watcher.hpp

/*
 * live_cells_cpp
 * Copyright (C) 2024  Alexander Gutev <alex.gutev@gmail.com>
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you
 * may not use this file except in compliance with the License.  You
 * may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
 * implied.  See the License for the specific language governing
 * permissions and limitations under the License.
 */
 
#ifndef LIVE_CELLS_WATCHER_HPP
#define LIVE_CELLS_WATCHER_HPP
 
#include <utility>
#include <memory>
#include <unordered_set>
 
#include "keys.hpp"
#include "observable.hpp"
#include "tracker.hpp"
 
namespace live_cells {
 
    class watcher : public std::enable_shared_from_this<watcher> {
    public:
 
        template <typename F>
        watcher(F callback) :
            observer(
                std::static_pointer_cast<base_observer>(
                    std::make_shared<watch_observer<F>>(callback)
                )
            ) {
            observer->init();
        }
 
        watcher(const watcher &) = delete;
 
        ~watcher() {
            stop();
        }
 
        watcher &operator=(const watcher &) = delete;
 
        void stop() {
            observer->stop();
        }
 
    private:
 
        struct base_observer : public observer, public std::enable_shared_from_this<base_observer> {
 
            std::unordered_set<cell> arguments;
 
            virtual ~base_observer() = default;
 
            virtual void init() = 0;
 
            void stop() {
                for (auto arg : arguments) {
                    arg.remove_observer(
                        std::static_pointer_cast<observer>(this->shared_from_this())
                    );
                }
 
                arguments.clear();
            }
        };
 
        template <typename F>
        struct watch_observer : public base_observer {
            const F callback;
 
            watch_observer(F callback) : callback(callback) {}
 
            watch_observer(const watch_observer &) = delete;
 
            void init() override {
                call_with_tracker();
            }
 
            bool is_updating = false;
 
            bool waiting_for_change = false;
 
 
            /* observer methods */
 
            void will_update(const key_ref &k) override {
                if (!is_updating) {
                    is_updating = true;
                    waiting_for_change = false;
                }
            }
 
            void update(const key_ref &k, bool changed) override {
                if (is_updating || (changed && waiting_for_change)) {
                    is_updating = false;
                    waiting_for_change = !changed;
 
                    if (changed) {
                        call_with_tracker();
                    }
                }
            }
 
            void call_with_tracker() {
                auto t = argument_tracker::global().with_tracker([this] (auto cell) {
                    if (!this->arguments.count(cell)) {
                        this->arguments.emplace(cell);
                        cell.add_observer(
                            std::static_pointer_cast<observer>(this->shared_from_this())
                        );
                    }
                });
 
                try {
                    callback();
                }
                catch (...) {
                    // TODO: Log exception to stderr in debug mode
                    // Prevent exception from being propagated to caller
                }
            }
        };
 
        std::shared_ptr<base_observer> observer;
    };
 
    template <typename F>
    std::shared_ptr<watcher> watch(F fn) {
        auto w = std::make_shared<watcher>(fn);
 
        return w;
    }
 
}  // live_cells
 
#endif /* LIVE_CELLS_WATCHER_HPP */