UE4 绘制Gizmo

Unity的Gizmos可以很方便的在编辑器下进行调试，Unreal中也有一些办法可以达到效果。

本文主要参考：https://zhuanlan.zhihu.com/p/363625037，进行了一些简化。并在Unreal 4.27中实现。

具体流程如下：

1. 需要绘制Gizmo的Actor挂载继承UPrimitiveComponent的组件；该组件重写了CreateSceneProxy方法，这个方法里可以拿到PDI绘制
2. 然后进行这个组件的编写(继承UPrimitiveComponent实际上也继承了USceneComponent)，手动挂载到Actor上就可以绘制Gizmo了
3. 再在Actor的构造函数中编写自动挂载该组件的逻辑，方便使用

先来编写绘制Gizmo的组件，这里命名为UMyPrimitiveComponent：

MyPrimitiveComponent.h (MYPROJECT_API注意替换)

//MyPrimitiveComponent.h

#pragma once

#include "CoreMinimal.h"
#include "Components/PrimitiveComponent.h"
#include "PrimitiveSceneProxy.h"
#include "MyPrimitiveComponent.generated.h"

UCLASS(ClassGroup = (Custom), meta = (BlueprintSpawnableComponent))
class MYPROJECT_API UMyPrimitiveComponent : public UPrimitiveComponent
{
    GENERATED_BODY()
    
public:
    //绘制逻辑主要在这里
    virtual FPrimitiveSceneProxy* CreateSceneProxy() override;

    //如果要在非选中情况下始终绘制的话，需要有Bounds信息，所以要重写该函数
    virtual FBoxSphereBounds CalcBounds(const FTransform& LocalToWorld) const;
};

MyPrimitiveComponent.cpp

//MyPrimitiveComponent.cpp
#include "MyPrimitiveComponent.h"

FPrimitiveSceneProxy* UMyPrimitiveComponent::CreateSceneProxy()
{
    class FMySceneProxy : public FPrimitiveSceneProxy
    {
    public:

        SIZE_T GetTypeHash() const override
        {
            static size_t UniquePointer;
            return reinterpret_cast(&UniquePointer);
        }

        FMySceneProxy(const UPrimitiveComponent* InComponent) : FPrimitiveSceneProxy(InComponent)
        {
            CacheInComponent = InComponent;
            bWillEverBeLit = false;
        }

        virtual void GetDynamicMeshElements(const TArray<const FSceneView*>& Views, const FSceneViewFamily& ViewFamily, uint32 VisibilityMap, FMeshElementCollector& Collector) const override
        {
            QUICK_SCOPE_CYCLE_COUNTER(STAT_Draw3DAgentSceneProxy_GetDynamicMeshElements);

            for (int32 ViewIndex = 0; ViewIndex < Views.Num(); ViewIndex++)
            {
                if (VisibilityMap & (1 << ViewIndex))
                {
                    const FSceneView* View = Views[ViewIndex];
                    FPrimitiveDrawInterface* PDI = Collector.GetPDI(ViewIndex);

                    //拿到Actor的矩阵绘制，而不是Component自己的
                    const FMatrix& LocalToWorld = CacheInComponent->GetTypedOuter()->GetTransform().ToMatrixWithScale();

                    //绘制函数可以去看下PrimitiveDrawingUtils.cpp
                    DrawOrientedWireBox(PDI
                        , LocalToWorld.TransformPosition(FVector::ZeroVector)
                        , LocalToWorld.GetScaledAxis(EAxis::X)
                        , LocalToWorld.GetScaledAxis(EAxis::Y)
                        , LocalToWorld.GetScaledAxis(EAxis::Z)
                        , FVector(100.0, 100.0, 100.0)
                        , FLinearColor(1.0, 0.0, 0.0, 1.0)
                        , SDPG_World
                        , 1.0);
                }
            }
        }
        virtual FPrimitiveViewRelevance GetViewRelevance(const FSceneView* View) const override
        {
            /*const bool bVisibleForSelection = IsSelected();
            const bool bShowForCollision = View->Family->EngineShowFlags.Collision && IsCollisionEnabled();

            FPrimitiveViewRelevance Result;
            Result.bDrawRelevance = (IsShown(View) && bVisibleForSelection) || bShowForCollision;
            Result.bDynamicRelevance = true;
            Result.bShadowRelevance = IsShadowCast(View);
            Result.bEditorPrimitiveRelevance = true;
            Result.bEditorNoDepthTestPrimitiveRelevance = true; */
            //上面这段表示选中绘制Gizmo

            FPrimitiveViewRelevance Result;
            Result.bDrawRelevance = true;
            Result.bDynamicRelevance = true;
            Result.bShadowRelevance =false;
            Result.bEditorPrimitiveRelevance = true;
            Result.bEditorNoDepthTestPrimitiveRelevance = true;
            //这段表示始终绘制

            return Result;
        }

        virtual uint32 GetMemoryFootprint(void) const override { return(sizeof(*this) + GetAllocatedSize()); }
        uint32 GetAllocatedSize(void) const { return(FPrimitiveSceneProxy::GetAllocatedSize()); }

    private:
        const UPrimitiveComponent* CacheInComponent;
    };

    return new FMySceneProxy(this);
}

FBoxSphereBounds UMyPrimitiveComponent::CalcBounds(const FTransform& LocalToWorld) const
{
    return FBoxSphereBounds(FBox(FVector(-50, -50, -50), FVector(50, 50, 50))).TransformBy(LocalToWorld);
    //因为缩放是1，这里填的就是实际尺寸，也可以遍历所有组件取最大Bounds.
}

然后手动再挂载一下组件，就有效果了：

如果需要像Unity那样；直接就有Gizmo，可以在Actor构造函数中编写逻辑自动挂载组件：

AMyTestActor::AMyTestActor()
{
    UBBoxActorGizmoComponent* Gizmo = CreateDefaultSubobject(TEXT("Gizmo"));
    if (Gizmo)
    {
        Gizmo->SetupAttachment(GetRootComponent());
    }
}