UE4.27实现UMG长截图

原理是将UI组件渲染到材质，再将材质保存为图片。

    FString SavePath = FPaths::ProjectDir() + TEXT("Saved/Screenshots/screenshot.png");

    FWidgetRenderer* wr = new FWidgetRenderer(true, false);
    UUserWidget * widget = FindChild<UUserWidget>(GetRootWidget(), TEXT("SBox_XXXX")); //要截图的组件
    TSharedPtr<SWidget> SlateWidget(widget->TakeWidget());
    FVector2D boxsize = widget->GetDesiredSize();
    UTextureRenderTarget2D* RenderTarget = wr->DrawWidget(SlateWidget.ToSharedRef(),boxsize);
    TArray<FColor> OutBMP;
    FRenderTarget* RTResource = RenderTarget->GameThread_GetRenderTargetResource();
    RTResource->ReadPixels(OutBMP);
    FIntPoint destSize(RenderTarget->GetSurfaceWidth(), RenderTarget->GetSurfaceHeight());
    TArray<uint8> CompressedBitmap;
    FImageUtils::CompressImageArray(destSize.X, destSize.Y, OutBMP, CompressedBitmap);
    bool imageSavedOk = FFileHelper::SaveArrayToFile(CompressedBitmap, *SavePath);