1、通过MeshDescription在运行时构建StaticMesh并设置碰撞、修改材质

1、MyActor.h

//MyActor.h

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "MyActor.generated.h"

UCLASS()
class CONTROLLER_API AMyActor : public AActor
{
	GENERATED_BODY()
	
public:	
	// Sets default values for this actor's properties
	AMyActor();
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "Polygon")
	TObjectPtr<class UStaticMeshComponent> PolygonComponent;
	FPolygonGroupID GroupId;
	UPROPERTY(EditAnywhere, Category = "Polygon")
	TArray<FVector> Coodinates;
	TObjectPtr<class UStaticMesh> Polygon;
	UPROPERTY(EditAnywhere, Category = "Polygon")
	UMaterialInterface* Material;
protected:
	// Called when the game starts or when spawned
	virtual void BeginPlay() override;
	virtual void OnConstruction(const FTransform& Transform) override;
public:	
	// Called every frame
	virtual void Tick(float DeltaTime) override;
	TObjectPtr<class UStaticMesh> BuildPolygon(TArray<FVector> InPositions);

};

2、MyActor.cpp

//MyActor.cpp


#include "MyActor.h"

#include "StaticMeshAttributes.h"

#include "CompGeom/PolygonTriangulation.h"
#include <StaticMeshOperations.h>
#include <MeshDescriptionBuilder.h>

// Sets default values
AMyActor::AMyActor()
{
 	// Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
	PrimaryActorTick.bCanEverTick = true;
	PolygonComponent = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("PolygonComponent"), false);
	SetRootComponent(PolygonComponent);
}

// Called when the game starts or when spawned
void AMyActor::BeginPlay()
{
	Super::BeginPlay();
	if(Coodinates.Num())
	{
		Material = LoadObject<UMaterial>(this, TEXT("/Engine/BasicShapes/BasicShapeMaterial.BasicShapeMaterial"));
		PolygonComponent->SetStaticMesh(BuildPolygon(Coodinates));
		PolygonComponent->SetMaterial(0,Material);
		//PolygonComponent->SetForcedLodModel(1);

	}
}

void AMyActor::OnConstruction(const FTransform& Transform)
{
	if (!IsTemplate()) {
		if (Coodinates.Num())
		{
			//加载虚幻引擎自带的材质
			Material = LoadObject<UMaterial>(this, TEXT("/Engine/BasicShapes/BasicShapeMaterial.BasicShapeMaterial"));
			//将生成的StaticMesh挂载到StaticMeshComponent上
			BuildPolygon(Coodinates);
			PolygonComponent->SetStaticMesh(Polygon);
			//在StaticMeshComponent上设置材质，坑1：StaticMesh也有一个SetMaterial方法，但是它是仅在编辑器状态下可用的，调用了这个方法打包时会报错，打包就失败了，而StaticMeshComponent的SetMaterial方法没有这个问题，且材质能够成功应用到StaticMesh上
			PolygonComponent->SetMaterial(0, Material);
		}
	}
}

// Called every frame
void AMyActor::Tick(float DeltaTime)
{
	Super::Tick(DeltaTime);

}
FBox2D computeMinRectangle(TArray<FVector2D> InPositions)
{
	FBox2D result;
	uint8 num = InPositions.Num();
	if (num < 2)
	{
		return result;
	}
	const double doubleMin = std::numeric_limits<double>::min();
	const double doubleMax = std::numeric_limits<double>::max();

	double minX = doubleMax, minY = doubleMax, maxX = doubleMin, maxY = doubleMin;
	for (uint8 i = 0; i < num; ++i)
	{
		FVector2D currentPoint = InPositions[i];

		maxX = maxX > currentPoint.X ? maxX : currentPoint.X;
		maxY = maxY > currentPoint.Y ? maxY : currentPoint.Y;

		minX = minX < currentPoint.X ? minX : currentPoint.X;
		minY = minY < currentPoint.Y ? minY : currentPoint.Y;
	}
	result.Max = FVector2D(maxX, maxY);
	result.Min = FVector2D(minX, minY);

	return result;
}
TObjectPtr<class UStaticMesh> AMyActor::BuildPolygon(TArray<FVector> InPositions)
{
	if (InPositions.Num()) {
		TSet<FVector> UniquePoints;

		for (int32 i = 0; i < InPositions.Num(); ++i)
		{
			if (!UniquePoints.Contains(InPositions[i]))
			{
				UniquePoints.Add(InPositions[i]);
			}
		}
		InPositions.Empty();
		InPositions = UniquePoints.Array();
	}
	if (InPositions.Num() < 3) {
		UE_LOG(LogTemp, Error, TEXT("Error:After removing duplicate points,the length of the coordinates array must be greater than or equal to 3!!"))
			return nullptr;
	}
	FVector4f Color(1.0, 1.0, 1.0, 1.0);
	Polygon = NewObject<UStaticMesh>(this, FName("Polygon"));
	//坑2：添加材质插槽，StaticMesh有个名为AddMaterial的方法，看名字像是添加材质插槽，但是实际使用上，打包后不生效，而采用“GetStaticMaterials().Add(FStaticMaterial())”这种方式可以
	Polygon->GetStaticMaterials().Add(FStaticMaterial());

	FMeshDescription meshDesc;
	FStaticMeshAttributes attributes(meshDesc);
	//注册默认的静态模型属性
	attributes.Register();

	//FMeshDescriptionBuilder用于设置顶点、法线、UV坐标等信息
	FMeshDescriptionBuilder meshDescBuilder;
	meshDescBuilder.SetMeshDescription(&meshDesc);
	//允许使用多边形组
	meshDescBuilder.EnablePolyGroups();
	//设置UV坐标的级数为1
	meshDescBuilder.SetNumUVLayers(1);

