UE5的引擎初始化流程

首先跟着UE的官方文档[1]获取到UE的源代码，然后在参考GitHub上repo的readme，将UE引擎从源码build出来。以Windows平台为例，先找到引擎的入口函数：

int32 WINAPI WinMain(_In_ HINSTANCE hInInstance, _In_opt_ HINSTANCE hPrevInstance, _In_ char* pCmdLine, _In_ int32 nCmdShow)
{
	int32 Result = LaunchWindowsStartup(hInInstance, hPrevInstance, pCmdLine, nCmdShow, nullptr);
	LaunchWindowsShutdown();
	return Result;
}

可以看到入口函数很简单，就分为startup和shutdown两部分，那么想必引擎的主循环是在这个startup里。startup函数主要负责初始化各种调用环境，解析命令行参数，调用真正的main函数：

LAUNCH_API int32 LaunchWindowsStartup( HINSTANCE hInInstance, HINSTANCE hPrevInstance, char*, int32 nCmdShow, const TCHAR* CmdLine )
{
	// Setup common Windows settings
	SetupWindowsEnvironment();

	int32 ErrorLevel			= 0;
	hInstance				= hInInstance;

	if (!CmdLine)
	{
		CmdLine = ::GetCommandLineW();

		// Attempt to process the command-line arguments using the standard Windows implementation
		// (This ensures behavior parity with other platforms where argc and argv are used.)
		if ( ProcessCommandLine() )
		{
			CmdLine = *GSavedCommandLine;
		}
	}

    ErrorLevel = GuardedMain( CmdLine );

	return ErrorLevel;
}

简化后的main函数如下：

int32 GuardedMain( const TCHAR* CmdLine )
{
    // Super early init code. DO NOT MOVE THIS ANYWHERE ELSE!
	FCoreDelegates::GetPreMainInitDelegate().Broadcast();

	// make sure GEngineLoop::Exit() is always called.
	struct EngineLoopCleanupGuard 
	{ 
		~EngineLoopCleanupGuard()
		{
			// Don't shut down the engine on scope exit when we are running embedded
			// because the outer application will take care of that.
			if (!GUELibraryOverrideSettings.bIsEmbedded)
			{
				EngineExit();
			}
		}
	} CleanupGuard;

	int32 ErrorLevel = EnginePreInit( CmdLine );

	// exit if PreInit failed.
	if ( ErrorLevel != 0 || IsEngineExitRequested() )
	{
		return ErrorLevel;
	}

#if WITH_EDITOR
    if (GIsEditor)
    {
        ErrorLevel = EditorInit(GEngineLoop);
    }
    else
#endif
    {
        ErrorLevel = EngineInit();
    }

	// Don't tick if we're running an embedded engine - we rely on the outer
	// application ticking us instead.
	if (!GUELibraryOverrideSettings.bIsEmbedded)
	{
		while( !IsEngineExitRequested() )
		{
			EngineTick();
		}
	}

#if WITH_EDITOR
	if( GIsEditor )
	{
		EditorExit();
	}
#endif
	return ErrorLevel;
}

梳理一下大体的逻辑，在一切初始化逻辑开始之前，函数会先把这个消息广播出去，如果有更早需要执行的逻辑，可以注册到这个委托上。另外，为了保证引擎的退出逻辑一定会被调用到，UE在这里定义了一个局部的struct，这个struct定义了一个析构函数，析构函数的实现就是调用引擎的退出逻辑。有了这个局部struct，再定义一个局部的对象，这个对象不是动态分配的，那么C++会保证它离开作用域时就会销毁它，从而调用到引擎的退出逻辑。这也是C++ RAII的常见做法了。

再往下，可以发现引擎的初始化分为PreInit和Init两个步骤。Init会根据是否是Editor环境，调用不同的逻辑。在初始化完成后，就进入到了引擎的主循环。

顺便一提，PreInit，Init，Tick，Exit都是简单的包装函数，它们的实现就是调用全局变量GEngineLoop的相应接口。

int32 EngineInit()
{
	int32 ErrorLevel = GEngineLoop.Init();

	return( ErrorLevel );
}

LAUNCH_API void EngineTick( void )
{
	GEngineLoop.Tick();
}

LAUNCH_API void EngineExit( void )
{
	// Make sure this is set
	RequestEngineExit(TEXT("EngineExit() was called"));

	GEngineLoop.Exit();
}

PreInit初始化了各种底层模块[4]，非常繁多。

UE5的引擎初始化流程1

除此之外，还完成了许多模块的加载：

UE5的引擎初始化流程2

Init阶段，UE会根据是否为editor环境调用EditorInit或者EngineInit。而EditorInit本身的实现里就是包含EngineInit的。Init简化后的版本如下。

int32 FEngineLoop::Init()
{
	UClass* EngineClass = nullptr;
	if( !GIsEditor )
	{
		// We're the game.
		FString GameEngineClassName;
		GConfig->GetString(TEXT("/Script/Engine.Engine"), TEXT("GameEngine"), GameEngineClassName, GEngineIni);
		EngineClass = StaticLoadClass( UGameEngine::StaticClass(), nullptr, *GameEngineClassName);
		GEngine = NewObject<UEngine>(GetTransientPackage(), EngineClass);
	}
	else
	{
#if WITH_EDITOR
		// We're UnrealEd.
		FString UnrealEdEngineClassName;
		GConfig->GetString(TEXT("/Script/Engine.Engine"), TEXT("UnrealEdEngine"), UnrealEdEngineClassName, GEngineIni);
		EngineClass = StaticLoadClass(UUnrealEdEngine::StaticClass(), nullptr, *UnrealEdEngineClassName);
		GEngine = GEditor = GUnrealEd = NewObject<UUnrealEdEngine>(GetTransientPackage(), EngineClass);
#endif
	}

    GEngine->ParseCommandline();
    GEngine->Init(this);
    FCoreDelegates::OnPostEngineInit.Broadcast();

    if (!IProjectManager::Get().LoadModulesForProject(ELoadingPhase::PostEngineInit) || !IPluginManager::Get().LoadModulesForEnabledPlugins(ELoadingPhase::PostEngineInit))
    {
        RequestEngineExit(TEXT("One or more modules failed PostEngineInit"));
        return 1;
    }

	// Call module loading phases completion callbacks
	SetEngineStartupModuleLoadingComplete();
    GEngine->Start();

	GIsRunning = true;

    FCoreDelegates::OnFEngineLoopInitComplete.Broadcast();
    return 0;
}

Init阶段主要做了如下的事情[3]：