ADD: callback function

src/CMakeLists.txt

@@ -114,6 +114,12 @@ add_executable (vulkan_engine
   # compute
   compute/vk_compute.h
   compute/vk_compute.cpp
+  # runtime
+  runtime/i_game_callbacks.h
+  runtime/time_manager.h
+  runtime/time_manager.cpp
+  runtime/game_runtime.h
+  runtime/game_runtime.cpp
 )
 
 set_property(TARGET vulkan_engine PROPERTY CXX_STANDARD 20)
@@ -270,6 +276,7 @@ target_include_directories(vulkan_engine PUBLIC
   "${CMAKE_CURRENT_SOURCE_DIR}/render/graph"
   "${CMAKE_CURRENT_SOURCE_DIR}/scene"
   "${CMAKE_CURRENT_SOURCE_DIR}/compute"
+  "${CMAKE_CURRENT_SOURCE_DIR}/runtime"
 )
 
 option(ENABLE_MIKKTS "Use MikkTSpace for tangent generation" ON)

src/core/engine.cpp

@@ -594,61 +594,9 @@ void VulkanEngine::init_default_data()
         sphereMesh = _assetManager->createMesh(ci);
     }
 
-    // Register default primitives as dynamic scene instances
-    if (_sceneManager)
-    {
-        _sceneManager->addMeshInstance("default.cube", cubeMesh,
-                                       glm::translate(glm::mat4(1.f), glm::vec3(-2.f, 0.f, -2.f)));
-        _sceneManager->addMeshInstance("default.sphere", sphereMesh,
-                                       glm::translate(glm::mat4(1.f), glm::vec3(2.f, 0.f, -2.f)),
-                                       BoundsType::Sphere);
-    }
-
-    // Test textured primitives
-    {
-        AssetManager::MeshMaterialDesc matDesc;
-        matDesc.kind = AssetManager::MeshMaterialDesc::Kind::Textured;
-        matDesc.options.albedoPath = "textures/grass_albedo.png";
-        matDesc.options.normalPath = "textures/grass_normal.png";
-        matDesc.options.metalRoughPath = "textures/grass_mro.png";
-        matDesc.options.occlusionPath = "textures/grass_ao.png";
-
-        addPrimitiveInstance("textured.cube",
-                             AssetManager::MeshGeometryDesc::Type::Cube,
-                             glm::translate(glm::mat4(1.f), glm::vec3(0.f, 1.f, -4.f)),
-                             matDesc);
-
-        addPrimitiveInstance("textured.sphere",
-                             AssetManager::MeshGeometryDesc::Type::Sphere,
-                             glm::translate(glm::mat4(1.f), glm::vec3(3.f, 1.f, -4.f)),
-                             matDesc);
-
-        addPrimitiveInstance("textured.plane",
-                             AssetManager::MeshGeometryDesc::Type::Plane,
-                             glm::scale(glm::translate(glm::mat4(1.f), glm::vec3(0.f, 0.f, -6.f)), glm::vec3(4.f)),
-                             matDesc);
-    }
-
-    if (addGLTFInstance("mirage", "mirage2000/scene.gltf", glm::mat4(1.0f)))
-    {
-        preloadInstanceTextures("mirage");
-    }
-
-    // Windmill animation test
-    {
-        glm::mat4 windmillTransform = glm::translate(glm::mat4(1.0f), glm::vec3(10.0f, 0.0f, 0.0f));
-        windmillTransform = glm::scale(windmillTransform, glm::vec3(0.5f));
-        if (addGLTFInstance("windmill", "windmill/scene.gltf", windmillTransform))
-        {
-            preloadInstanceTextures("windmill");
-            // Enable first animation (index 0) with looping
-            if (_sceneManager)
-            {
-                _sceneManager->setGLTFInstanceAnimation("windmill", 0, true);
-                _sceneManager->setGLTFInstanceAnimationLoop("windmill", true);
-            }
-        }
-    }
+    // Note: Default primitive meshes (cubeMesh, sphereMesh) are created above
+    // but no longer spawned as scene instances. Use GameRuntime callbacks
+    // or GameAPI to add objects to the scene at runtime.
 
