override NativeOnDragDetected() in the class which is draping from.

example source:

void UItemSlotWidget::NativeOnDragDetected(const FGeometry& InGeometry,
    const FPointerEvent& InMouseEvent,
    UDragDropOperation*& OutOperation)
{
    Super::NativeOnDragDetected(InGeometry, InMouseEvent, OutOperation);
    RTS_PlayerController->SetInputMode_UI_Only();
    OutOperation = CreateItemDrag();
}


FReply UItemSlotWidget::NativeOnMouseButtonDown(const FGeometry & InGeometry, const FPointerEvent & InMouseEvent)
{
    FReply reply = Super::NativeOnMouseButtonDown(InGeometry, InMouseEvent);
    SetTextToInformationBar();
    if (InMouseEvent.GetEffectingButton() == EKeys::LeftMouseButton) {
        reply.DetectDrag(TakeWidget(), EKeys::LeftMouseButton);
    }
    return reply;
}

UDragDropOperation* UItemSlotWidget::CreateItemDrag() const {
    auto drag_drop_operation = NewObject<UItemDrag>();
    if (RTS_PlayerController && !IsEmpty()) {
        RTS_PlayerController->DragDropOp_property = drag_drop_operation; // for garbage collector
        drag_drop_operation->DraggedItem = const_cast<UItemSlotWidget*>(this);
        drag_drop_operation->ItemImage = currentImage_Texture2D;

        auto DragVisual = CreateWidget<UDraggedItem>(RTS_PlayerController, URTS_FunctionLibrary::GetClassPtrOf_DraggedItem_BP());
        DragVisual->ImageOfDraggedItem = NewObject<UImage>();
        DragVisual->ImageOfDraggedItem->SetBrushFromTexture(currentImage_Texture2D);

        drag_drop_operation->DefaultDragVisual = DragVisual;
        drag_drop_operation->Pivot = EDragPivot::MouseDown;
    }
    return drag_drop_operation;
}

Reference:
https://github.com/absentje/UE4/blob/master/RTS_Game/Source/RTS_Game/Private/UI/Inventory/ItemSlotWidget.cpp

一段路，走了很久依然看不到希望，那就改变方向。生活个性说说大全。