GameFeature浅析

一套支持动态增删游戏玩法的框架，往 CoreGame 里 Add/Remove 其它游戏资源/逻辑；

以 Lyra(UE5.3) 为例，GameFeature 提供了很好的解耦资源/逻辑的方案，可以看出这里的 ShooterCore 被实现为了一个相对比较独立的 Plugin。

image-20240220173041938

初始化

首先在游戏内，GameFeaturesSubsystem 管理着所有的 GameFeature，看到对应的初始化 UGameFeaturesSubsystem::Initialize，可以分为几步：

  1. 加载Policy：根据 GameFeaturesSubsystemSettings->GameFeaturesManagerClassName 加载对应的 GameSpecificPolicies

  2. 注册资源回调：UAssetManager::CallOrRegister_OnAssetManagerCreated

  3. 注册 ConsoleCommand 方便 Debug

在资源加载完的回调后，会走到 UGameFeaturesSubsystem::OnAssetManagerCreated，进行 GameSpecificPolicies->InitGameFeatureManager，根据 Policies 执行初始化逻辑；

这里的 GameFeaturesProjectPolicies 决定了一些 GameFeaturePlugin 的加载规则，比如需要在什么地方(Server/Client) 加载什么 GameFeature，同时可以在这里的 Init 扩展需要的额外功能，比如注册一些 Observers、Subsystems。

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virtual void InitGameFeatureManager() override;
virtual void ShutdownGameFeatureManager() override;
virtual TArray<FPrimaryAssetId> GetPreloadAssetListForGameFeature(const UGameFeatureData* GameFeatureToLoad, bool bIncludeLoadedAssets = false) const override;
virtual bool IsPluginAllowed(const FString& PluginURL) const override;
virtual const TArray<FName> GetPreloadBundleStateForGameFeature() const override;
virtual void GetGameFeatureLoadingMode(bool& bLoadClientData, bool& bLoadServerData) const override;

在 UDefaultGameFeaturesProjectPolicies::InitGameFeatureManager 中会执行 UGameFeaturesSubsystem::Get().LoadBuiltInGameFeaturePlugins(AdditionalFilter)，根据 Filter 来加载 GameFeaturePlugin，可以发现，会进行状态机 GameFeaturePluginStateMachine 的初始状态设置并初始化。

状态机的底层的数据都存在 GameFeatureData 上，可以从这里面开始分析：

image-20240220173210508

（需要注意的是，GameFeatureData 本身只记录了 Actions 和 PrimaryAssetTypesToScan，这里的 FeatureState 对应的是 GameFeaturePlugin 的对应状态机的信息，由 FGameFeaturesEditorModule::StartModule -> FGameFeatureDataDetailsCustomization::CustomizeDetails 注册进 Details 面板）

状态机

GameFeature 由一个 UGameFeaturesSubsystem : UEngineSubsystem 管理其生命周期：

classDiagram
    class UGameFeaturesSubsystem {
        GameFeaturePluginStateMachines : TMap[GameFeature, StateMachine] 
        ListGameFeaturePlugins()
        LoadGameFeaturePlugin()
        DeactivateGameFeaturePlugin()
        UnloadGameFeaturePlugin()
        ReleaseGameFeaturePlugin()
        UninstallGameFeaturePlugin()
        TerminateGameFeaturePlugin()
    }
    
    UGameFeaturesSubsystem..>UGameFeaturePluginStateMachine
class UGameFeaturePluginStateMachine {
        AllStates : TUniquePtr[FGameFeaturePluginState] 
        CurrentStateInfo : FGameFeaturePluginStateInfo
        SetDestination()
    }
    
    UGameFeaturePluginStateMachine..>FGameFeaturePluginState
class FGameFeaturePluginState {
        TickHandle : FTSTicker-FDelegateHandle 
        BeginState()
        UpdateState()
        TryCancelState()
        EndState()
    }
    
    UGameFeaturePluginStateMachine..>FGameFeaturePluginStateInfo
class FGameFeaturePluginStateInfo {
    	State : EGameFeaturePluginState
    }

最基本的有 Load、Deactivate、Unload、Release、Uninstall、Terminate 几个功能，可以由外部调用。

核心内容围绕着每个 GameFeaturePlugin 对应的 StateMachine 展开；GameFeatureSubSystem 上记录着 TMap<FString, TObjectPtr<UGameFeaturePluginStateMachine>> GameFeaturePluginStateMachines ，也就是 PluginIdentifier->StateMachine 的多个映射。

最后都会走到 UGameFeaturesSubsystem::ChangeGameFeatureDestination 修改状态机的状态：

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void ChangeGameFeatureDestination(UGameFeaturePluginStateMachine* Machine, const FGameFeaturePluginStateRange& StateRange, FGameFeaturePluginChangeStateComplete CompleteDelegate);

