热更新Pak
,
UE4.22
lipengzha
New
Edit

引擎启动时 Pak 的加载

当在 UE 的 Project Setting 里Project-Packaging-UsePakFile启用时，会打包出来 pak 文件，以 Windows 平台为例，打包出来的 pak 路径为：

1
WindowsNoEditor/PROJECT_NAME/Content/Paks

该目录下的 pak 文件在游戏启动时会自动加载，在引擎的 FEngineLoop::PreInit 中调用 LaunchCheckForFileOverride(LaunchEngineLoop.cpp) 又调用 ConditionallyCreateFileWrapper 来加载 PakFile 的PlatformFile，但是在 ConditionallyCreateFileWrapper 的代码中做了一层 WrapperFile->ShouldBeUsed 判断：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
// Runtime/Launch/Private/LaunchEngineLoop.cpp
static IPlatformFile* ConditionallyCreateFileWrapper(const TCHAR* Name, IPlatformFile* CurrentPlatformFile, const TCHAR* CommandLine, bool* OutFailedToInitialize = nullptr, bool* bOutShouldBeUsed = nullptr )
{
	if (OutFailedToInitialize)
	{
		*OutFailedToInitialize = false;
	}
	if (bOutShouldBeUsed)
	{
		*bOutShouldBeUsed = false;
	}
	IPlatformFile* WrapperFile = FPlatformFileManager::Get().GetPlatformFile(Name);
	if (WrapperFile != nullptr && WrapperFile->ShouldBeUsed(CurrentPlatformFile, CommandLine))
	{
		if (bOutShouldBeUsed)
		{
			*bOutShouldBeUsed = true;
		}
		if (WrapperFile->Initialize(CurrentPlatformFile, CommandLine) == false)
		{
			if (OutFailedToInitialize)
			{
				*OutFailedToInitialize = true;
			}
			// Don't delete the platform file. It will be automatically deleted by its module.
			WrapperFile = nullptr;
		}
	}
	else
	{
		// Make sure it won't be used.
		WrapperFile = nullptr;
	}
	return WrapperFile;
}

如果为 false 不会对该 PlatformFile 调用 Initialize，而FPakPlatformFile 的一些成员就是在这里被设置的。FPakPlatformFile::ShoubleBeUsed的定义如下(UE_4.22.3)：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
// Runtime/PakFile/Private/IPlatformFilePak.cpp
bool FPakPlatformFile::ShouldBeUsed(IPlatformFile* Inner, const TCHAR* CmdLine) const
{
	bool Result = false;
#if !WITH_EDITOR
	if (!FParse::Param(CmdLine, TEXT("NoPak")))
	{
		TArray<FString> PakFolders;
		GetPakFolders(CmdLine, PakFolders);
		Result = CheckIfPakFilesExist(Inner, PakFolders);
	}
#endif
	return Result;
}

编辑器模式下直接就是 false，即编辑器模式下不可以使用 pak 的 mount 操作，因为 mount 中需要用到 LowerLevel 成员，而该成员在 FPakPlatformFile::Initialize 中被设置，所以不可以调用 mount，否则必 Crash。

Mount 所有 pak 的相关代码在：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
// Runtime\PakFile\Private\IPlatformFilePak.cpp
bool FPakPlatformFile::Initialize(IPlatformFile* Inner, const TCHAR* CmdLine)
{
  LLM_SCOPE(ELLMTag::FileSystem);
  SCOPED_BOOT_TIMING("FPakPlatformFile::Initialize");
  // Inner is required.
  check(Inner != NULL);
  LowerLevel = Inner;

#if EXCLUDE_NONPAK_UE_EXTENSIONS
  // Extensions for file types that should only ever be in a pak file. Used to stop unnecessary access to the lower level platform file
  ExcludedNonPakExtensions.Add(TEXT("uasset"));
  ExcludedNonPakExtensions.Add(TEXT("umap"));
  ExcludedNonPakExtensions.Add(TEXT("ubulk"));
  ExcludedNonPakExtensions.Add(TEXT("uexp"));
#endif

#if DISABLE_NONUFS_INI_WHEN_COOKED
  IniFileExtension = TEXT(".ini");
  GameUserSettingsIniFilename = TEXT("GameUserSettings.ini");
#endif

  // signed if we have keys, and are not running with fileopenlog (currently results in a deadlock).
  bSigned = GetPakSigningKey().IsValid() && !FParse::Param(FCommandLine::Get(), TEXT("fileopenlog"));;

  // Find and mount pak files from the specified directories.
  TArray<FString> PakFolders;
  GetPakFolders(FCommandLine::Get(), PakFolders);
  MountAllPakFiles(PakFolders);

#if !UE_BUILD_SHIPPING
  GPakExec = MakeUnique<FPakExec>(*this);
#endif // !UE_BUILD_SHIPPING

  FCoreDelegates::OnMountAllPakFiles.BindRaw(this, &FPakPlatformFile::MountAllPakFiles);
  FCoreDelegates::OnMountPak.BindRaw(this, &FPakPlatformFile::HandleMountPakDelegate);
  FCoreDelegates::OnUnmountPak.BindRaw(this, &FPakPlatformFile::HandleUnmountPakDelegate);

