UnrealEngineUWP/Engine/Plugins/Runtime/StateTree/Source/StateTreeModule/Public/StateTreeLinker.h

// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "StateTreeSchema.h"
#include "StateTreeTypes.h"
#include "StateTreeLinker.generated.h"

UENUM()
enum class EStateTreeLinkerStatus : uint8
{
	Succeeded,
	Failed,
};

/**
 * The StateTree linker is used to resolved references to various StateTree data at load time.
 * @see TStateTreeExternalDataHandle<> for example usage.
 */
struct FStateTreeLinker
{
	explicit FStateTreeLinker(const UStateTreeSchema* InSchema) : Schema(InSchema) {}
	
	/** Sets base index for all external data handles. */
	void SetExternalDataBaseIndex(const int32 InExternalDataBaseIndex) { ExternalDataBaseIndex = InExternalDataBaseIndex; }

	/** Sets currently linked item's instance data type and index. */ 
	void SetCurrentInstanceDataType(const UStruct* Struct, const int32 Index)
	{
		CurrentInstanceStruct = Struct;
		CurrentInstanceIndex = Index;
	}

	EStateTreeLinkerStatus GetStatus() const { return Status; }
	
	/**
	 * Links reference to an external UObject.
	 * @param Handle Reference to TStateTreeExternalDataHandle<> with UOBJECT type to link to.
	 */
	template <typename T>
	typename TEnableIf<TIsDerivedFrom<typename T::DataType, UObject>::IsDerived, void>::Type LinkExternalData(T& Handle)
	{
		LinkExternalData(Handle, T::DataType::StaticClass(), T::DataRequirement);
	}

	/**
	 * Links reference to an external UObject.
	 * @param Handle Reference to TStateTreeExternalDataHandle<> with USTRUCT type to link to.
	 */
	template <typename T>
	typename TEnableIf<!TIsDerivedFrom<typename T::DataType, UObject>::IsDerived, void>::Type LinkExternalData(T& Handle)
	{
		LinkExternalData(Handle, T::DataType::StaticStruct(), T::DataRequirement);
	}

	/**
	 * Links reference to an external Object or Struct.
	 * This function should only be used when TStateTreeExternalDataHandle<> cannot be used, i.e. the Struct is based on some data.
	 * @param Handle Reference to link to.
	 * @param Struct Expected type of the Object or Struct to link to.
	 * @param Requirement Describes if the external data is expected to be required or optional.
	 */
	void LinkExternalData(FStateTreeExternalDataHandle& Handle, const UStruct* Struct, const EStateTreeExternalDataRequirement Requirement)
	{
		if (!ensureMsgf(Schema == nullptr || Schema->IsExternalItemAllowed(*Struct),
				TEXT("External data of type '%s' used by current node is not allowed by schema '%s' (i.e. rejected by IsExternalItemAllowed)"),
				*Struct->GetName(),
				*Schema->GetClass()->GetName()))
		{
			Handle = FStateTreeExternalDataHandle();
			Status = EStateTreeLinkerStatus::Failed;
			return;
		}
		
		const FStateTreeExternalDataDesc Desc(Struct, Requirement);
		int32 Index = ExternalDataDescs.Find(Desc);
		if (Index == INDEX_NONE)
		{
			Index = ExternalDataDescs.Add(Desc);
			check(FStateTreeExternalDataHandle::IsValidIndex(Index + ExternalDataBaseIndex));
			ExternalDataDescs[Index].Handle.DataViewIndex = FStateTreeIndex8(Index + ExternalDataBaseIndex);
		}
		Handle.DataViewIndex = FStateTreeIndex8(Index + ExternalDataBaseIndex);
	}
	/**
	 * Links reference to a property in instance data.
	 * Usage:
	 * 	  Linker.LinkRuntimeDataProperty(HitPointsHandle, STATETREE_INSTANCEDATA_PROPERTY(FHitPointLayout, HitPoints));
	 *
	 * @param Handle Reference to TStateTreeExternalDataHandle<> with USTRUCT type to link to.
	 * @param DummyProperty Do not use directly.
	 * @param ScriptStruct Do not use directly.
	 * @param PropertyName Do not use directly.
	 */
	template <typename T, typename S>
	void LinkInstanceDataProperty(T& Handle, const S& DummyProperty, const UScriptStruct* ScriptStruct, const TCHAR* PropertyName)
	{
		static_assert(TIsSame<typename T::DataType, S>::Value, "Expecting linked handle to have same type as the instance data struct member.");
		LinkInstanceDataPropertyInternal(Handle, ScriptStruct, PropertyName);
	}

	/** @return linked external data descriptors. */
	TConstArrayView<FStateTreeExternalDataDesc> GetExternalDataDescs() const { return ExternalDataDescs; }

protected:

	void LinkInstanceDataPropertyInternal(FStateTreeInstanceDataPropertyHandle& Handle, const UScriptStruct* ScriptStruct, const TCHAR* PropertyName)
	{
		check(CurrentInstanceStruct != nullptr);
		check(CurrentInstanceIndex != INDEX_NONE);

		const FProperty* Property = ScriptStruct->FindPropertyByName(FName(PropertyName));
		if (Property == nullptr)
		{
			Handle = FStateTreeInstanceDataPropertyHandle();
			Status = EStateTreeLinkerStatus::Failed;
			return;
		}

		check(FStateTreeIndex8::IsValidIndex(CurrentInstanceIndex));
		check(Property->GetOffset_ForInternal() < MAX_uint16);
		
		Handle.DataViewIndex = FStateTreeIndex8(CurrentInstanceIndex);
		Handle.Type = EStateTreePropertyIndirection::Offset;
		Handle.PropertyOffset = (uint16)Property->GetOffset_ForInternal();
	}

	const UStateTreeSchema* Schema = nullptr;
	EStateTreeLinkerStatus Status = EStateTreeLinkerStatus::Succeeded;
	const UStruct* CurrentInstanceStruct = nullptr;
	int32 CurrentInstanceIndex = INDEX_NONE;
	int32 ExternalDataBaseIndex = 0;
	TArray<FStateTreeExternalDataDesc> ExternalDataDescs;
};