     _mainDeletionQueue.push_function([&]() {
         _resourceManager->destroy_image(_whiteImage);

src/core/engine_ui.cpp

@@ -1840,80 +1840,171 @@ namespace
     }
 } // namespace
 
+// Window visibility states for menu-bar toggles
+namespace
+{
+    struct DebugWindowStates
+    {
+        bool show_overview{false};
+        bool show_window{false};
+        bool show_background{false};
+        bool show_particles{false};
+        bool show_shadows{false};
+        bool show_render_graph{false};
+        bool show_pipelines{false};
+        bool show_ibl{false};
+        bool show_postfx{false};
+        bool show_scene{false};
+        bool show_async_assets{false};
+        bool show_textures{false};
+    };
+    static DebugWindowStates g_debug_windows;
+} // namespace
+
 void vk_engine_draw_debug_ui(VulkanEngine *eng)
 {
     if (!eng) return;
 
     ImGuizmo::BeginFrame();
 
-    // Consolidated debug window with tabs
-    if (ImGui::Begin("Debug"))
+    // Main menu bar at the top
+    if (ImGui::BeginMainMenuBar())
     {
-        const ImGuiTabBarFlags tf =
-                ImGuiTabBarFlags_Reorderable | ImGuiTabBarFlags_AutoSelectNewTabs;
-        if (ImGui::BeginTabBar("DebugTabs", tf))
+        if (ImGui::BeginMenu("View"))
         {
-            if (ImGui::BeginTabItem("Overview"))
-            {
-                ui_overview(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Window"))
-            {
-                ui_window(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Background"))
-            {
-                ui_background(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Particles"))
-            {
-                ui_particles(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Shadows"))
-            {
-                ui_shadows(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Render Graph"))
-            {
-                ui_render_graph(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Pipelines"))
-            {
-                ui_pipelines(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("IBL"))
-            {
-                ui_ibl(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("PostFX"))
-            {
-                ui_postfx(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Scene"))
-            {
-                ui_scene(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Async Assets"))
-            {
-                ui_async_assets(eng);
-                ImGui::EndTabItem();
-            }
-            if (ImGui::BeginTabItem("Textures"))
-            {
-                ui_textures(eng);
-                ImGui::EndTabItem();
-            }
-            ImGui::EndTabBar();
+            ImGui::MenuItem("Overview", nullptr, &g_debug_windows.show_overview);
+            ImGui::MenuItem("Window", nullptr, &g_debug_windows.show_window);
+            ImGui::Separator();
+            ImGui::MenuItem("Scene", nullptr, &g_debug_windows.show_scene);
+            ImGui::MenuItem("Render Graph", nullptr, &g_debug_windows.show_render_graph);
+            ImGui::MenuItem("Pipelines", nullptr, &g_debug_windows.show_pipelines);
+            ImGui::Separator();
+            ImGui::MenuItem("Shadows", nullptr, &g_debug_windows.show_shadows);
+            ImGui::MenuItem("IBL", nullptr, &g_debug_windows.show_ibl);
+            ImGui::MenuItem("PostFX", nullptr, &g_debug_windows.show_postfx);
+            ImGui::MenuItem("Background", nullptr, &g_debug_windows.show_background);
+            ImGui::Separator();
+            ImGui::MenuItem("Particles", nullptr, &g_debug_windows.show_particles);
+            ImGui::MenuItem("Textures", nullptr, &g_debug_windows.show_textures);
+            ImGui::MenuItem("Async Assets", nullptr, &g_debug_windows.show_async_assets);
+            ImGui::EndMenu();
+        }
+
+        // Quick stats in menu bar
+        ImGui::Separator();
+        ImGui::Text("%.1f ms | %d tris | %d draws",
+                    eng->stats.frametime,
+                    eng->stats.triangle_count,
+                    eng->stats.drawcall_count);
+
+        ImGui::EndMainMenuBar();
+    }
+
+    // Individual debug windows (only shown when toggled)
+    if (g_debug_windows.show_overview)
+    {
+        if (ImGui::Begin("Overview", &g_debug_windows.show_overview))
+        {
+            ui_overview(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_window)
+    {
+        if (ImGui::Begin("Window Settings", &g_debug_windows.show_window))
+        {
+            ui_window(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_background)
+    {
+        if (ImGui::Begin("Background", &g_debug_windows.show_background))
+        {
+            ui_background(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_particles)
+    {
+        if (ImGui::Begin("Particles", &g_debug_windows.show_particles))
+        {
+            ui_particles(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_shadows)
+    {
+        if (ImGui::Begin("Shadows", &g_debug_windows.show_shadows))
+        {
+            ui_shadows(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_render_graph)
+    {
+        if (ImGui::Begin("Render Graph", &g_debug_windows.show_render_graph))
+        {
+            ui_render_graph(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_pipelines)
+    {
+        if (ImGui::Begin("Pipelines", &g_debug_windows.show_pipelines))
+        {
+            ui_pipelines(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_ibl)
+    {
+        if (ImGui::Begin("IBL", &g_debug_windows.show_ibl))
+        {
+            ui_ibl(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_postfx)
+    {
+        if (ImGui::Begin("PostFX", &g_debug_windows.show_postfx))
+        {
+            ui_postfx(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_scene)
+    {
+        if (ImGui::Begin("Scene", &g_debug_windows.show_scene))
+        {
+            ui_scene(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_async_assets)
+    {
+        if (ImGui::Begin("Async Assets", &g_debug_windows.show_async_assets))
+        {
+            ui_async_assets(eng);
+        }
+        ImGui::End();
+    }
+
+    if (g_debug_windows.show_textures)
+    {
+        if (ImGui::Begin("Textures", &g_debug_windows.show_textures))
+        {
+            ui_textures(eng);
         }
         ImGui::End();
     }

