前言

当前版本：4.26.2
本文原创
说明：有些名词保持英文可能更容易理解
目录
- 游戏流程总览
- 各个平台启动引擎的入口函数
- 引擎入口函数 GuardedMain()
- EnginePreInit 引擎预初始化
- EngineInit 引擎初始化
- EngineLoop 引擎主循环
- EngineExit 引擎退出

游戏流程总览

解释启用引擎和运行游戏的过程。
此处说明两种主要路径：编辑器路径、standalone 路径。
事件的一般顺序为初始化引擎、创建并初始化 GameInstance、加载关卡，最后开始游戏。然而在部分被调用函数的准确排序和函数被调用的位置方面，standalone 模式和编辑器模式之间存在差异。以下流程图展示两个平行运行的路径，它们在游戏开始之前汇聚。

各个平台启动引擎的入口函数

代码路径：Engine\Source\Runtime\Launch\Private\
作为入口函数，然后进入引擎的入口函数GuardedMain。如下图所示

引擎入口函数 GuardedMain()

代码路径：Runtime/Launch/Private/Launch.cpp
执行引擎四大阶段的流程：EnginePreInit 、EngineInit 、EngineLoop、EngineExit 。执行流程图如下所示

EnginePreInit 引擎预初始化

实际上调用 FEngineLoop::PreInit ，再调用 PreInitPreStartupScreen 和 PreInitPostStartupScreen ，从而实现许多初始化和核心模块相关的事情

PreInitPreStartupScreen 里的执行流程

先进行各种初始化，如解析command参数、trace、stat（性能分析工具）、FModuleManager、IFileManager、FPlatformFileManager、各种路径等
加载CoreUObject模块，启动FTaskGraphInterface
通过LoadPreInitModules()
- 加载 Engine、Renderer、AnimGraphRuntime 模块
- 通过 FPlatformApplicationMisc::LoadPreInitModules()加载 D3D12RHI、D3D11RHI、OpenGLDrv等模块（根据所在平台）
- 加载 SlateRHIRenderer模块（非服务器下）
- 加载 Landscape、RenderCore 模块
- 加载 TextureCompressor、AudioEditor、AnimationModifiers 模块
通过 FEngineLoop::AppInit() 进行
- 本地化文本
- Init logging to disk
- init config system
  - 通过 FConfigCacheIni::LoadGlobalIniFile 加载Engine、Game、Input 配置文件
  - 其他配置文件还有：Editor、EditorPerProjectUserSettings、EditorSettings、EditorKeyBindings、EditorLayout、Lightmass、Scalability、Hardware、DeviceProfiles、GameUserSettings等
- Load "asap" plugin modules，为项目和插件加载类型为 EarliestPossible 的模块
- 检查项目和插件模块数据，是否需要编译
- 为项目和插件加载类型为 PostConfigInit 的模块
- 初始化头显 PreInitHMDDevice();
- 打印初始化日志
- 初始化其他系统 FCoreDelegates::OnInit.Broadcast();
设置游戏主线程
线程池初始化（如果支持多线程）
异步IO系统初始化（如果支持多线程）
系统设置system settings和 cvar 初始化，cvar从 ini 文件获取，含渲染参数、分辨率、窗口模式等
Scalability系统初始化 InitScalabilitySystem()
渲染线程 UseRenderThread()
平台特定的初始化，如 FPlatformMisc::PlatformInit() 、FPlatformApplicationMisc::Init() 、FPlatformMemory::Init()
IPlatformFeaturesModule::Get()
物理引擎初始化 InitGamePhys()
引擎本地化初始化 InitEngineTextLocalization()
开启音频线程 UseAudioThread()
启动界面显示 FPlatformSplash::Show();
平台应用程序创建 FSlateApplication::Create()；服务器模式下EKeys::Initialize()、FCoreStyle::ResetToDefault();
RHI 初始化 RHIInit()
RenderUtils初始化 RenderUtilsInit()
FShaderCodeLibrary::InitForRuntime()
FShaderPipelineCache::Initialize()
Shader hash cache: InitializeShaderHashCache()
GetRendererModule
InitializeShaderTypes
CompileGlobalShaderMap
CreateMoviePlayer
etc