#if !(IS_PROGRAM || WITH_EDITOR)
  FCoreDelegates::OnFEngineLoopInitComplete.AddLambda([this] {
      FPlatformMisc::LowLevelOutputDebugStringf(TEXT("Checking Pak Config"));
      bool bUnloadPakEntryFilenamesIfPossible = false;
      GConfig->GetBool(TEXT("Pak"), TEXT("UnloadPakEntryFilenamesIfPossible"), bUnloadPakEntryFilenamesIfPossible, GEngineIni);

      if (bUnloadPakEntryFilenamesIfPossible)
      {
        // With [Pak] UnloadPakEntryFilenamesIfPossible enabled, [Pak] DirectoryRootsToKeepInMemoryWhenUnloadingPakEntryFilenames
        // can contain pak entry directory wildcards of which the entire recursive directory structure of filenames underneath a
        // matching wildcard will be kept.
        //
        // Example:
        //   [Pak]
        //   DirectoryRootsToKeepInMemoryWhenUnloadingPakEntryFilenames="*/Config/Tags/"
        //   +DirectoryRootsToKeepInMemoryWhenUnloadingPakEntryFilenames="*/Content/Localization/*"
        TArray<FString> DirectoryRootsToKeep;
        GConfig->GetArray(TEXT("Pak"), TEXT("DirectoryRootsToKeepInMemoryWhenUnloadingPakEntryFilenames"), DirectoryRootsToKeep, GEngineIni);

        FPakPlatformFile* PakPlatformFile = (FPakPlatformFile*)(FPlatformFileManager::Get().FindPlatformFile(FPakPlatformFile::GetTypeName()));
        PakPlatformFile->UnloadPakEntryFilenames(&DirectoryRootsToKeep);
      }

      bool bShrinkPakEntriesMemoryUsage = false;
      GConfig->GetBool(TEXT("Pak"), TEXT("ShrinkPakEntriesMemoryUsage"), bShrinkPakEntriesMemoryUsage, GEngineIni);
      if (bShrinkPakEntriesMemoryUsage)
      {
        FPakPlatformFile* PakPlatformFile = (FPakPlatformFile*)(FPlatformFileManager::Get().FindPlatformFile(FPakPlatformFile::GetTypeName()));
        PakPlatformFile->ShrinkPakEntriesMemoryUsage();
      }
    });
#endif

  return !!LowerLevel;
}

UE 自动加载的 pak 路径：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
// Runtime\PakFile\Private\IPlatformFilePak.cpp
void FPakPlatformFile::GetPakFolders(const TCHAR* CmdLine, TArray<FString>& OutPakFolders)
{
#if !UE_BUILD_SHIPPING
  // Command line folders
  FString PakDirs;
  if (FParse::Value(CmdLine, TEXT("-pakdir="), PakDirs))
  {
    TArray<FString> CmdLineFolders;
    PakDirs.ParseIntoArray(CmdLineFolders, TEXT("*"), true);
    OutPakFolders.Append(CmdLineFolders);
  }
#endif

  // @todo plugin urgent: Needs to handle plugin Pak directories, too
  // Hardcoded locations
  OutPakFolders.Add(FString::Printf(TEXT("%sPaks/"), *FPaths::ProjectContentDir()));
  OutPakFolders.Add(FString::Printf(TEXT("%sPaks/"), *FPaths::ProjectSavedDir()));
  OutPakFolders.Add(FString::Printf(TEXT("%sPaks/"), *FPaths::EngineContentDir()));
}

在非 Shipping 打包的时候可以通过才命令行加启动参数 -pakdir 来添加额外的 pak 路径。

引擎默认添加的路径为：

1
2
3
4
5
# relative to Project Path
Content/Paks/
Saved/Paks/
# relative to Engine Path
Content/Paks

之后又调用了 FPakPlatformFile::MountAllPakFiles 来把挂载所有 pak（默认对 pak 的名字进行了个降序排序，但是这里的排序没用），在该函数中 mount 的时候会给不同的路径加载的 pak 设置不同的Order，其函数在：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
// Runtime\PakFile\Private\IPlatformFilePak.cpp
int32 FPakPlatformFile::GetPakOrderFromPakFilePath(const FString& PakFilePath)
{
	if (PakFilePath.StartsWith(FString::Printf(TEXT("%sPaks/%s-"), *FPaths::ProjectContentDir(), FApp::GetProjectName())))
	{
		return 4;
	}
	else if (PakFilePath.StartsWith(FPaths::ProjectContentDir()))
	{
		return 3;
	}
	else if (PakFilePath.StartsWith(FPaths::EngineContentDir()))
	{
		return 2;
	}
	else if (PakFilePath.StartsWith(FPaths::ProjectSavedDir()))
	{
		return 1;
	}

	return 0;
}

概括来说：

1
2
3
4
5
6
7
# relative to project path
4 Content/Paks/PROJECT_NAME-*.pak
3 Content/Paks/
1 Saved/Paks