src/main.cpp

@@ -1,14 +1,86 @@
+// Main entry point for Vulkan Engine
+//
+// Two modes are available:
+// 1. Legacy mode: Uses VulkanEngine::run() directly (simple, no game separation)
+// 2. GameRuntime mode: Uses GameRuntime for clean game/engine separation
+//
+// Set USE_GAME_RUNTIME to 1 to enable GameRuntime with example callbacks.
+
+#define USE_GAME_RUNTIME 1
+
 #include "core/engine.h"
 
+#if USE_GAME_RUNTIME
+#include "runtime/game_runtime.h"
+#include <glm/gtx/transform.hpp>
+
+// Example game implementation using IGameCallbacks
+class ExampleGame : public GameRuntime::IGameCallbacks
+{
+public:
+    void on_init(GameRuntime::Runtime& runtime) override
+    {
+        // Example: Set up initial scene
+        auto& api = runtime.api();
+
+        // Load a glTF model asynchronously
+        // api.load_gltf_async("example_model", "models/example.gltf",
+        //     GameAPI::Transform{}.with_position({0, 0, 0}));
+
+        // Spawn a primitive
+        // api.spawn_mesh_instance("test_cube", api.primitive_cube(),
+        //     GameAPI::Transform{}.with_position({2, 0, 0}));
+
+        // Set up camera
+        // api.set_camera_position({0, 5, -10});
+        // api.set_camera_rotation({-20, 0, 0});
+    }
+
+    void on_update(float dt) override
+    {
+        // Called every frame with variable delta time
+        // Use for rendering-dependent logic, input handling, camera control
+        _elapsed += dt;
+    }
+
+    void on_fixed_update(float fixed_dt) override
+    {
+        // Called at fixed intervals (default: 60Hz)
+        // Use for physics updates, AI tick, game state simulation
+        // Example: Apply physics forces, update AI state machines
+    }
+
+    void on_shutdown() override
+    {
+        // Called before engine shutdown
+        // Use for cleanup, saving game state, etc.
+    }
+
+private:
+    float _elapsed{0.0f};
+};
+#endif // USE_GAME_RUNTIME
+
 int main(int argc, char* argv[])
 {
-	VulkanEngine engine;
+    (void)argc;
+    (void)argv;
 
-	engine.init();	
+    VulkanEngine engine;
+    engine.init();
 
-	engine.run();	
+#if USE_GAME_RUNTIME
+    // GameRuntime mode: clean separation between engine and game logic
+    {
+        GameRuntime::Runtime runtime(&engine);
+        ExampleGame game;
+        runtime.run(&game);
+    }
+#else
+    // Legacy mode: simple direct engine loop
+    engine.run();
+#endif
 