PreInitPostStartupScreen 里的执行流程

播放启动动画 GetMoviePlayer()->SetupLoadingScreenFromIni()
加载PreEarlyLoadingScreen类型的项目模块
- 通过 FPreLoadScreenManager::Get()->Initialize() 调用 UGameEngine::CreateGameWindow(); 创建窗口
Pak 文件获取和挂载
Config 文件获取和参数读取
SlateRenderer 初始化
加载 AssetRegistry 模块
UObject classes 注册和初始化
PIEPreviewDeviceProfileSelector
InitDefaultMaterials
Initialize the texture streaming system
加载核心启动模块 LoadStartupCoreModules，包括
- Core、Networking、Messaging
- MRMesh、UnrealEd、EditorStyle、LandscapeEditorUtilities
- SlateCore、Slate、SlateReflector、UMG
- MessageLog、CollisionAnalyzer、FunctionalTesting
- BehaviorTreeEditor、GameplayTasksEditor、StringTableEditor、VREditor、IntroTutorials、Blutility
- Overlay、MediaAssets、ClothingSystemRuntimeNv、ClothingSystemEditor、PacketHandler、NetworkReplayStreaming
通过LoadStartupModules 加载PreDefault、Default、PostDefault类型的模块
加载 PreLoadingScreen 类型的模块
为项目和插件加载 PostEngineInit 类型的模块
Online模块
etc

EngineInit 引擎初始化

执行GEngineLoop.Init() 。如果事编辑器会执行 EditorInit() ，本质上事执行 GEngineLoop.Init() 后再进行Editor 的一些资源加载

GEngineLoop.Init()

分析主要以Game非编辑器模式为主

非编辑器模式：GEngine = NewObject<UEngine>(GetTransientPackage(), EngineClass);
编辑器模式：GEngine = GEditor = GUnrealEd = NewObject<UUnrealEdEngine>(GetTransientPackage(), EngineClass);
GEngine->Init()
- UGameEngine::Init() 非编辑器模式
  - UEngine::Init()
    - Subsystems 初始化 EngineSubsystemCollection.Initialize(this);
    - 头显设备初始化 InitializeHMDDevice();
    - 眼动追踪设备初始化 InitializeEyeTrackingDevice();
    - FSlateSoundDevice
    - 加载引擎类 LoadObject<UClass>(UEngine::StaticClass()->GetOuter()...
    - Engine.ini LoadConfig();
    - 初始化 Object 引用 InitializeObjectReferences();
    - 为编辑器 CreateNewWorldContext()
    - InitializeAudioDeviceManager();
    - networking 相关的一些东西
    - Initialise buffer visualization system data GetBufferVisualizationData().Initialize();
    - FEngineAnalytics::Initialize();
    - 加载引擎 runtime modules：ImageWriteQueue、StreamingPauseRendering、MovieScene、MovieSceneTracks、LevelSequence
    - Finish asset manager loading AssetManager->FinishInitialLoading();
    - RecordHMDAnalytics();
  - GetGameUserSettings()->LoadSettings();
    GetGameUserSettings()->ApplyNonResolutionSettings();
  - 创建 game instance
    GameInstance = NewObject<UGameInstance>(this, GameInstanceClass);
    GameInstance->InitializeStandalone(); —— CreateNewWorldContext
  - GameInstance->Init();
    - OnlineSession = NewObject<UOnlineSession>(this, SpawnClass);
      OnlineSession->RegisterOnlineDelegates();
    - SubsystemCollection.Initialize(this);
  - 初始化 viewport client
加载 PostEngineInit 类型的模块
GEngine->Start()
- UGameEngine::Start() 非编辑器模式
  - GameInstance->StartGameInstance();
  - const UGameMapsSettings* GameMapsSettings = GetDefault<UGameMapsSettings>();
  - const FString& DefaultMap = GameMapsSettings->GetGameDefaultMap();
  - Engine->Browse() 服务器相关
  - UEngine->LoadMap
    - 做一些清理工作，Unload 当前world、显示loading screen、清理 networking等
    - LoadPackage() ,加载level
    - 设置new world、CurrentWorld
      - WorldContext.SetCurrentWorld(NewWorld);
      - WorldContext.World()->WorldType = WorldContext.WorldType;
    - WorldContext.World()->InitWorld();
      - InitializeSubsystems()
      - FWorldDelegates::OnPreWorldInitialization.Broadcast(this, IVS);
      - AWorldSettings* WorldSettings = GetWorldSettings();
      - CreatePhysicsScene(WorldSettings);
      - CreateAISystem();
      - SetupParameterCollectionInstances();
      - Levels.Add( PersistentLevel );
      - 场景物理特性的一些设置，如重力、碰撞
    - 一些网络相关如 NetDriver、listen 等
    - WorldContext.World()->SetGameMode();
    - GShaderCompilingManager->ProcessAsyncResults()
    - WorldContext.World()->CreateAISystem();
    - WorldContext.World()->InitializeActorsForPlay();
    - FNavigationSystem::AddNavigationSystemToWorld()
    - SpawnPlayActor()
    - WorldContext.World()->BeginPlay();
      - GameMode->StartPlay();
        GameState->HandleBeginPlay();
        GetWorldSettings()->NotifyBeginPlay();
        为每个Actor调用BeginPlay()， FActorIterator It(World); It->DispatchBeginPlay(bFromLevelLoad);
        GetWorldSettings()->NotifyMatchStarted();
      - GetAISystem()->StartPlay();
      - PhysicsScene->OnWorldBeginPlay();
    - WorldContext.World()->bWorldWasLoadedThisTick = true;
    - WorldContext.OwningGameInstance->LoadComplete()；
WaitForEngineLoadingScreenToFinish()
WaitForMovieToFinish()
加载模块： Media、AutomationWorker、AutomationController、ProfilerClient 、SequenceRecorder、SequenceRecorderSections
线程心跳包 FThreadHeartBeat::Get().Start();
外部分析器
etc

