UniformGrid for Silverlight

<UserControl x:Class="UniformGrid.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:local="clr-namespace:System.Windows.Controls.Primitives">
<Grid x:Name="LayoutRoot" Width="600" Height="450">
    <local:UniformGrid Columns="2">
        <Image Source="/UniformGrid;component/Chicago.jpg" Stretch="Uniform" />
        <Grid>
            <Image Source="/UniformGrid;component/London.jpg" Stretch="Uniform" />
            <TextBlock VerticalAlignment="Center" HorizontalAlignment="Center" Foreground="White" Opacity=".5" FontSize="52">london</TextBlock>
        </Grid>
        <Image Source="/UniformGrid;component/Neuschwanstein.jpg" Stretch="Uniform" />
        <local:UniformGrid Columns="3" Margin="2">
            <Button Content="Blue" Background="Blue" />
            <Button Content="Red" Background="Red" />
            <Button Content="Yellow" Background="Yellow" />
            <Button Content="Green" Background="Green" />
            <Button Content="Purple" Background="Purple" />
            <Button Content="Teal" Background="Teal" />
        </local:UniformGrid>
    </local:UniformGrid>
</Grid>
</UserControl>

// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License (Ms-PL).
// Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
// All other rights reserved.

using System;
using System.Windows;
using System.Windows.Controls;

namespace System.Windows.Controls.Primitives
{
    /// <summary>
    /// A UniformGrid implementation for Silverlight that arranges children in a
    /// grid with all equal cell sizes.
    /// </summary>
    public class UniformGrid : Panel
    {
        /// <summary>
        /// Gets or sets the computed row value.
        /// </summary>
        private int ComputedRows { get; set; }

        /// <summary>
        /// Gets or sets the computed column value.
        /// </summary>
        private int ComputedColumns { get; set; }

        /// <summary>
        /// Initializes a new instance of UniformGrid.
        /// </summary>
        public UniformGrid()
        {
        }

        /// <summary>
        /// Gets or sets the number of first columns to leave blank.
        /// </summary>
        public int FirstColumn
        {
            get { return (int)GetValue(FirstColumnProperty); }
            set { SetValue(FirstColumnProperty, value); }
        }

        /// <summary>
        /// The FirstColumnProperty dependency property.
        /// </summary>
        public static readonly DependencyProperty FirstColumnProperty =
                DependencyProperty.Register(
                        "FirstColumn",
                        typeof(int),
                        typeof(UniformGrid),
                        new PropertyMetadata(0, OnIntegerDependencyPropertyChanged));

        /// <summary>
        /// Gets or sets the number of columns in the grid. A value of zero 
        /// indicates that the count should be dynamically computed based on the
        /// number of rows and the number of non-collapsed children in the grid.
        /// </summary>
        public int Columns
        {
            get { return (int)GetValue(ColumnsProperty); }
            set { SetValue(ColumnsProperty, value); }
        }

        /// <summary>
        /// DependencyProperty for the Columns property.
        /// </summary>
        public static readonly DependencyProperty ColumnsProperty =
                DependencyProperty.Register(
                        "Columns",
                        typeof(int),
                        typeof(UniformGrid),
                        new PropertyMetadata(0, OnIntegerDependencyPropertyChanged));

        /// <summary>
        /// Validate the new property value and silently revert if the new value
        /// is not appropriate. Used in place of WPF value coercian by the 
        /// dependency properties in UniformGrid.
        /// </summary>
        /// <param name="o">The dependency object.</param>
        /// <param name="e">The dependency property.</param>
        private static void OnIntegerDependencyPropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            // Silently coerce the value back to >= 0 if negative.
            if (!(e.NewValue is int) || (int)e.NewValue < 0)
            {
                o.SetValue(e.Property, e.OldValue);
            }
        }

        /// <summary>
        /// Gets or sets the number of rows in the grid. A value of zero 
        /// indicates that the row count should be dynamically computed based on
        /// the number of columns and the number of non-collapsed children in
        /// the grid.
        /// </summary>
        public int Rows
        {
            get { return (int)GetValue(RowsProperty); }
            set { SetValue(RowsProperty, value); }
        }

        /// <summary>
        /// The Rows DependencyProperty.
        /// </summary>
        public static readonly DependencyProperty RowsProperty =
                DependencyProperty.Register(
                        "Rows",
                        typeof(int),
                        typeof(UniformGrid),
                        new PropertyMetadata(0, OnIntegerDependencyPropertyChanged));

