internal WrapItem GetItemAt(int index)
        {
            if (index < 0)
            {
                throw new IndexOutOfRangeException();
            }

            if (index <= (_items.Count - 1))
            {
                return(_items[index]);
            }
            else
            {
                WrapItem item = new WrapItem(index);
                _items.Add(item);
                return(item);
            }
        }
        /// <inheritdoc />
        protected override Size MeasureOverride(VirtualizingLayoutContext context, Size availableSize)
        {
            var totalMeasure      = UvMeasure.Zero;
            var parentMeasure     = new UvMeasure(Orientation, availableSize.Width, availableSize.Height);
            var spacingMeasure    = new UvMeasure(Orientation, HorizontalSpacing, VerticalSpacing);
            var realizationBounds = new UvBounds(Orientation, context.RealizationRect);
            var position          = UvMeasure.Zero;

            var state = (WrapLayoutState)context.LayoutState;

            if (state.Orientation != Orientation)
            {
                state.SetOrientation(Orientation);
            }

            if (spacingMeasure.Equals(state.Spacing) == false)
            {
                state.ClearPositions();
                state.Spacing = spacingMeasure;
            }

            if (state.AvailableU != parentMeasure.U)
            {
                state.ClearPositions();
                state.AvailableU = parentMeasure.U;
            }

            double currentV = 0;

            for (int i = 0; i < context.ItemCount; i++)
            {
                bool     measured = false;
                WrapItem item     = state.GetItemAt(i);
                if (item.Measure == null)
                {
                    item.Element = context.GetOrCreateElementAt(i);
                    item.Element.Measure(availableSize);
                    item.Measure = new UvMeasure(Orientation, item.Element.DesiredSize.Width, item.Element.DesiredSize.Height);
                    measured     = true;
                }

                UvMeasure currentMeasure = item.Measure.Value;
                if (currentMeasure.U == 0)
                {
                    continue; // ignore collapsed items
                }

                if (item.Position == null)
                {
                    if (parentMeasure.U < position.U + currentMeasure.U)
                    {
                        // New Row
                        position.U  = 0;
                        position.V += currentV + spacingMeasure.V;
                        currentV    = 0;
                    }

                    item.Position = position;
                }

                position = item.Position.Value;

                double vEnd = position.V + currentMeasure.V;
                if (vEnd < realizationBounds.VMin)
                {
                    // Item is "above" the bounds
                    if (item.Element != null)
                    {
                        context.RecycleElement(item.Element);
                        item.Element = null;
                    }
                }
                else if (position.V > realizationBounds.VMax)
                {
                    // Item is "below" the bounds.
                    if (item.Element != null)
                    {
                        context.RecycleElement(item.Element);
                        item.Element = null;
                    }

                    // We don't need to measure anything below the bounds
                    break;
                }
                else if (measured == false)
                {
                    // Always measure elements that are within the bounds
                    item.Element = context.GetOrCreateElementAt(i);
                    item.Element.Measure(availableSize);

                    currentMeasure = new UvMeasure(Orientation, item.Element.DesiredSize.Width, item.Element.DesiredSize.Height);
                    if (currentMeasure.Equals(item.Measure) == false)
                    {
                        // this item changed size; we need to recalculate layout for everything after this
                        state.RemoveFromIndex(i + 1);
                        item.Measure = currentMeasure;

                        // did the change make it go into the new row?
                        if (parentMeasure.U < position.U + currentMeasure.U)
                        {
                            // New Row
                            position.U  = 0;
                            position.V += currentV + spacingMeasure.V;
                            currentV    = 0;
                        }

                        item.Position = position;
                    }
                }

                position.U += currentMeasure.U + spacingMeasure.U;
                currentV    = Math.Max(currentMeasure.V, currentV);
            }

            // update value with the last line
            // if the the last loop is(parentMeasure.U > currentMeasure.U + lineMeasure.U) the total isn't calculated then calculate it
            // if the last loop is (parentMeasure.U > currentMeasure.U) the currentMeasure isn't added to the total so add it here
            // for the last condition it is zeros so adding it will make no difference
            // this way is faster than an if condition in every loop for checking the last item
            totalMeasure.U = parentMeasure.U;

            // Propagating an infinite size causes a crash. This can happen if the parent is scrollable and infinite in the opposite
            // axis to the panel. Clearing to zero prevents the crash.
            // This is likely an incorrect use of the control by the developer, however we need stability here so setting a default that wont crash.
            if (double.IsInfinity(totalMeasure.U))
            {
                totalMeasure.U = 0.0;
            }

            totalMeasure.V = state.GetHeight();

            totalMeasure.U = Math.Ceiling(totalMeasure.U);
            return(Orientation == Orientation.Horizontal ? new Size(totalMeasure.U, totalMeasure.V) : new Size(totalMeasure.V, totalMeasure.U));
        }