DoesPackageExists 分析

该函数用来检测 Package 是否 在磁盘上存在：

class COREUOBJECT_API FPackageName
{
public:
  /**
   * Checks if the package exists on disk.
   * 
   * @param LongPackageName Package name.
   * @param OutFilename Package filename on disk.
   * @param InAllowTextFormats Detect text format packages as well as binary (priority to text)
   * @return true if the specified package name points to an existing package, false otherwise.
   **/
  static bool DoesPackageExist(const FString& LongPackageName, const FGuid* Guid = NULL, FString* OutFilename = NULL, bool InAllowTextFormats = true);
};

注意：在磁盘上存在在 UE 里有两个情况：

1. Editor 下存在 uasset 文件
2. 打包模式下 uasset 是否在 Mounted 的 Pak 中存在

事实上，UE 也是这么做检测的：

1. 首先把要检测的 LongPackageName 根据规则转换为文件路径
2. 通过 FileManager 来检测文件路径是否存在

在 Editor 和打包模式下，FileManager 通过 GetLowLevel() 拿到IPlatformFile，之后再进行 FileExist 的检测，UE 针对各个平台封装了IPlatformFile，而且也具有 PakPlatformFile 的实现，可以实现从 Pak 中读取文件与在普通文件系统中访问一样的接口。

UE 的跨平台写法，声明在通用接口里，定义在各个平台的单独文件中：

在引擎启动时会把这些 IPlatformFile 的对象创建：

Runtime/Launch/Private/LaunchEngineLoop.cpp

/**
 * Look for any file overrides on the command line (i.e. network connection file handler)
 */
bool LaunchCheckForFileOverride(const TCHAR* CmdLine, bool& OutFileOverrideFound)
{
  OutFileOverrideFound = false;

  // Get the physical platform file.
  IPlatformFile* CurrentPlatformFile = &FPlatformFileManager::Get().GetPlatformFile();

  // Try to create pak file wrapper
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("PakFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
    PlatformFile = ConditionallyCreateFileWrapper(TEXT("CachedReadFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }

  // Try to create sandbox wrapper
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("SandboxFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }

#if !UE_BUILD_SHIPPING // UFS clients are not available in shipping builds.
  // Streaming network wrapper (it has a priority over normal network wrapper)
  bool bNetworkFailedToInitialize = false;
  do
  {
    bool bShouldUseStreamingFile = false;
    IPlatformFile* NetworkPlatformFile = ConditionallyCreateFileWrapper(TEXT("StreamingFile"), CurrentPlatformFile, CmdLine, &bNetworkFailedToInitialize, &bShouldUseStreamingFile);
    if (NetworkPlatformFile)
    {
      CurrentPlatformFile = NetworkPlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }

    bool bShouldUseCookedIterativeFile = false;
    if (!bShouldUseStreamingFile && !NetworkPlatformFile)
    {
      NetworkPlatformFile = ConditionallyCreateFileWrapper(TEXT("CookedIterativeFile"), CurrentPlatformFile, CmdLine, &bNetworkFailedToInitialize, &bShouldUseCookedIterativeFile);
      if (NetworkPlatformFile)
      {
        CurrentPlatformFile = NetworkPlatformFile;
        FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
      }
    }

    // if streaming network platform file was tried this loop don't try this one
    // Network file wrapper (only create if the streaming wrapper hasn't been created)
    if (!bShouldUseStreamingFile && !bShouldUseCookedIterativeFile && !NetworkPlatformFile)
    {
      NetworkPlatformFile = ConditionallyCreateFileWrapper(TEXT("NetworkFile"), CurrentPlatformFile, CmdLine, &bNetworkFailedToInitialize);
      if (NetworkPlatformFile)
      {
        CurrentPlatformFile = NetworkPlatformFile;
        FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
      }
    }

    if (bNetworkFailedToInitialize)
    {
      FString HostIpString;
      FParse::Value(CmdLine, TEXT("-FileHostIP="), HostIpString);
#if PLATFORM_REQUIRES_FILESERVER
      FPlatformMisc::LowLevelOutputDebugStringf(TEXT("Failed to connect to file server at %s. RETRYING in 5s.\n"), *HostIpString);
      FPlatformProcess::Sleep(5.0f);
      uint32 Result = 2;
#else //PLATFORM_REQUIRES_FILESERVER
      // note that this can't be localized because it happens before we connect to a filserver - localizing would cause ICU to try to load.... from over the file server connection!
      FString Error = FString::Printf(TEXT("Failed to connect to any of the following file servers:\n\n    %s\n\nWould you like to try again? No will fallback to local disk files, Cancel will quit."), *HostIpString.Replace(TEXT("+"), TEXT("\n    ")));
      uint32 Result = FMessageDialog::Open(EAppMsgType::YesNoCancel, FText::FromString(Error) );
#endif  //PLATFORM_REQUIRES_FILESERVER

      if (Result == EAppReturnType::No)
      {
        break;
      }
      else if (Result == EAppReturnType::Cancel)
      {
        // Cancel - return a failure, and quit
        return false;
      }
    }
  }
  while (bNetworkFailedToInitialize);
#endif

#if !UE_BUILD_SHIPPING
  // Try to create file profiling wrapper
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("ProfileFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("SimpleProfileFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }
  // Try and create file timings stats wrapper
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("FileReadStats"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }
  // Try and create file open log wrapper (lists the order files are first opened)
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("FileOpenLog"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }
#endif  //#if !UE_BUILD_SHIPPING

  // Wrap the above in a file logging singleton if requested
  {
    IPlatformFile* PlatformFile = ConditionallyCreateFileWrapper(TEXT("LogFile"), CurrentPlatformFile, CmdLine);
    if (PlatformFile)
    {
      CurrentPlatformFile = PlatformFile;
      FPlatformFileManager::Get().SetPlatformFile(*CurrentPlatformFile);
    }
  }

  // If our platform file is different than it was when we started, then an override was used
  OutFileOverrideFound = (CurrentPlatformFile != &FPlatformFileManager::Get().GetPlatformFile());

  return true;
}