GetWidgetSize.cpp

#include "Framework/Application/SlateApplication.h"
#include "Layout/LayoutUtils.h"
#include "Layout/ArrangedChildren.h"

void ArrangeChildrenRecursive(FArrangedChildren& Childs, FArrangedWidget& ParentWidget)
{
	FArrangedChildren arrangedChilds (EVisibility::All);
	
	ParentWidget.Widget->ArrangeChildren(ParentWidget.Geometry, arrangedChilds);
	Childs.Append(arrangedChilds); // add arranged children to array
	
	for (auto& arrangedChild: arrangedChilds.GetInternalArray())
	{
		ArrangeChildrenRecursive(Childs, arrangedChild); // walk through recursively
	}
}

FVector2D UMyWidget::GetWidgetSize()
{
	FVector2D DesiredWidgetSize;
	FArrangedChildren Childs(EVisibility::All);
	auto thisWidget = TakeWidget();
	auto parent = GetCachedWidget()->GetParentWidget();
	
	// search for first parent with valid geometry
	while (parent.IsValid() && parent->GetCachedGeometry().Size.SizeSquared()==0)
	{
		parent = parent->GetParentWidget();
	}
	
	// calculate size for all children down form found parent
	FArrangedWidget parentArrangedWidget(parent.ToSharedRef(), parent->GetCachedGeometry());

	ArrangeChildrenRecursive(Childs, parentArrangedWidget);

	// search for desired widget (thisWidget)
	for (auto& child: Childs.GetInternalArray())
	{
		if (child.Widget == thisWidget)
		{
			// desired size found
			DesiredWidgetSize = child.Geometry.Size;
			break;
		}
	}
	
	return DesiredWidgetSize;
 }