虚幻引擎程序化资源生成框架PCG 之 UPCGBlueprintElement源码笔记（一）

UPCGBlueprintElement是PCGGraph中自定义节点的基类，但官方目前还没有给出详细的文档，所以从源代码里找点答案。

可覆盖函数（Override Functions）

UPCGBlueprintElement是PCGGraph中所有自定义节点的基类，有如下几个可供蓝图覆盖的函数：

• Excute
• Excute with Context
• Iteration Loop Body
• Point Loop Body
• Variable Loop Body
• Nested Loop Body
• Node Color Override
• Node Title Override
• Node Type Override

其中 Excute、Excute with Context、NodeTitleOverride、NodeColorOverride、NodeTypeOverride是覆盖后被自动触发的；而剩余的几个“Loop Body函数”则需要在蓝图中显示调用对应的“Loop函数”才可以执行循环，比如：要调用PointLoop才可以执行Point Loop Body

Excute 和 Excute with Context

这两个函数可以理解为自定义节点的“主函数”,区别就是一个有PCGContext输入另一个没有，ExecuteWithContext会调用Execute

Excute

	UFUNCTION(BlueprintImplementableEvent, BlueprintCallable, Category = "PCG|Execution")void Execute(const FPCGDataCollection& Input, FPCGDataCollection& Output);

Excute with Context

	UFUNCTION(BlueprintNativeEvent, Category = "PCG|Execution")void ExecuteWithContext(UPARAM(ref)FPCGContext& InContext, const FPCGDataCollection& Input, FPCGDataCollection& Output);

void UPCGBlueprintElement::ExecuteWithContext_Implementation(FPCGContext& InContext, const FPCGDataCollection& Input, FPCGDataCollection& Output)
{Execute(Input, Output);
}

其实还有个C++ 函数ExecuteInternal，Excute with Context是由它调用的。

virtual bool ExecuteInternal(FPCGContext* Context) const override;

	/** Finally, execute the actual blueprint */Context->BlueprintElementInstance->ExecuteWithContext(*Context, Context->InputData, Context->OutputData);

Loop Body函数和Loop函数

Loop Body函数都是在Loop函数中定义的lambda表达式调用的。

• Point Loop Body、Nested Loop Body、Iteration Loop Body通过FPCGAsync::AsyncPointProcessing调用；
• Variable Loop Body通过FPCGAsync::AsyncMultiPointProcessing调用
• 上述节点中的各种Data参数都是PCGPointData类型，需要的话可以在调用Loop函数的时候传入，它们会直接传入Loop Body函数

Point Loop Body和PointLoop

Point Loop Body需要通过PointLoop执行循环

Point Loop Body

PointLoop

	FPCGAsync::AsyncPointProcessing(&InContext, InPoints.Num(), OutPoints, [this, &InContext, InData, OutData, &InPoints](int32 Index, FPCGPoint& OutPoint){return PointLoopBody(InContext, InData, InPoints[Index], OutPoint, OutData->Metadata);});

Variable Loop Body和VariableLoop

Variable Loop Body需要通过VariableLoop执行循环

VariableLoop

	FPCGAsync::AsyncMultiPointProcessing(&InContext, InPoints.Num(), OutPoints, [this, &InContext, InData, OutData, &InPoints](int32 Index){return VariableLoopBody(InContext, InData, InPoints[Index], OutData->Metadata);});
}

FPCGAsync::AsyncPointProcessing和FPCGAsync::AsyncMultiPointProcessing都是异步的，它们的区别就在于处理输入:输出为1:1还是1:N的循环。Point Loop Body和VariableLoopBody,前者输入一个point返回一个point；后者输入一个point可以返回一个point的数组。比如：如果你想把一个point看作一块地基，想在它的垂直上方再生成一串点作为楼层，然后把这些点返回，那么你就需要用到VariableLoopBody。

Nested Loop Body和NestedLoop

Nested Loop Body需要通过NestedLoop执行循环

关键代码如下，但使用情景没想明白。

NestedLoop

	const TArray<FPCGPoint>& InOuterPoints = InOuterData->GetPoints();const TArray<FPCGPoint>& InInnerPoints = InInnerData->GetPoints();TArray<FPCGPoint>& OutPoints = OutData->GetMutablePoints();FPCGAsync::AsyncPointProcessing(&InContext, InOuterPoints.Num() * InInnerPoints.Num(), OutPoints, [this, &InContext, InOuterData, InInnerData, OutData, &InOuterPoints, &InInnerPoints](int32 Index, FPCGPoint& OutPoint){return NestedLoopBody(InContext, InOuterData, InInnerData, InOuterPoints[Index / InInnerPoints.Num()], InInnerPoints[Index % InInnerPoints.Num()], OutPoint, OutData->Metadata);});

Iteration Loop Body和IterationLoop

Iteration Loop Body需要通过IterationLoop执行循环

和Point Loop Body一样，但它会提供一个NumIterations用于指定迭代次数

IterationLoop

	FPCGAsync::AsyncPointProcessing(&InContext, NumIterations, OutPoints, [this, &InContext, InA, InB, OutData](int32 Index, FPCGPoint& OutPoint){return IterationLoopBody(InContext, Index, InA, InB, OutPoint, OutData->Metadata);});