resourceLoader.h

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// SPDX-License-Identifier: Apache-2.0
// Copyright 2025-2026 Arnis Lektauers
//
// Created by Arnis Lektauers on 12.10.2025.
//
#pragma once
 
#include <atomic>
#include <condition_variable>
#include <deque>
#include <functional>
#include <memory>
#include <mutex>
#include <optional>
#include <string>
#include <thread>
#include <unordered_map>
#include <vector>
 
namespace visutwin::canvas
{
    class Engine;
 
    // ── Loaded data produced by background thread ──────────────────────────

    struct LoadedData
    {
        std::string url;

        std::vector<uint8_t> bytes;

        struct PixelData
        {
            std::vector<uint8_t>  pixels;      
            std::vector<float>    hdrPixels;   
            int  width    = 0;
            int  height   = 0;
            int  channels = 0;
            bool isHdr    = false;
        };
 
        std::optional<PixelData> pixelData;

        std::shared_ptr<void> preparsed;

        std::shared_ptr<void> preparedData;
    };
 
    // ── Callback types ─────────────────────────────────────────────────────
 
    using LoadSuccessCallback = std::function<void(std::unique_ptr<LoadedData>)>;
    using LoadErrorCallback   = std::function<void(const std::string& error)>;
 
    // ── Resource handler base ──────────────────────────────────────────────

    class ResourceHandler
    {
    public:
        virtual ~ResourceHandler() = default;

        virtual std::unique_ptr<LoadedData> load(const std::string& url) = 0;
    };
 
    // ── Async resource loader ──────────────────────────────────────────────

    class ResourceLoader
    {
    public:
        explicit ResourceLoader(const std::shared_ptr<Engine>& engine);
        ~ResourceLoader();
 
        // Non-copyable, non-movable (owns a thread).
        ResourceLoader(const ResourceLoader&) = delete;
        ResourceLoader& operator=(const ResourceLoader&) = delete;

        void addHandler(const std::string& type, std::unique_ptr<ResourceHandler> handler);

        void removeHandler(const std::string& type);

        void load(const std::string& url, const std::string& type,
                  LoadSuccessCallback onSuccess, LoadErrorCallback onError = nullptr);

        void processCompletions(int maxCompletions = 0);

        void shutdown();

        bool hasPending() const;
 
    private:
        void workerLoop();
 
        struct Request
        {
            std::string url;
            std::string type;
            LoadSuccessCallback onSuccess;
            LoadErrorCallback   onError;
        };
 
        struct Completion
        {
            std::unique_ptr<LoadedData> data;
            std::string                 error;
            LoadSuccessCallback         onSuccess;
            LoadErrorCallback           onError;
        };
 
        std::weak_ptr<Engine> _engine;
 
        // Handler registry — written from the main thread only, read from worker.
        std::unordered_map<std::string, std::unique_ptr<ResourceHandler>> _handlers;
 
        // Worker thread.
        std::thread      _worker;
        std::atomic<bool> _running{false};
 
        // Request queue  (main thread → worker).
        std::mutex              _requestMutex;
        std::condition_variable _requestCV;
        std::deque<Request>     _requests;
 
        // Completion queue  (worker → main thread).
        std::mutex          _completionMutex;
        std::deque<Completion> _completions;
 
        std::atomic<int> _pendingCount{0};
    };
 
    // ── Built-in resource handlers ─────────────────────────────────────────

    class TextureResourceHandler : public ResourceHandler
    {
    public:
        std::unique_ptr<LoadedData> load(const std::string& url) override;
    };

    class ContainerResourceHandler : public ResourceHandler
    {
    public:
        std::unique_ptr<LoadedData> load(const std::string& url) override;
    };

    class FontResourceHandler : public ResourceHandler
    {
    public:
        std::unique_ptr<LoadedData> load(const std::string& url) override;
    };
}