-	engine.cleanup();	
-
-	return 0;
+    engine.cleanup();
+    return 0;
 }

src/runtime/game_runtime.cpp

@@ -0,0 +1,211 @@
+#include "game_runtime.h"
+#include "core/engine.h"
+#include "core/input/input_system.h"
+#include "core/ui/imgui_system.h"
+#include "scene/vk_scene.h"
+
+#include "SDL2/SDL.h"
+
+#include <thread>
+#include <chrono>
+
+namespace GameRuntime
+{
+    Runtime::Runtime(VulkanEngine *renderer)
+        : _renderer(renderer)
+    {
+        _api = std::make_unique<GameAPI::Engine>(renderer);
+    }
+
+    Runtime::~Runtime() = default;
+
+    void Runtime::set_physics_world(IPhysicsWorld *physics)
+    {
+        _physics = physics;
+    }
+
+    void Runtime::set_audio_system(IAudioSystem *audio)
+    {
+        _audio = audio;
+    }
+
+    void Runtime::sync_physics_to_render()
+    {
+        // TODO: When physics integration is added, sync physics body transforms
+        // to their corresponding render instances here.
+        // For each physics body with a render instance:
+        //   glm::mat4 transform;
+        //   _physics->get_body_transform(bodyId, transform);
+        //   _api->set_mesh_instance_transform(instanceName, GameAPI::Transform::from_matrix(transform));
+    }
+
+    void Runtime::update_audio_listener()
+    {
+        if (!_audio || !_renderer || !_renderer->_sceneManager)
+        {
+            return;
+        }
+
+        auto &cam = _renderer->_sceneManager->getMainCamera();
+        glm::vec3 pos = _renderer->_sceneManager->get_camera_local_position();
+
+        // Calculate forward and up from camera orientation quaternion
+        glm::vec3 forward = glm::normalize(cam.orientation * glm::vec3(0.0f, 0.0f, -1.0f));
+        glm::vec3 up = glm::normalize(cam.orientation * glm::vec3(0.0f, 1.0f, 0.0f));
+
+        _audio->set_listener(pos, forward, up);
+    }
+
+    void Runtime::run(IGameCallbacks *game)
+    {
+        if (!game || !_renderer)
+        {
+            return;
+        }
+
+        _quit_requested = false;
+
+        // Call game initialization
+        game->on_init(*this);
+
+        // Main game loop
+        while (!_quit_requested)
+        {
+            // --- Begin frame: time, input ---
+            _time.begin_frame();
+
+            InputSystem *input = _renderer->input();
+            if (input)
+            {
+                input->begin_frame();
+                input->pump_events();
+
+                if (input->quit_requested())
+                {
+                    _quit_requested = true;
+                }
+
+                _renderer->freeze_rendering = input->window_minimized();
+
+                if (input->resize_requested())
+                {
+                    _renderer->resize_requested = true;
+                    input->clear_resize_request();
+                }
+            }
+
+            // --- Process UI and input capture ---
+            const bool ui_capture_mouse = _renderer->ui() && _renderer->ui()->want_capture_mouse();
+            const bool ui_capture_keyboard = _renderer->ui() && _renderer->ui()->want_capture_keyboard();
+
+            // Dispatch native events to UI and picking
+            if (input)
+            {
+                struct DispatchCtx
+                {
+                    VulkanEngine *engine;
+                    bool ui_capture_mouse;
+                } ctx{_renderer, ui_capture_mouse};
+
+                input->for_each_native_event([](void *user, InputSystem::NativeEventView view) {
+                    auto *c = static_cast<DispatchCtx *>(user);