# relative to engine path
2 Content/Paks/

可以看到 Saved/Paks 下的 pak 文件加载的优先级是最低的。

在 Mount 的时候如果上述路径中有打出来 Patch 包，以 _Num_P.pak 结尾的文件，其中 Num 是数字，Patch 包的优先级高于普通的 pak，在 IPlatformFilePak.cpp 中默认给 _P.pak 的PakOrder加了 100，_P.pak前面的数字越大，其加载的优先级就越高。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
// Runtime\PakFile\Private\IPlatformFilePak.cpp
bool FPakPlatformFile::Mount(const TCHAR* InPakFilename, uint32 PakOrder, const TCHAR* InPath /*= NULL*/)
{
  bool bSuccess = false;
  TSharedPtr<IFileHandle> PakHandle = MakeShareable(LowerLevel->OpenRead(InPakFilename));
  if (PakHandle.IsValid())
  {
    FPakFile* Pak = new FPakFile(LowerLevel, InPakFilename, bSigned);
    if (Pak->IsValid())
    {
      if (InPath != NULL)
      {
        Pak->SetMountPoint(InPath);
      }
      FString PakFilename = InPakFilename;
      if (PakFilename.EndsWith(TEXT("_P.pak")))
      {
        // Prioritize based on the chunk version number
        // Default to version 1 for single patch system
        uint32 ChunkVersionNumber = 1;
        FString StrippedPakFilename = PakFilename.LeftChop(6);
        int32 VersionEndIndex = PakFilename.Find("_", ESearchCase::CaseSensitive, ESearchDir::FromEnd);
        if (VersionEndIndex != INDEX_NONE && VersionEndIndex > 0)
        {
          int32 VersionStartIndex = PakFilename.Find("_", ESearchCase::CaseSensitive, ESearchDir::FromEnd, VersionEndIndex - 1);
          if (VersionStartIndex != INDEX_NONE)
          {
            VersionStartIndex++;
            FString VersionString = PakFilename.Mid(VersionStartIndex, VersionEndIndex - VersionStartIndex);
            if (VersionString.IsNumeric())
            {
              int32 ChunkVersionSigned = FCString::Atoi(*VersionString);
              if (ChunkVersionSigned >= 1)
              {
                // Increment by one so that the first patch file still gets more priority than the base pak file
                ChunkVersionNumber = (uint32)ChunkVersionSigned + 1;
              }
            }
          }
        }
        PakOrder += 100 * ChunkVersionNumber;
      }
      {
        // Add new pak file
        FScopeLock ScopedLock(&PakListCritical);
        FPakListEntry Entry;
        Entry.ReadOrder = PakOrder;
        Entry.PakFile = Pak;
        PakFiles.Add(Entry);
        PakFiles.StableSort();
      }
      bSuccess = true;
    }
    else
    {
      if (Pak->GetInfo().EncryptionKeyGuid.IsValid())
      {
        UE_LOG(LogPakFile, Log, TEXT("Deferring mount of pak \"%s\" until encryption key '%s' becomes available"), InPakFilename, *Pak->GetInfo().EncryptionKeyGuid.ToString());

        check(!GetRegisteredEncryptionKeys().HasKey(Pak->GetInfo().EncryptionKeyGuid));
        FPakListDeferredEntry& Entry = PendingEncryptedPakFiles[PendingEncryptedPakFiles.Add(FPakListDeferredEntry())];
        Entry.Filename = InPakFilename;
        Entry.Path = InPath;
        Entry.ReadOrder = PakOrder;
        Entry.EncryptionKeyGuid = Pak->GetInfo().EncryptionKeyGuid;
        Entry.ChunkID = Pak->ChunkID;

        delete Pak;
        PakHandle.Reset();
        return false;
      }
      else
      {
        UE_LOG(LogPakFile, Warning, TEXT("Failed to mount pak \"%s\", pak is invalid."), InPakFilename);
      }
    }
  }
  else
  {
    UE_LOG(LogPakFile, Warning, TEXT("Pak \"%s\" does not exist!"), InPakFilename);
  }
  return bSuccess;
}

另外，腾讯的和平精英的热更新的 pak 就是放在 Saved/Paks 里面。

而且，和平精英的 apk 的大小是 1.6G，我下载完之后又热更新了 1.2G 左右的内容，总共占了 3 个多 G…这样的 apk 里面 obb 的数据估计和外部的 pak 中的内容都是覆盖的，但是 apk 内容又没办法改，只能越更新越多了（但是可以在用户更新 APK 之后删除多余的 pak）。

注：腾讯的吃鸡用 sluaunreal，脚本的热更也是通过 pak 方式来加载的，这点在 sluaunreal 中有写：sluaunreal 增量打包问题

如果从省空间的角度考虑，最好的方式就是基础 apk+ 更新的方式修改游戏内容，但是玩家下载完 apk 之后还需要再更新可能会造成用户流失，这是个要考虑的问题。