EditorInit()

调用GEngineLoop.Init() 后执行

一些命令行、通知、消息、日志、ui等注册
加载关卡 startup map
Process global shader results before we try to render anything
InitEngineAnalytics
etc

EngineLoop 引擎主循环

执行 FEngineLoop::Tick()

TickRenderingTickables();
Make sure something is ticking the rendering tickables in -onethread mode to avoid leaks/bugs.
ActiveProfiler->FrameSync();
外部分析器帧同步
FCoreDelegates::OnBeginFrame.Broadcast();
GLog->FlushThreadedLogs();
刷新线程日志
FlushRenderingCommands()
渲染命令更新
Scene->UpdateAllPrimitiveSceneInfos()
BeginFrameRenderThread()
beginning of RHI frame
Scene->StartFrame();
FlushPendingDeleteRHIResources_RenderThread();
渲染线程里一些需要帧处理的任务
一些 stats 相关的事情
GEngine->Tick()
main game engine tick (world, game objects, etc.)
- log 更新
- CleanupGameViewport();
- 更新 subsystems
- FEngineAnalytics、FStudioAnalytics 的 tick() 更新
- Context.World()->Tick()
- USkyLightComponent::UpdateSkyCaptureContents(Context.World());
  UReflectionCaptureComponent::UpdateReflectionCaptureContents(Context.World());
- ULocalPlayer 处理
- LevelStreaming 处理
- FTickableGameObject::TickObjects()
- MediaModule->TickPostEngine();
- GameViewport->Tick(DeltaSeconds);
- RedrawViewports();
  Render everything.
- IStreamingManager、GameAudioDeviceManager 更新
- 渲染线程命令相关更新 GRenderingRealtimeClock、GRenderTargetPool、FRDGBuilder、ICustomResourcePool
GShaderCompilingManager->ProcessAsyncResults(true, false);
GDistanceFieldAsyncQueue->ProcessAsyncTasks();
MediaModule->TickPreSlate();
tick media framework
slate 的 tick 和 task
ReplicatedProperties 属性同步
FTaskGraphInterface 一些并行任务的处理
RHITick()
帧计数、帧间隔
Objects 下一帧回收比标记
FrameEndSync 帧结束同步事件
EndFrameRenderThread()
FCoreDelegates::OnEndFrame.Broadcast();