+                    if (!c || !c->engine || view.backend != InputSystem::NativeBackend::SDL2 || view.data == nullptr)
+                    {
+                        return;
+                    }
+                    const SDL_Event &e = *static_cast<const SDL_Event *>(view.data);
+                    if (c->engine->ui())
+                    {
+                        c->engine->ui()->process_event(e);
+                    }
+                    if (c->engine->picking())
+                    {
+                        c->engine->picking()->process_event(e, c->ui_capture_mouse);
+                    }
+                }, &ctx);
+            }
+
+            // --- Camera input (if not captured by UI) ---
+            if (_renderer->_sceneManager && input)
+            {
+                _renderer->_sceneManager->getMainCamera().process_input(*input, ui_capture_keyboard, ui_capture_mouse);
+            }
+
+            // --- Throttle when minimized ---
+            if (_renderer->freeze_rendering)
+            {
+                std::this_thread::sleep_for(std::chrono::milliseconds(100));
+                continue;
+            }
+
+            // --- Handle resize ---
+            if (_renderer->resize_requested)
+            {
+                if (_renderer->_swapchainManager)
+                {
+                    _renderer->_swapchainManager->resize_swapchain(_renderer->_window);
+                    if (_renderer->ui())
+                    {
+                        _renderer->ui()->on_swapchain_recreated();
+                    }
+                    _renderer->resize_requested = false;
+                }
+            }
+
+            // --- Fixed update loop (physics) ---
+            while (_time.consume_fixed_step())
+            {
+                game->on_fixed_update(_time.fixed_delta_time());
+
+                if (_physics)
+                {
+                    _physics->step(_time.fixed_delta_time());
+                }
+            }
+
+            // --- Sync physics transforms to render ---
+            sync_physics_to_render();
+
+            // --- Variable update ---
+            game->on_update(_time.delta_time());
+
+            // --- Audio listener update ---
+            update_audio_listener();
+            if (_audio)
+            {
+                _audio->update();
+            }
+
+            // --- Wait for GPU and prepare frame ---
+            VK_CHECK(vkWaitForFences(_renderer->_deviceManager->device(), 1,
+                &_renderer->get_current_frame()._renderFence, true, 1000000000));
+
+            if (_renderer->_rayManager)
+            {
+                _renderer->_rayManager->flushPendingDeletes();
+                _renderer->_rayManager->pump_blas_builds(1);
+            }
+
+            // --- Flush per-frame resources ---
+            _renderer->get_current_frame()._deletionQueue.flush();
+            if (_renderer->_renderGraph)
+            {
+                _renderer->_renderGraph->resolve_timings();
+            }
+            _renderer->get_current_frame()._frameDescriptors.clear_pools(_renderer->_deviceManager->device());
+
+            // --- ImGui ---
+            if (_renderer->ui())
+            {
+                _renderer->ui()->begin_frame();
+                _renderer->ui()->end_frame();
+            }
+
+            // --- Draw ---
+            _renderer->draw();
+        }
+
+        // Call game shutdown
+        game->on_shutdown();
+    }
+} // namespace GameRuntime

