DoesPackageExists 分析

该函数用来检测 Package 是否 在磁盘上存在

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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
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/**
* 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;
}