	TArray<FVertexID> vertexIDs;
	const int count = InPositions.Num();
	vertexIDs.SetNum(count);
	TArray<FVector2D> Positions2D;
	for (int32 i = 0; i < count; ++i) {
		FVector& position = InPositions[i];
		vertexIDs[i] = meshDescBuilder.AppendVertex(position);
		Positions2D.Add(FVector2D(position.X, position.Y));
	}
	//创建一个多边形组,每个多边形组可以单独赋予不同的材质
	GroupId = meshDescBuilder.AppendPolygonGroup();


	TArray<UE::Geometry::FIndex3i> polyTriangleIndices;
	TArray<int32> PolyTriangleIndices;
	bool bOrientAsHoleFill = true;
	//耳切法生成多边形三角网格的顶点索引
	PolygonTriangulation::TriangulateSimplePolygon(InPositions, polyTriangleIndices, bOrientAsHoleFill);
	for (UE::Geometry::FIndex3i& indice : polyTriangleIndices) {
		PolyTriangleIndices.Add(indice.A);
		PolyTriangleIndices.Add(indice.B);
		PolyTriangleIndices.Add(indice.C);

	}
	//用于计算纹理坐标
	const FBox2D box = computeMinRectangle(Positions2D);
	auto ComputeUV = [&box](FVector InPosition) {
		FVector2D coord;
		coord.X = (InPosition.X - box.Min.X) / 100.0;
		coord.Y = (InPosition.Y - box.Min.Y) / 100.0;
		return coord;
		};
		
	for (int32 i = 0; i < polyTriangleIndices.Num(); ++i) {
		//将顶点添加到meshDescBuild中并获得顶点索引
		FVertexInstanceID index1 = meshDescBuilder.AppendInstance(vertexIDs[PolyTriangleIndices[i * 3]]);
		//设置指定索引的顶点的uv坐标
		meshDescBuilder.SetInstanceUV(index1, ComputeUV(meshDescBuilder.GetPosition(vertexIDs[PolyTriangleIndices[i * 3]])), 0);
		//设置指定索引的顶点的颜色
		meshDescBuilder.SetInstanceColor(index1, Color);

		FVertexInstanceID index2 = meshDescBuilder.AppendInstance(vertexIDs[PolyTriangleIndices[i * 3 + 1]]);
		meshDescBuilder.SetInstanceUV(index2, ComputeUV(meshDescBuilder.GetPosition(vertexIDs[PolyTriangleIndices[i * 3 + 1]])), 0);
		meshDescBuilder.SetInstanceColor(index2, Color);

		FVertexInstanceID index3 = meshDescBuilder.AppendInstance(vertexIDs[PolyTriangleIndices[i * 3 + 2]]);
		meshDescBuilder.SetInstanceUV(index3, ComputeUV(meshDescBuilder.GetPosition(vertexIDs[PolyTriangleIndices[i * 3 + 2]])), 0);
		meshDescBuilder.SetInstanceColor(index3, Color);
		//设置三角形网格的顶点索引，并指定该网格属于哪个多边形组
		meshDescBuilder.AppendTriangle(index1, index2, index3, GroupId);

	}

	UStaticMesh::FBuildMeshDescriptionsParams mdParams;
	mdParams.bBuildSimpleCollision = true;//构建简单碰撞
	mdParams.bFastBuild = true;//快速构建
	
	//计算三角形网格的切线和法线
	FStaticMeshOperations::ComputeTriangleTangentsAndNormals(meshDesc);
	
	TArray<const FMeshDescription*> meshDescPtrs;
	meshDescPtrs.Emplace(&meshDesc);
	//构建StaticMesh(构建Mesh的方法不止这一种，其中有种方法要调用InitResources，然后还要设置Lod，但是通过BuildFromMeshDescriptions方法不需要这么做，其方法内部已经封装好了)
	Polygon->BuildFromMeshDescriptions(meshDescPtrs, mdParams);
	
	//坑3：如果要给StaticMeshComponent绑定鼠标的click、hover等时间的话，需要调用下面这段代码以启用复杂碰撞，因为click等事件也是基于射线检测进行判断的，但是它检测的是复杂碰撞(TraceComplex参数值为true)，因此如果不调用下面这段代码，click等事件是不会生效的。注意：这里不要创建BodySetup,在前面的BuildFromMeshDescriptions方法的函数实现里面已经为StaticMesh创建了一个BodySetup，并为其赋予了一些值。
		if (UBodySetup* bodySetup = Polygon->GetBodySetup())
	{
		bodySetup->DefaultInstance.SetCollisionProfileName(UCollisionProfile::BlockAll_ProfileName);
		bodySetup->CollisionTraceFlag = CTF_UseSimpleAsComplex;
		bodySetup->bGenerateMirroredCollision = false;
	}
	return Polygon;
}

3、总结
1）运行时修改StaticMesh的材质要调用StaticMeshComponent的SetMaterial方法而不是StaticMesh的SetMaterial方法，因为StaticMesh的SetMaterial方法仅在编辑器状态下可用，会导致你打包失败；
2）想要在运行时修改StaticMesh的材质，首先要确保StaticMesh有材质插槽，为StaticMesh增加材质插槽的方法如上，一定不能使用StaticMesh的AddMaterial方法，运行时状态下无效；
3）想要给StaticMesh绑定click、hover等事件，需要为其启用复杂碰撞，代码如上，但是不使用那段代码也有碰撞，只是click、hover等事件不生效。