UnrealEngine4源码分析 1 - 入口

习惯上，分析一个程序的源码，需要从其入口开始，即启动后执行的第一个函数开始。
虚幻引擎是用C++写的，对于每一个C++程序，都有一个入口函数 main，虚幻引擎想也不可能例外。
经过一番探索，发现，虚幻引擎编辑器软件启动的源码目录在

https://github.com/EpicGames/UnrealEngine/tree/release/Engine/Source/Runtime/Launch/

虚幻引擎源码启动目录

进入/Private目录，可见里面封装了各个操作系统下的启动代码，Launch.cpp显然就是总的平台无关的启动依赖文件了。
我们只分析下Windows下的启动情况就好了：

https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Runtime/Launch/Private/Windows/LaunchWindows.cpp

在这个文件里我们看到了熟悉的Win32应用程序的main函数

int32 WINAPI WinMain( _In_ HINSTANCE hInInstance, _In_opt_ HINSTANCE hPrevInstance, _In_ char*, _In_ int32 nCmdShow )
{
    // Setup common Windows settings
    SetupWindowsEnvironment();

    int32 ErrorLevel            = 0;
    hInstance               = hInInstance;
    const TCHAR* CmdLine = ::GetCommandLineW();

#if !(UE_BUILD_SHIPPING && WITH_EDITOR)
    // Named mutex we use to figure out whether we are the first instance of the game running. This is needed to e.g.
    // make sure there is no contention when trying to save the shader cache.
    GIsFirstInstance = MakeNamedMutex( CmdLine );

    if ( FParse::Param( CmdLine,TEXT("crashreports") ) )
    {
        GAlwaysReportCrash = true;
    }
#endif

    // Using the -noinnerexception parameter will disable the exception handler within native C++, which is call from managed code,
    // which is called from this function.
    // The default case is to have three wrapped exception handlers 
    // Native: WinMain() -> Native: GuardedMainWrapper().
    // The inner exception handler in GuardedMainWrapper() catches crashes/asserts in native C++ code and is the only way to get the
    // correct callstack when running a 64-bit executable. However, XAudio2 sometimes (?) don't like this and it may result in no sound.
#ifdef _WIN64
    if ( FParse::Param(CmdLine,TEXT("noinnerexception")) || FApp::IsBenchmarking() )
    {
        GEnableInnerException = false;
    }
#endif  

#if WINVER > 0x502  // Windows Error Reporting is not supported on Windows XP
    if (FParse::Param(CmdLine, TEXT("useautoreporter")))
#endif
    {
        GUseCrashReportClient = false;
    }

#if UE_BUILD_DEBUG
    if( true && !GAlwaysReportCrash )
#else
    if( FPlatformMisc::IsDebuggerPresent() && !GAlwaysReportCrash )
#endif
    {
        // Don't use exception handling when a debugger is attached to exactly trap the crash. This does NOT check
        // whether we are the first instance or not!
        ErrorLevel = GuardedMain( CmdLine, hInInstance, hPrevInstance, nCmdShow );
    }
    else
    {
        // Use structured exception handling to trap any crashes, walk the the stack and display a crash dialog box.
#if !PLATFORM_SEH_EXCEPTIONS_DISABLED
        __try
#endif
        {
            GIsGuarded = 1;
            // Run the guarded code.
            ErrorLevel = GuardedMainWrapper( CmdLine, hInInstance, hPrevInstance, nCmdShow );
            GIsGuarded = 0;
        }
#if !PLATFORM_SEH_EXCEPTIONS_DISABLED
        __except( GEnableInnerException ? EXCEPTION_EXECUTE_HANDLER : ReportCrash( GetExceptionInformation( ) ) )
        {
#if !(UE_BUILD_SHIPPING && WITH_EDITOR)
            // Release the mutex in the error case to ensure subsequent runs don't find it.
            ReleaseNamedMutex();
#endif
            // Crashed.
            ErrorLevel = 1;
            GError->HandleError();
            LaunchStaticShutdownAfterError();
            FPlatformMallocCrash::Get().PrintPoolsUsage();
            FPlatformMisc::RequestExit( true );
        }
#endif
    }

    // Final shut down.
    FEngineLoop::AppExit();

#if !(UE_BUILD_SHIPPING && WITH_EDITOR)
    // Release the named mutex again now that we are done.
    ReleaseNamedMutex();
#endif

    

    // pause if we should
    if (GShouldPauseBeforeExit)
    {
        Sleep(INFINITE);
    }

    return ErrorLevel;
}

这个函数基本上管理了一个Win32界面的生存周期。而引擎的生存周期则内部跳转给了GuardedMain函数。去看看

https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Runtime/Launch/Private/Launch.cpp

几个主要函数为

int32 EnginePreInit( const TCHAR* CmdLine );
int32 EngineInit();
void EngineTick( void );
void EngineExit( void );

这显然就是游戏引擎完整的生命周期了，包括 预初始化、初始化、时钟循环、退出4个阶段，但是实现主要是通过GEngineLoop类来实现的。
界面的启动和引擎的启动就先看到这里了，下一节再看引擎的生命周期到底干了些什么。