// <copyright>
// This control is created by Ashley Davis and copyrighted under CPOL, and available as part of
// a CodeProject article at
//    http://www.codeproject.com/KB/WPF/zoomandpancontrol.aspx
// <date>This code is based on the article dated: 29 Jun 2010</date>
// </copyright>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Controls;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Diagnostics;
using System.Windows.Threading;
using System.Windows.Controls.Primitives;

namespace SharpVectors.Runtime
{
    /// <summary>
    /// A class that wraps up zooming and panning of it's content.
    /// </summary>
    public partial class ZoomPanControl : ContentControl, IScrollInfo
    {
        #region General Private Fields

        /// <summary>
        /// Reference to the underlying content, which is named PART_Content in the template.
        /// </summary>
        private FrameworkElement content;

        /// <summary>
        /// The transform that is applied to the content to scale it by 'ContentScale'.
        /// </summary>
        private ScaleTransform contentScaleTransform;

        /// <summary>
        /// The transform that is applied to the content to offset it by 'ContentOffsetX' and 'ContentOffsetY'.
        /// </summary>
        private TranslateTransform contentOffsetTransform;

        /// <summary>
        /// Enable the update of the content offset as the content scale changes.
        /// This enabled for zooming about a point (Google-maps style zooming) and zooming to a rect.
        /// </summary>
        private bool enableContentOffsetUpdateFromScale;

        /// <summary>
        /// Used to disable synchronization between IScrollInfo interface and ContentOffsetX/ContentOffsetY.
        /// </summary>
        private bool disableScrollOffsetSync;

        /// <summary>
        /// Normally when content offsets changes the content focus is automatically updated.
        /// This synchronization is disabled when 'disableContentFocusSync' is set to 'true'.
        /// When we are zooming in or out we 'disableContentFocusSync' is set to 'true' because 
        /// we are zooming in or out relative to the content focus we don't want to update the focus.
        /// </summary>
        private bool disableContentFocusSync;

        /// <summary>
        /// The width of the viewport in content coordinates, clamped to the width of the content.
        /// </summary>
        private double constrainedContentViewportWidth;

        /// <summary>
        /// The height of the viewport in content coordinates, clamped to the height of the content.
        /// </summary>
        private double constrainedContentViewportHeight;

        #endregion General Private Fields

        #region IScrollInfo Private Fields

        //
        // These data members are for the implementation of the IScrollInfo interface.
        // This interface works with the ScrollViewer such that when ZoomPanControl is 
        // wrapped (in XAML) with a ScrollViewer the IScrollInfo interface allows the ZoomPanControl to
        // handle the the scrollbar offsets.
        //
        // The IScrollInfo properties and member functions are implemented in ZoomAndPanControl_IScrollInfo.cs.
        //
        // There is a good series of articles showing how to implement IScrollInfo starting here:
        //     http://blogs.msdn.com/bencon/archive/2006/01/05/509991.aspx
        //

        /// <summary>
        /// Set to 'true' when the vertical scrollbar is enabled.
        /// </summary>
        private bool canVerticallyScroll;

        /// <summary>
        /// Set to 'true' when the vertical scrollbar is enabled.
        /// </summary>
        private bool canHorizontallyScroll;

        /// <summary>
        /// Records the unscaled extent of the content.
        /// This is calculated during the measure and arrange.
        /// </summary>
        private Size unScaledExtent;

        /// <summary>
        /// Records the size of the viewport (in viewport coordinates) onto the content.
        /// This is calculated during the measure and arrange.
        /// </summary>
        private Size viewport;

        /// <summary>
        /// Reference to the ScrollViewer that is wrapped (in XAML) around the ZoomPanControl.
        /// Or set to null if there is no ScrollViewer.
        /// </summary>
        private ScrollViewer scrollOwner;

        #endregion IScrollInfo Private Fields

        #region Dependency Property Definitions

        //
        // Definitions for dependency properties.
        //

        public static readonly DependencyProperty ContentScaleProperty =
                DependencyProperty.Register("ContentScale", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(1.0, ContentScale_PropertyChanged, ContentScale_Coerce));

        public static readonly DependencyProperty MinContentScaleProperty =
                DependencyProperty.Register("MinContentScale", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.01, MinOrMaxContentScale_PropertyChanged));

        public static readonly DependencyProperty MaxContentScaleProperty =
                DependencyProperty.Register("MaxContentScale", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(10.0, MinOrMaxContentScale_PropertyChanged));

        public static readonly DependencyProperty ContentOffsetXProperty =
                DependencyProperty.Register("ContentOffsetX", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0, ContentOffsetX_PropertyChanged, ContentOffsetX_Coerce));

        public static readonly DependencyProperty ContentOffsetYProperty =
                DependencyProperty.Register("ContentOffsetY", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0, ContentOffsetY_PropertyChanged, ContentOffsetY_Coerce));

        public static readonly DependencyProperty AnimationDurationProperty =
                DependencyProperty.Register("AnimationDuration", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.4  )); 

        public static readonly DependencyProperty ContentZoomFocusXProperty =
                DependencyProperty.Register("ContentZoomFocusX", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty ContentZoomFocusYProperty =
                DependencyProperty.Register("ContentZoomFocusY", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty ViewportZoomFocusXProperty =
                DependencyProperty.Register("ViewportZoomFocusX", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty ViewportZoomFocusYProperty =
                DependencyProperty.Register("ViewportZoomFocusY", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty ContentViewportWidthProperty =
                DependencyProperty.Register("ContentViewportWidth", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty ContentViewportHeightProperty =
                DependencyProperty.Register("ContentViewportHeight", typeof(double), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(0.0));

        public static readonly DependencyProperty IsMouseWheelScrollingEnabledProperty =
                DependencyProperty.Register("IsMouseWheelScrollingEnabled", typeof(bool), typeof(ZoomPanControl),
                                            new FrameworkPropertyMetadata(false));