src/runtime/game_runtime.h

@@ -0,0 +1,145 @@
+#pragma once
+
+// GameRuntime: High-level game loop manager
+// Provides a clean separation between engine and game logic with proper
+// time management, fixed timestep for physics, and game callbacks.
+
+#include "i_game_callbacks.h"
+#include "time_manager.h"
+#include "core/game_api.h"
+
+#include <memory>
+
+class VulkanEngine;
+
+namespace GameRuntime
+{
+
+// Forward declarations for future integrations
+class IPhysicsWorld;
+class IAudioSystem;
+
+class Runtime
+{
+public:
+    explicit Runtime(VulkanEngine* renderer);
+    ~Runtime();
+
+    // Non-copyable
+    Runtime(const Runtime&) = delete;
+    Runtime& operator=(const Runtime&) = delete;
+
+    // ------------------------------------------------------------------------
+    // External System Integration (optional)
+    // ------------------------------------------------------------------------
+
+    // Set physics world (e.g., Jolt, Bullet, PhysX wrapper)
+    void set_physics_world(IPhysicsWorld* physics);
+    IPhysicsWorld* physics() const { return _physics; }
+
+    // Set audio system (e.g., FMOD, OpenAL wrapper)
+    void set_audio_system(IAudioSystem* audio);
+    IAudioSystem* audio() const { return _audio; }
+
+    // ------------------------------------------------------------------------
+    // Time Management
+    // ------------------------------------------------------------------------
+
+    // Get time manager for direct access
+    TimeManager& time() { return _time; }
+    const TimeManager& time() const { return _time; }
+
+    // Convenience accessors
+    float delta_time() const { return _time.delta_time(); }
+    float fixed_delta_time() const { return _time.fixed_delta_time(); }
+    float time_scale() const { return _time.time_scale(); }
+    void set_time_scale(float scale) { _time.set_time_scale(scale); }
+    void set_fixed_delta_time(float dt) { _time.set_fixed_delta_time(dt); }
+
+    // ------------------------------------------------------------------------
+    // Game API Access
+    // ------------------------------------------------------------------------
+
+    // Get the high-level game API for engine interaction
+    GameAPI::Engine& api() { return *_api; }
+    const GameAPI::Engine& api() const { return *_api; }
+
+    // Get the underlying Vulkan engine (for advanced use)
+    VulkanEngine* renderer() const { return _renderer; }
+
+    // ------------------------------------------------------------------------
+    // Main Loop
+    // ------------------------------------------------------------------------
+
+    // Run the game loop with the given callback handler.
+    // Blocks until the game exits.
+    void run(IGameCallbacks* game);
+
+    // Request quit (sets quit flag, loop will exit next frame)
+    void request_quit() { _quit_requested = true; }
+
+    // Check if quit was requested
+    bool quit_requested() const { return _quit_requested; }
+
+private:
+    VulkanEngine* _renderer{nullptr};
+    std::unique_ptr<GameAPI::Engine> _api;
+    TimeManager _time;
+
+    IPhysicsWorld* _physics{nullptr};
+    IAudioSystem* _audio{nullptr};
+
+    bool _quit_requested{false};
+
+    // Internal helpers
+    void sync_physics_to_render();
+    void update_audio_listener();
+};
+
+// ============================================================================
+// Physics Interface (for future integration)
+// ============================================================================
+
+class IPhysicsWorld
+{
+public:
+    virtual ~IPhysicsWorld() = default;
+
+    // Step the physics simulation by dt seconds
+    virtual void step(float dt) = 0;
+
+    // Get transform of a physics body by ID
+    virtual void get_body_transform(uint32_t id, glm::mat4& out) = 0;
+
+    // Raycast into the physics world
+    struct RayHit
+    {
+        bool hit{false};
+        glm::vec3 position{0.0f};
+        glm::vec3 normal{0.0f};
+        float distance{0.0f};
+        uint32_t bodyId{0};
+    };
+    virtual RayHit raycast(const glm::vec3& origin, const glm::vec3& direction, float maxDistance) = 0;
+};
+
+// ============================================================================
+// Audio Interface (for future integration)
+// ============================================================================
+
+class IAudioSystem
+{
+public:
+    virtual ~IAudioSystem() = default;
+
+    // Set listener position and orientation
+    virtual void set_listener(const glm::vec3& position, const glm::vec3& forward, const glm::vec3& up) = 0;
+
+    // Play a 3D sound at a position
+    virtual void play_3d(const std::string& event, const glm::vec3& position) = 0;
+
+    // Update audio system (call once per frame)
+    virtual void update() = 0;
+};
+
+} // namespace GameRuntime

src/runtime/i_game_callbacks.h

@@ -0,0 +1,35 @@
+#pragma once
+
+// IGameCallbacks: Interface for game logic callbacks
+// Implement this interface and pass to GameRuntime::run() to receive game loop events.
+
+namespace GameRuntime
+{
+
+class Runtime;
+
+class IGameCallbacks
+{
+public:
+    virtual ~IGameCallbacks() = default;
+
+    // Called once after runtime initialization, before the first update.
+    // Use this to load initial assets, spawn entities, set up the camera, etc.
+    virtual void on_init(Runtime& runtime) = 0;
+
+    // Called every frame with variable delta time.
+    // Use for rendering-dependent logic, input handling, camera control, etc.
+    // @param dt: Frame delta time in seconds (clamped to 0.0-0.1)
+    virtual void on_update(float dt) = 0;
+
+    // Called at fixed intervals for physics/simulation.
+    // Use for physics updates, AI tick, game state simulation, etc.
+    // @param fixed_dt: Fixed delta time in seconds (typically 1/60)
+    virtual void on_fixed_update(float fixed_dt) = 0;
+
+    // Called once before shutdown.
+    // Use for cleanup, saving state, etc.
+    virtual void on_shutdown() = 0;
+};
+
+} // namespace GameRuntime

