueT)>>
    class value_signal : public signal<FunctionT> {
    public:
        explicit value_signal (ValueT _value): m_value (_value) { ; }
        value_signal () = default;

        operator const ValueT&() const { return m_value; }

        value_signal<ValueT>&
        operator= (const ValueT &t)
        {
            m_value = t;
            (*this) (m_value);
            return *this;
        }

    private:
        ValueT m_value;
    };
}