Mount Point 的作用

在 Mount Pak 的时候，有一个参数可以指定 MountPoint:

/**
* Mounts a pak file at the specified path.
*
* @param InPakFilename Pak filename.
* @param InPath Path to mount the pak at.
*/
bool Mount(const TCHAR* InPakFilename, uint32 PakOrder, const TCHAR* InPath = NULL, bool bLoadIndex = true);

那么它是干什么的呢？
首先从 Mount 函数开始：

if (InPath != NULL)
{
  Pak->SetMountPoint(InPath);
}

如果在调用 Mount 时传递了 InPath，则通过加载 Pak 的 FPakFile 实例调用SetMountPoint，把 InPath 设置给它。
其实在 FPakFile 中，MountPath 是有默认值的（从 Pak 文件中读取），在 FPakFile 的构造函数中调用了 Initialize(Reader, bLoadIndex);，Initialize 中又调用了LoadIndex，在LoadIndex 中从 Pak 中读取 Pak 的 Mount Point 的逻辑：

// Runtime/PakFile/Private/IPlatformFilePak.cpp
void FPakFile::LoadIndex(FArchive* Reader)
{
  if (CachedTotalSize < (Info.IndexOffset + Info.IndexSize))
  {
    UE_LOG(LogPakFile, Fatal, TEXT("Corrupted index offset in pak file."));
  }
  else
  {
    if (Info.Version >= FPakInfo::PakFile_Version_FrozenIndex && Info.bIndexIsFrozen)
    {
      SCOPED_BOOT_TIMING("PakFile_LoadFrozen");

      // read frozen data
      Reader->Seek(Info.IndexOffset);
      int32 FrozenSize = Info.IndexSize;

      // read in the index, etc data in one lump
      void* DataMemory = FMemory::Malloc(FrozenSize);
      Reader->Serialize(DataMemory, FrozenSize);
      Data = TUniquePtr<FPakFileData>((FPakFileData*)DataMemory);

      // cache the number of entries
      NumEntries = Data->Files.Num();
      // @todo loadtime: it is nice to serialize the mountpoint right into the Data so that IndexSize is right here
      // but it takes this to copy it out, because it's too painful for the string manipulation when dealing with
      // MemoryImageString everywhere MountPoint is used
      MountPoint = Data->MountPoint;
    }
      // ...
    }
  // ...
}

简单的可以理解为：如果 Mount 时不传递 Mount Point 就会从 Pak 文件中读取，如果有传入就设置为传入的值（Pak 文件中的 MountPoint 是 Pak 中所有文件的公共路径）。

那么，给 Pak 设置 MountPoint 的作用是什么呢？
真实目的是，检测要加载的文件是否存在于当前 Pak 中！因为 Pak 的 Mount Point 的默认含义是当前 Pak 中所有文件的公共路径，所以只需要检测要读取的文件是否以这个路径开头，就可以首先排除掉基础路径不对的文件（基础路径都不对，意味着这个文件在 Pak 中也不存在）。

具体逻辑可以看这个函数的实现：

// Runtime/PakFile/Public/IPlatformFilePak.h
/**
* Finds a file in the specified pak files.
*
* @param Paks Pak files to find the file in.
* @param Filename File to find in pak files.
* @param OutPakFile Optional pointer to a pak file where the filename was found.
* @return Pointer to pak entry if the file was found, NULL otherwise.
*/
static bool FindFileInPakFiles(TArray<FPakListEntry>& Paks,const TCHAR* Filename,FPakFile** OutPakFile,FPakEntry* OutEntry = nullptr)
{
    FString StandardFilename(Filename);
    FPaths::MakeStandardFilename(StandardFilename);

    int32 DeletedReadOrder = -1;

    for (int32 PakIndex = 0; PakIndex < Paks.Num(); PakIndex++)
    {
        int32 PakReadOrder = Paks[PakIndex].ReadOrder;
        if (DeletedReadOrder != -1 && DeletedReadOrder > PakReadOrder)
        {
            //found a delete record in a higher priority patch level, but now we're at a lower priority set - don't search further back or we'll find the original, old file.
            UE_LOG(LogPakFile, Verbose, TEXT("Delete Record: Accepted a delete record for %s"), Filename );
            return false;
        }

        FPakFile::EFindResult FindResult = Paks[PakIndex].PakFile->Find(*StandardFilename, OutEntry);
        if (FindResult == FPakFile::EFindResult::Found)
        {
            if (OutPakFile != NULL)
            {
                *OutPakFile = Paks[PakIndex].PakFile;
            }
            UE_CLOG(DeletedReadOrder != -1, LogPakFile, Verbose, TEXT("Delete Record: Ignored delete record for %s - found it in %s instead (asset was moved between chunks)"), Filename, *Paks[PakIndex].PakFile->GetFilename());
            return true;
        }
        else if (FindResult == FPakFile::EFindResult::FoundDeleted)
        {
            DeletedReadOrder = PakReadOrder;
            UE_LOG(LogPakFile, Verbose, TEXT("Delete Record: Found a delete record for %s in %s"), Filename, *Paks[PakIndex].PakFile->GetFilename());
        }
    }

    UE_CLOG(DeletedReadOrder != -1, LogPakFile, Warning, TEXT("Delete Record: No lower priority pak files looking for %s. (maybe not downloaded?)"), Filename );
    return false;
}

当我们从 Pak 中读取文件时，通过对游戏中所有 Mount 的 Pak 调用 Find 函数，而 FPakFile::Find 的函数就实现了上述我说的逻辑：

// Runtime/PakFile/Private/IPlatformFilePak.cpp
FPakFile::EFindResult FPakFile::Find(const FString& Filename, FPakEntry* OutEntry) const
{
  QUICK_SCOPE_CYCLE_COUNTER(PakFileFind);
  if (Filename.StartsWith(MountPoint))
  {
    FString Path(FPaths::GetPath(Filename));
      // ...
    }
  // ...
}

所以，MountPoint 的作用就是在从 Pak 中查找文件时，首先判断文件的路径是否与 Pak 中所有文件的 基础路径 相匹配（StartWith），如果不存在也就不会进入后续的流程了。