// 暴露给外部的主要目标状态
enum class EGameFeatureTargetState : uint8
{
	Installed,
	Registered,
	Loaded,
	Active,
	Count	UMETA(Hidden)
};
void ChangeGameFeatureTargetState(const FString& PluginURL, EGameFeatureTargetState TargetState, const FGameFeaturePluginChangeStateComplete& CompleteDelegate);

GameFeaturePluginStateMachine

主要维护对应 GameFeaturePlugin 的状态。

最重要的设置状态入口是 UGameFeaturePluginStateMachine::SetDestination。

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// UE5.0: 传入的目标状态为一个 GameFeaturePluginState
void UGameFeaturePluginStateMachine::SetDestinationState(EGameFeaturePluginState InDestinationState, FGameFeatureStateTransitionComplete OnFeatureStateTransitionComplete)
{
	check(IsValidDestinationState(InDestinationState));

	// TODO: If we aren't in a destination state and our new destination is in the opposite direction of 
	// our current destination, cancel the current state transition (if possible)
	// The completion delegate may be stomped in these cases.  Should probably callback with a cancelled error

	StateProperties.DestinationState = InDestinationState;
	StateProperties.OnFeatureStateTransitionComplete = OnFeatureStateTransitionComplete;

	UpdateStateMachine();
}

// UE5.1-5.3：将传入的状态改为了一个 FGameFeaturePluginStateRange(MinState, MaxState)
bool UGameFeaturePluginStateMachine::SetDestination(FGameFeaturePluginStateRange InDestination, FGameFeatureStateTransitionComplete OnFeatureStateTransitionComplete, FDelegateHandle* OutCallbackHandle /*= nullptr*/)

可以发现 StateMachine 维护了一个 FGameFeaturePluginStateMachineProperties& StateProperties，记录了状态机运行中可以切换到的状态的一些属性，其中的 DestinationState 表示在状态变化中期望到达的状态（UE5.3 改为了状态区间，为了适配 Terminal 以及状态转移中不丢失对应的 CompletionHandle）

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/** 当前状态是否不在 Destination 内，不在的话说明需要尝试到一个新的 Destination，标记为 IsRunning */
bool IsRunning() const
{
	return !StateProperties.Destination.Contains(CurrentStateInfo.State);
}


// 已经在 Destination 内，那么可以尝试到任意的 Destination
if (!IsRunning())
{
    if (InDestination.Contains(CurrentStateInfo.State)) return true;
    StateProperties.Destination = InDestination;
    UpdateStateMachine();
    return true;
}

// 不在 Destination 内，说明状态还在过渡状态，这时如果收到了新的 Destination，为了保证原有状态可以顺利过渡，与原有的 Destination 求交，继续尝试切换状态
if (TOptional<FGameFeaturePluginStateRange> NewDestination = StateProperties.Destination.Intersect(InDestination))
{
    // The machine is already running so we can only transition to this range if it overlaps with our current range.
    // We can satisfy both ranges in this case.

    if (CurrentStateInfo.State < StateProperties.Destination)
    {
        StateProperties.Destination = *NewDestination;

        if (InDestination.Contains(CurrentStateInfo.State)) return true;
    }
    else if(CurrentStateInfo.State > StateProperties.Destination)
    {
        StateProperties.Destination = *NewDestination;

        if (InDestination.Contains(CurrentStateInfo.State)) return true;
    }

    return true;
}

FeaturePluginState

内部维护了很多状态：

image-20240221105036685

image-20240220225411173

可以被作为DestinationState的状态有：UnknownStatus、Uninstalled、StatusKnown、Installed、Registered、Load、Active、Terminal、其它ErrorStates。

状态间的关系如图所示：

flowchart LR

Uninitialized-->*UnknownStatus

*UnknownStatus-->CheckingStatus
*UnknownStatus-->*Terminal

CheckingStatus-->*StatusKnown

*StatusKnown-->*Installed
*StatusKnown-->Downloading
*StatusKnown-->Uninstalling
*StatusKnown-->*Terminal

Downloading-->*Installed

Uninstalling-->*Uninstalled

*Uninstalled-->*Terminal
*Uninstalled-->CheckingStatus

Releasing-->*StatusKnown

*Installed-->Mounting
*Installed-->Releasing

Mounting-->WaitingForDependencies

Unmounting-->*Installed

WaitingForDependencies-->Registering

Registering-->*Registered

*Registered-->Unregistering
*Registered-->Loading

Unregistering-->Unmounting

Loading-->*Loaded

*Loaded-->Activating
*Loaded-->Unloading

Unloading-->*Registered

Activating-->*Active

*Active-->Deactivating

Deactivating-->*Loaded

状态修改完会通过UGameFeaturesSubsystem::Get().OnGameFeature... 进行通知，GameFeatureSubSystem 会调用到 Actions 的回调，同时提供了一个 CallbackObservers 通知已注册的 Observers。

