Методы ArrangeOverride и MeasureOverride для позиционирования элементов

/// <summary>
        /// Measure the children
        /// </summary>
        /// <param name="availableSize">Size available</param>
        /// <returns>Size desired</returns>
        protected override Size MeasureOverride(Size availableSize) {
            try {
                this.UpdateScrollInfo(availableSize);
 
                // Figure out range that's visible based on layout algorithm
                int firstVisibleItemIndex = 0;
                int lastVisibleItemIndex = 0;
                GetVisibleRange(ref firstVisibleItemIndex, ref lastVisibleItemIndex);
 
                // We need to access InternalChildren before the generator to work around a bug
                UIElementCollection children = this.InternalChildren;
                IItemContainerGenerator generator = this.ItemContainerGenerator;
 
                // Get the generator position of the first visible data item
                GeneratorPosition startPos = generator.GeneratorPositionFromIndex(firstVisibleItemIndex);
 
                // Get index where we'd insert the child for this position. If the item is realized
                // (position.Offset == 0), it's just position.Index, otherwise we have to add one to
                // insert after the corresponding child
                int childIndex = (startPos.Offset == 0) ? startPos.Index : startPos.Index + 1;
 
                using (generator.StartAt(startPos, GeneratorDirection.Forward, true)) {
                    int itemIndex = firstVisibleItemIndex;
                    while (itemIndex <= lastVisibleItemIndex) {
                        bool newlyRealized = false;
 
                        // Get or create the child
                        UIElement child = generator.GenerateNext(out newlyRealized) as UIElement;
                        if (newlyRealized) {
                            // Figure out if we need to insert the child at the end or somewhere in the middle
                            if (childIndex >= children.Count) {
                                base.AddInternalChild(child);
                            } else {
                                base.InsertInternalChild(childIndex, child);
                            }
                            generator.PrepareItemContainer(child);
                        } else {
                            // The child has already been created, let's be sure it's in the right spot
                            Debug.Assert(child.Equals(children[childIndex]), "Wrong child was generated");
                        }
 
                        // Measurements will depend on layout algorithm
                        child.Measure(GetChildSize());
                        itemIndex += 1;
                        childIndex += 1;
                    }
                }
 
                // Note: this could be deferred to idle time for efficiency
                CleanUpItems(firstVisibleItemIndex, lastVisibleItemIndex);
            } catch (ArgumentOutOfRangeException) {
                // No idea if we can ignore this
            } catch (NullReferenceException) {
                // No idea if we can ignore this
            }
 
            // Guard against possible infinity if exiting measure early
            return new Size(double.IsInfinity(availableSize.Width) ? 0 : availableSize.Width, double.IsInfinity(availableSize.Height) ? 0 : availableSize.Height);
        }
 
        /// <summary>
        /// Arrange the children
        /// </summary>
        /// <param name="finalSize">Size available</param>
        /// <returns>Size used</returns>
        protected override Size ArrangeOverride(Size finalSize) {
            IItemContainerGenerator generator = this.ItemContainerGenerator;
 
            UpdateScrollInfo(finalSize);
 
            for (int i = 0; i <= this.Children.Count - 1; i++) {
                UIElement child = this.Children[i];
 
                // Map the child offset to an item offset
                int itemIndex = generator.IndexFromGeneratorPosition(new GeneratorPosition(i, 0));
 
                ArrangeChild(itemIndex, child, finalSize);
            }
 
            return finalSize;
        }