src/runtime/time_manager.cpp

@@ -0,0 +1,52 @@
+#include "time_manager.h"
+
+#include <algorithm>
+
+namespace GameRuntime
+{
+    TimeManager::TimeManager()
+    {
+        _start_time = Clock::now();
+        _last_time = _start_time;
+    }
+
+    void TimeManager::begin_frame()
+    {
+        TimePoint now = Clock::now();
+        auto elapsed = std::chrono::duration<float>(now - _last_time);
+        _last_time = now;
+
+        // Clamp delta time to avoid spiral of death
+        _unscaled_delta_time = std::min(elapsed.count(), k_max_delta_time);
+        _delta_time = _unscaled_delta_time * _time_scale;
+
+        // Update total times
+        _total_time += _delta_time;
+        _unscaled_total_time += _unscaled_delta_time;
+
+        // Accumulate for fixed timestep
+        _fixed_accumulator += _delta_time;
+
+        ++_frame_count;
+    }
+
+    void TimeManager::set_fixed_delta_time(float dt)
+    {
+        _fixed_delta_time = std::clamp(dt, 1.0f / 240.0f, 1.0f / 10.0f);
+    }
+
+    void TimeManager::set_time_scale(float scale)
+    {
+        _time_scale = std::max(0.0f, scale);
+    }
+
+    bool TimeManager::consume_fixed_step()
+    {
+        if (_fixed_accumulator >= _fixed_delta_time)
+        {
+            _fixed_accumulator -= _fixed_delta_time;
+            return true;
+        }
+        return false;
+    }
+} // namespace GameRuntime

src/runtime/time_manager.h

@@ -0,0 +1,71 @@
+#pragma once
+
+// TimeManager: Manages game time, time scale, and fixed timestep accumulation.
+
+#include <chrono>
+
+namespace GameRuntime
+{
+
+class TimeManager
+{
+public:
+    TimeManager();
+
+    // Begin a new frame. Call at the start of each frame.
+    void begin_frame();
+
+    // Get delta time in seconds since last frame (scaled by time_scale, clamped).
+    float delta_time() const { return _delta_time; }
+
+    // Get unscaled delta time in seconds since last frame (clamped).
+    float unscaled_delta_time() const { return _unscaled_delta_time; }
+
+    // Get fixed delta time for physics updates.
+    float fixed_delta_time() const { return _fixed_delta_time; }
+
+    // Set fixed delta time for physics updates (default: 1/60).
+    void set_fixed_delta_time(float dt);
+
+    // Get time scale multiplier (default: 1.0).
+    float time_scale() const { return _time_scale; }
+
+    // Set time scale multiplier (0 = paused, 0.5 = half speed, 2 = double speed).
+    void set_time_scale(float scale);
+
+    // Get total elapsed time in seconds since start.
+    float total_time() const { return _total_time; }
+
+    // Get total unscaled elapsed time in seconds since start.
+    float unscaled_total_time() const { return _unscaled_total_time; }
+
+    // Get accumulated fixed time (for physics step loop).
+    float fixed_accumulator() const { return _fixed_accumulator; }
+
+    // Consume fixed timestep from accumulator. Returns true if step should run.
+    bool consume_fixed_step();
+
+    // Get frame count since start.
+    uint64_t frame_count() const { return _frame_count; }
+
+private:
+    using Clock = std::chrono::high_resolution_clock;
+    using TimePoint = std::chrono::time_point<Clock>;
+
+    TimePoint _last_time;
+    TimePoint _start_time;
+
+    float _delta_time{0.0f};
+    float _unscaled_delta_time{0.0f};
+    float _fixed_delta_time{1.0f / 60.0f};
+    float _time_scale{1.0f};
+    float _total_time{0.0f};
+    float _unscaled_total_time{0.0f};
+    float _fixed_accumulator{0.0f};
+
+    uint64_t _frame_count{0};
+
+    static constexpr float k_max_delta_time = 0.1f; // 100ms cap
+};
+
+} // namespace GameRuntime