EngineExit 引擎退出

执行 GEngineLoop.Exit() ，主要是关闭和释放引擎的各个模块，具体可看源码
最后调用 FEngineLoop::AppExit(); 退出应用

源码


void FEngineLoop::Exit()
{STAT_ADD_CUSTOMMESSAGE_NAME( STAT_NamedMarker, TEXT( "EngineLoop.Exit" ) );TRACE_BOOKMARK(TEXT("EngineLoop.Exit"));GIsRunning	= 0;GLogConsole	= nullptr;IInstallBundleManager::InstallBundleCompleteDelegate.RemoveAll(this);// shutdown visual logger and flush all data
#if ENABLE_VISUAL_LOGFVisualLogger::Get().Shutdown();
#endif// Make sure we're not in the middle of loading something.{bool bFlushOnExit = true;if (GConfig){FBoolConfigValueHelper FlushStreamingOnExitHelper(TEXT("/Script/Engine.StreamingSettings"), TEXT("s.FlushStreamingOnExit"), GEngineIni);bFlushOnExit = FlushStreamingOnExitHelper;			}if (bFlushOnExit){FlushAsyncLoading();}else{CancelAsyncLoading();}}// Block till all outstanding resource streaming requests are fulfilled.if (!IStreamingManager::HasShutdown()){UTexture2D::CancelPendingTextureStreaming();IStreamingManager::Get().BlockTillAllRequestsFinished();}#if WITH_ENGINE// shut down messagingdelete EngineService;EngineService = nullptr;if (SessionService.IsValid()){SessionService->Stop();SessionService.Reset();}if (GDistanceFieldAsyncQueue){GDistanceFieldAsyncQueue->Shutdown();delete GDistanceFieldAsyncQueue;}
#endif // WITH_ENGINEif ( GEngine != nullptr ){GEngine->ReleaseAudioDeviceManager();}if ( GEngine != nullptr ){GEngine->PreExit();}FAudioDeviceManager::Shutdown();// close all windowsFSlateApplication::Shutdown();#if !UE_SERVERif ( FEngineFontServices::IsInitialized() ){FEngineFontServices::Destroy();}
#endif#if WITH_EDITOR// These module must be shut down first because other modules may try to access them during shutdown.// Accessing these modules at shutdown causes instability since the object system will have been shut down and these modules uses uobjects internally.FModuleManager::Get().UnloadModule("AssetTools", true);#endif // WITH_EDITORFModuleManager::Get().UnloadModule("WorldBrowser", true);FModuleManager::Get().UnloadModule("AssetRegistry", true);#if !PLATFORM_ANDROID || PLATFORM_LUMIN 	// AppPreExit doesn't work on AndroidAppPreExit();TermGamePhys();ParticleVertexFactoryPool_FreePool();
#else// AppPreExit() stops malloc profiler, do it here insteadMALLOC_PROFILER( GMalloc->Exec(nullptr, TEXT("MPROF STOP"), *GLog);	);
#endif // !ANDROID// Stop the rendering thread.StopRenderingThread();// Disable the PSO cacheFShaderPipelineCache::Shutdown();// Close shader code map, if anyFShaderCodeLibrary::Shutdown();#if !PLATFORM_ANDROID || PLATFORM_LUMIN // UnloadModules doesn't work on Android
#if WITH_ENGINE// Save the hot reload stateIHotReloadInterface* HotReload = IHotReloadInterface::GetPtr();if(HotReload != nullptr){HotReload->SaveConfig();}
#endif// Unload all modules.  Note that this doesn't actually unload the module DLLs (that happens at// process exit by the OS), but it does call ShutdownModule() on all loaded modules in the reverse// order they were loaded in, so that systems can unregister and perform general clean up.FModuleManager::Get().UnloadModulesAtShutdown();
#endif // !ANDROIDIStreamingManager::Shutdown();// Tear down the RHI.RHIExitAndStopRHIThread();DestroyMoviePlayer();// Move earlier?
#if STATSFThreadStats::StopThread();
#endifFTaskGraphInterface::Shutdown();FPlatformMisc::ShutdownTaggedStorage();TRACE_CPUPROFILER_SHUTDOWN();
}