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控制 AssetRegistry 的序列化

AssetRegistry 其实主要是在 Editor 下用来方便进行资源的查找和过滤操作，它的主要使用者是 ContentBrowser，这一点在 UE 的文档中也有描述：Asset Registry。

对于项目而言在 Runtime 可能没有需求来使用它，但是在 AssetRegistry 模块一启动就会把 AssetRegistry.bin 加载到内存中，如果对它没有需求其实这部分内存是浪费的。

关闭 AssetRegistry 的影响：无法在运行时通过 AssetRegistry 模块进行资源的依赖分析、以及通过 AssetRegistry 检测资源是否存在的判断。

好在 UE 提供了不序列化或者部分序列化 AssetRegistry 数据的方法，在 UAssetRegistryImpl 的构造函数中会调用 InitializeSerializationOptionsFromIni 函数来读取 DefaultEngine.ini 中的配置，并会构造出一个 FAssetRegistrySerializationOptions 结构来存储，它会在后续的 Serialize 函数中使用，用来控制把哪部分的数据序列化到 AssetRegistry 中。

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// Runtime/AssetRegistry/Private/AssetRegistry.cpp
void UAssetRegistryImpl::InitializeSerializationOptionsFromIni(FAssetRegistrySerializationOptions& Options, const FString& PlatformIniName) const
{
  FConfigFile* EngineIni = nullptr;
#if WITH_EDITOR
  // Use passed in platform, or current platform if empty
  FConfigFile PlatformEngineIni;
  FConfigCacheIni::LoadLocalIniFile(PlatformEngineIni, TEXT("Engine"), true, (!PlatformIniName.IsEmpty() ? *PlatformIniName : ANSI_TO_TCHAR(FPlatformProperties::IniPlatformName())));
  EngineIni = &PlatformEngineIni;
#else
  // In cooked builds, always use the normal engine INI
  EngineIni = GConfig->FindConfigFile(GEngineIni);
#endif

  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeAssetRegistry"), Options.bSerializeAssetRegistry);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeDependencies"), Options.bSerializeDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeNameDependencies"), Options.bSerializeSearchableNameDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeManageDependencies"), Options.bSerializeManageDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializePackageData"), Options.bSerializePackageData);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bUseAssetRegistryTagsWhitelistInsteadOfBlacklist"), Options.bUseAssetRegistryTagsWhitelistInsteadOfBlacklist);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterAssetDataWithNoTags"), Options.bFilterAssetDataWithNoTags);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterDependenciesWithNoTags"), Options.bFilterDependenciesWithNoTags);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterSearchableNames"), Options.bFilterSearchableNames);
  // ...
}

这个控制方式可以在打包时控制是否生成 AssetRegistry.bin，以及控制在运行时反序列化哪些 AssetRegistry 的数据（但是不会对 DevelopmentAssetRegistry.bin 造成影响，可以用它来进行资产审计）。

它的反序列化流程为：

  1. 检测 bSerializeAssetRegistry，如果为true 则把 AssetRegistry.bin 以二进制形式加载到内存中
  2. 通过 Serialize 函数来把二进制数据反序列化
  3. 释放加载 AssetRegistry.bin 所占用的内存

所以，AssetRegistry 的内存占用是在序列化之后的数据，而 FAssetRegistrySerializationOptions 就是控制把哪些数据序列化的。

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// Runtime/AssetRegistry/Public/AssetRegistryState.h
/** Load/Save options used to modify how the cache is serialized. These are read out of the AssetRegistry section of Engine.ini and can be changed per platform. */
struct FAssetRegistrySerializationOptions
{
  /** True rather to load/save registry at all */
  bool bSerializeAssetRegistry;

  /** True rather to load/save dependency info. If true this will handle hard and soft package references */
  bool bSerializeDependencies;

  /** True rather to load/save dependency info for Name references,  */
  bool bSerializeSearchableNameDependencies;

  /** True rather to load/save dependency info for Manage references,  */
  bool bSerializeManageDependencies;

  /** If true will read/write FAssetPackageData */
  bool bSerializePackageData;

  /** True if CookFilterlistTagsByClass is a whitelist. False if it is a blacklist. */
  bool bUseAssetRegistryTagsWhitelistInsteadOfBlacklist;

  /** True if we want to only write out asset data if it has valid tags. This saves memory by not saving data for things like textures */
  bool bFilterAssetDataWithNoTags;

  /** True if we also want to filter out dependency data for assets that have no tags. Only filters if bFilterAssetDataWithNoTags is also true */
  bool bFilterDependenciesWithNoTags;

  /** Filter out searchable names from dependency data */
  bool bFilterSearchableNames;