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enum class EObserverCallback
{
    CheckingStatus,
    Terminating,
    Registering,
    Unregistering,
    Loading,
    Activating,
    Deactivating,
    PauseChanged,
    Count
};

Actions

对于一个 GameFeature ，在 Register、Unregister、Load、Activate、Deactive 等行为时（也可以自己添加）会调用到 GameFeatureData 上配置的 Actions 的对应回调：

image-20240221155727741 image-20240221155957459

可以自定义各种各样的 Actions：

image-20240221155442578

GameFeatureAction 中有一个关键 UGameFeatureAction_AddComponents ，其主要是将 Component 注册进对应的 Actor，应用到了另一个模块 ModularGameplay。

AddComponent 通过 ModularGameplay 的 UGameFrameworkComponentManager::AddComponentRequest 实现，简单介绍一下 ModularGameplay ：

ModularGameplay

一套往 Actor 上注册 Component/ExtensionHandler 的解决方案。

这套框架核心由一个 UGameFrameworkComponentManager : USubSystem 在全局管理各个 ActorClass 的 Component/ExtensionHandler，维护 Component 的生命周期，并且提供接口供外部调用（Add/Remove/SendEvent 等）

ActorReceiver

注册 Receiver

首先将需要使用这套框架维护 Component 的 ActorClass 作为 Receiver ，通过 UGameFrameworkComponentManager::AddGameFrameworkComponentReceiver 注册进 Manager；

同时，在注册的时候，会判断 ActorClass 是否已经提前注册的 ComponentClass 或者 EventDelegate ，如果有 ComponentClass 则会在此时创建对应实例 CreateComponentOnInstance，有 EventDelagte 则会通知一下 NAME_ReceiverAdded。

（一般在 AActor::PreInitializeComponents 调用）

移除 Receiver

通过 UGameFrameworkComponentManager::RemoveGameFrameworkComponentReceiver 移除；

调用对应 Component 的销毁，通知一下 NAME_ReceiverRemoved。

（一般在 AActor::EndPlay 调用）

Component

classDiagram
    class UGameFrameworkComponentManager 
    UGameFrameworkComponentManager : ReceiverClassToComponentClassMap
    UGameFrameworkComponentManager : AddComponentRequest()
    UGameFrameworkComponentManager : RemoveComponentRequest()

维护了一个 ActorClass->ComponentClass ，也就是 TMap<FComponentRequestReceiverClassPath, TSet<TObjectPtr<UClass>>> ReceiverClassToComponentClassMap，（其中 FComponentRequestReceiverClassPath 是一个结构体，用字符串数组记录 Class->Root 这条链路，作为 Class 的标识，不直接用 UClass 是因为以 Component 的视角进行操作，不期望在这里直接依赖 Receiver 对应模块的指针）

注册 Component

通过 UGameFrameworkComponentManager::AddComponentRequest 实现；

这里的 CompoentRequest 也就是一个 FComponentRequestHandle(OwningManager, ReceiverClass, ComponentClass)；

内部维护了一个 RequestTrackingMap 用于 CompoentRequest 的计数，如果 ==1 则说明需要尝试创建实例；

（有可能不同的模块都依赖对应的 ComponentClass ，所以需要计数一下）

注册的时候判断对应的 Actor 是否已经 Initialize ，如果是，则 CreateComponentOnInstance。

CreateComponentOnInstance：创建 ComponentInstance ，维护 TMap<UClass*, TSet<FObjectKey>> ComponentClassToComponentInstanceMap 用于记录这个 ComponentClass 有哪些对应的 Instance

移除 Component

通过 UGameFrameworkComponentManager::RemoveComponentRequest 实现；

修改计数，==0 则执行销毁。

销毁的时候通过 ComponentClassToComponentInstanceMap 找到所有的 Component，判断对应的 Actor 是否是期望的，如果是，则销毁；

ExtensionHandler

和 Component 类似，也就是注册 Event 的部分；

classDiagram
    class UGameFrameworkComponentManager 
	UGameFrameworkComponentManager  : ReceiverClassToEventMap
    UGameFrameworkComponentManager : AddExtensionHandler()
    UGameFrameworkComponentManager : RemoveExtensionHandler()
    UGameFrameworkComponentManager : SendExtensionEvent()

资源加载

image-20240221163647066

在 FGameFeaturePluginState_Registering 状态时，会根据配置的 GameFeatureToAdd->GetPrimaryAssetTypesToScan()，执行对应的 UGameFeaturesSubsystem::AddGameFeatureToAssetManager，进行资源是否存在等判定之后，尝试加载资源到 AssetManager。

类似的，在 FGameFeaturePluginState_Unregistering 状态时，通过 UGameFeaturesSubsystem::RemoveGameFeatureFromAssetManager 进行资源卸载。

参考

Lyra (UE5.0-5.3) GameFeature 部分源码

UE5：Game Feature 预研

《InsideUE5》GameFeatures架构