        /// <summary>
        /// Compute the desired size of the UniformGrid by measuring all of the
        /// children with a constraint equal to a cell's portion of the given
        /// constraint. The maximum child width and maximum child height are 
        /// tracked, and then the desired size is computed by multiplying these
        /// maximums by the row and column count.
        /// </summary>
        /// <param name="constraint">The size constraint.</param>
        /// <returns>Returns the desired size.</returns>
        protected override Size MeasureOverride(Size constraint)
        {
            UpdateComputedValues();

            Size childConstraint = new Size(constraint.Width / ComputedColumns, constraint.Height / ComputedRows);
            double maxChildDesiredWidth = 0.0;
            double maxChildDesiredHeight = 0.0;

            //  Measure each child, keeping track of max desired width & height.
            for (int i = 0, count = Children.Count; i < count; ++i)
            {
                UIElement child = Children[i];
                child.Measure(childConstraint);
                Size childDesiredSize = child.DesiredSize;
                if (maxChildDesiredWidth < childDesiredSize.Width)
                {
                    maxChildDesiredWidth = childDesiredSize.Width;
                }
                if (maxChildDesiredHeight < childDesiredSize.Height)
                {
                    maxChildDesiredHeight = childDesiredSize.Height;
                }
            }
            return new Size((maxChildDesiredWidth * ComputedColumns), (maxChildDesiredHeight * ComputedRows));
        }

        /// <summary>
        /// Arrange the children of the UniformGrid by distributing space evenly
        /// among the children, making each child the size equal to a cell
        /// portion of the arrangeSize parameter.
        /// </summary>
        /// <param name="arrangeSize">The arrange size.</param>
        /// <returns>Returns the updated Size.</returns>
        protected override Size ArrangeOverride(Size arrangeSize)
        {
            Rect childBounds = new Rect(0, 0, arrangeSize.Width / ComputedColumns, arrangeSize.Height / ComputedRows);
            double xStep = childBounds.Width;
            double xBound = arrangeSize.Width - 1.0;
            childBounds.X += childBounds.Width * FirstColumn;

            // Arrange and Position each child to the same cell size
            foreach (UIElement child in Children)
            {
                child.Arrange(childBounds);
                if (child.Visibility != Visibility.Collapsed)
                {
                    childBounds.X += xStep;
                    if (childBounds.X >= xBound)
                    {
                        childBounds.Y += childBounds.Height;
                        childBounds.X = 0;
                    }
                }
            }

            return arrangeSize;
        }

        /// <summary>
        /// If the Rows or Columns values are set to 0, dynamically compute the
        /// values based on the actual number of non-collapsed children.
        /// </summary>
        /// <remarks>
        /// In the case when both Rows and Columns are set to 0, the Rows and 
        /// Columns will be equal, laying out a square grid.
        /// </remarks>
        private void UpdateComputedValues()
        {
            ComputedColumns = Columns;
            ComputedRows = Rows;

            // Reset the first column. This is the same logic performed by WPF.
            if (FirstColumn >= ComputedColumns)
            {
                FirstColumn = 0;
            }

            if ((ComputedRows == 0) || (ComputedColumns == 0))
            {
                int nonCollapsedCount = 0;
                for (int i = 0, count = Children.Count; i < count; ++i)
                {
                    UIElement child = Children[i];
                    if (child.Visibility != Visibility.Collapsed)
                    {
                        nonCollapsedCount++;
                    }
                }
                if (nonCollapsedCount == 0)
                {
                    nonCollapsedCount = 1;
                }
                if (ComputedRows == 0)
                {
                    if (ComputedColumns > 0)
                    {
                        ComputedRows = (nonCollapsedCount + FirstColumn + (ComputedColumns - 1)) / ComputedColumns;
                    }
                    else
                    {
                        ComputedRows = (int)Math.Sqrt(nonCollapsedCount);
                        if ((ComputedRows * ComputedRows) < nonCollapsedCount)
                        {
                            ComputedRows++;
                        }
                        ComputedColumns = ComputedRows;
                    }
                }
                else if (ComputedColumns == 0)
                {
                    ComputedColumns = (nonCollapsedCount + (ComputedRows - 1)) / ComputedRows;
                }
            }
        }
    }
}