  /** The map of classname to tag set of tags that are allowed in cooked builds. This is either a whitelist or blacklist depending on bUseAssetRegistryTagsWhitelistInsteadOfBlacklist */
  TMap<FName, TSet<FName>> CookFilterlistTagsByClass;

  FAssetRegistrySerializationOptions()
    : bSerializeAssetRegistry(false)
    , bSerializeDependencies(false)
    , bSerializeSearchableNameDependencies(false)
    , bSerializeManageDependencies(false)
    , bSerializePackageData(false)
    , bUseAssetRegistryTagsWhitelistInsteadOfBlacklist(false)
    , bFilterAssetDataWithNoTags(false)
    , bFilterDependenciesWithNoTags(false)
    , bFilterSearchableNames(false)
  {}

  /** Options used to read/write the DevelopmentAssetRegistry, which includes all data */
  void ModifyForDevelopment()
  {
    bSerializeAssetRegistry = bSerializeDependencies = bSerializeSearchableNameDependencies = bSerializeManageDependencies = bSerializePackageData = true;
    DisableFilters();
  }

  /** Disable all filters */
  void DisableFilters()
  {
    bFilterAssetDataWithNoTags = false;
    bFilterDependenciesWithNoTags = false;
    bFilterSearchableNames = false;
  }
};

配置的读取在以下代码中：

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// Runtime/AssetRegistry/Private/AssetRegistry.cpp
void UAssetRegistryImpl::InitializeSerializationOptionsFromIni(FAssetRegistrySerializationOptions& Options, const FString& PlatformIniName) const
{
  FConfigFile* EngineIni = nullptr;
#if WITH_EDITOR
  // Use passed in platform, or current platform if empty
  FConfigFile PlatformEngineIni;
  FConfigCacheIni::LoadLocalIniFile(PlatformEngineIni, TEXT("Engine"), true, (!PlatformIniName.IsEmpty() ? *PlatformIniName : ANSI_TO_TCHAR(FPlatformProperties::IniPlatformName())));
  EngineIni = &PlatformEngineIni;
#else
  // In cooked builds, always use the normal engine INI
  EngineIni = GConfig->FindConfigFile(GEngineIni);
#endif

  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeAssetRegistry"), Options.bSerializeAssetRegistry);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeDependencies"), Options.bSerializeDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeNameDependencies"), Options.bSerializeSearchableNameDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializeManageDependencies"), Options.bSerializeManageDependencies);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bSerializePackageData"), Options.bSerializePackageData);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bUseAssetRegistryTagsWhitelistInsteadOfBlacklist"), Options.bUseAssetRegistryTagsWhitelistInsteadOfBlacklist);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterAssetDataWithNoTags"), Options.bFilterAssetDataWithNoTags);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterDependenciesWithNoTags"), Options.bFilterDependenciesWithNoTags);
  EngineIni->GetBool(TEXT("AssetRegistry"), TEXT("bFilterSearchableNames"), Options.bFilterSearchableNames);

  TArray<FString> FilterlistItems;
  if (Options.bUseAssetRegistryTagsWhitelistInsteadOfBlacklist)
  {
    EngineIni->GetArray(TEXT("AssetRegistry"), TEXT("CookedTagsWhitelist"), FilterlistItems);
  }
  else
  {
    EngineIni->GetArray(TEXT("AssetRegistry"), TEXT("CookedTagsBlacklist"), FilterlistItems);
  }

  {
    // this only needs to be done once, and only on builds using USE_COMPACT_ASSET_REGISTRY
    TArray<FString> AsFName;
    EngineIni->GetArray(TEXT("AssetRegistry"), TEXT("CookedTagsAsFName"), AsFName);
    TArray<FString> AsPathName;
    EngineIni->GetArray(TEXT("AssetRegistry"), TEXT("CookedTagsAsPathName"), AsPathName);
    TArray<FString> AsLocText;
    EngineIni->GetArray(TEXT("AssetRegistry"), TEXT("CookedTagsAsLocText"), AsLocText);
    FAssetRegistryState::IngestIniSettingsForCompact(AsFName, AsPathName, AsLocText);
  }
  // ...
  
}

在 Config/DefaultEngine.ini 中创建AssetRegistrySection 使用上面的名字就可以控制 AssetRegistry 的序列化，减少打包时的包体大小以及内存占用（AssetRegistry 在引擎启动时会加载到内存中）

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[AssetRegistry]
bSerializeAssetRegistry=false
bSerializeDependencies=false
bSerializeNameDependencies=false
bSerializeManageDependencies=false
bSerializePackageData=false

也可以对某个平台来单独指定，只需要修改平台相关的 Ini 文件：

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Config/Windows/WindowsEngine.ini
Config/Android/AndroidEngine.ini
Config/IOS/IOSEngine.ini