        #endregion Dependency Property Definitions

        #region Constructors and Destructor
        
        public ZoomPanControl()
        {
            unScaledExtent = new Size(0, 0);
            viewport = new Size(0, 0);
        }

        /// <summary>
        /// Static constructor to define metadata for the control (and link it to the style in Generic.xaml).
        /// </summary>
        static ZoomPanControl()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(ZoomPanControl), 
                new FrameworkPropertyMetadata(typeof(ZoomPanControl)));
        }

        #endregion

        #region Public Events

        /// <summary>
        /// Event raised when the ContentOffsetX property has changed.
        /// </summary>
        public event EventHandler ContentOffsetXChanged;

        /// <summary>
        /// Event raised when the ContentOffsetY property has changed.
        /// </summary>
        public event EventHandler ContentOffsetYChanged;

        /// <summary>
        /// Event raised when the ContentScale property has changed.
        /// </summary>
        public event EventHandler ContentScaleChanged;

        #endregion

        #region Public Properties

        /// <summary>
        /// Get/set the X offset (in content coordinates) of the view on the content.
        /// </summary>
        public double ContentOffsetX
        {
            get
            {
                return (double)GetValue(ContentOffsetXProperty);
            }
            set
            {
                SetValue(ContentOffsetXProperty, value);
            }
        }

        /// <summary>
        /// Get/set the Y offset (in content coordinates) of the view on the content.
        /// </summary>
        public double ContentOffsetY
        {
            get
            {
                return (double)GetValue(ContentOffsetYProperty);
            }
            set
            {
                SetValue(ContentOffsetYProperty, value);
            }
        }

        /// <summary>
        /// Get/set the current scale (or zoom factor) of the content.
        /// </summary>
        public double ContentScale
        {
            get
            {
                return (double)GetValue(ContentScaleProperty);
            }
            set
            {
                SetValue(ContentScaleProperty, value);
            }
        }

        /// <summary>
        /// Get/set the minimum value for 'ContentScale'.
        /// </summary>
        public double MinContentScale
        {
            get
            {
                return (double)GetValue(MinContentScaleProperty);
            }
            set
            {
                SetValue(MinContentScaleProperty, value);
            }
        }

        /// <summary>
        /// Get/set the maximum value for 'ContentScale'.
        /// </summary>
        public double MaxContentScale
        {
            get
            {
                return (double)GetValue(MaxContentScaleProperty);
            }
            set
            {
                SetValue(MaxContentScaleProperty, value);
            }
        }

        /// <summary>
        /// The X coordinate of the content focus, this is the point that we are focusing on when zooming.
        /// </summary>
        public double ContentZoomFocusX
        {
            get
            {
                return (double)GetValue(ContentZoomFocusXProperty);
            }
            set
            {
                SetValue(ContentZoomFocusXProperty, value);
            }
        }

        /// <summary>
        /// The Y coordinate of the content focus, this is the point that we are focusing on when zooming.
        /// </summary>
        public double ContentZoomFocusY
        {
            get
            {
                return (double)GetValue(ContentZoomFocusYProperty);
            }
            set
            {
                SetValue(ContentZoomFocusYProperty, value);
            }
        }

        /// <summary>
        /// The X coordinate of the viewport focus, this is the point in the viewport (in viewport coordinates) 
        /// that the content focus point is locked to while zooming in.
        /// </summary>
        public double ViewportZoomFocusX
        {
            get
            {
                return (double)GetValue(ViewportZoomFocusXProperty);
            }
            set
            {
                SetValue(ViewportZoomFocusXProperty, value);
            }
        }

        /// <summary>
        /// The Y coordinate of the viewport focus, this is the point in the viewport (in viewport coordinates) 
        /// that the content focus point is locked to while zooming in.
        /// </summary>
        public double ViewportZoomFocusY
        {
            get
            {
                return (double)GetValue(ViewportZoomFocusYProperty);
            }
            set
            {
                SetValue(ViewportZoomFocusYProperty, value);
            }
        }

        /// <summary>
        /// The duration of the animations (in seconds) started by calling AnimatedZoomTo and the other animation methods.
        /// </summary>
        public double AnimationDuration
        {
            get
            {
                return (double)GetValue(AnimationDurationProperty);
            }
            set
            {
                SetValue(AnimationDurationProperty, value);                
            }
        }

        /// <summary>
        /// Get the viewport width, in content coordinates.
        /// </summary>
        public double ContentViewportWidth
        {
            get
            {
                return (double)GetValue(ContentViewportWidthProperty);
            }
            set
            {
                SetValue(ContentViewportWidthProperty, value);
            }
        }

        /// <summary>
        /// Get the viewport height, in content coordinates.
        /// </summary>
        public double ContentViewportHeight
        {
            get
            {
                return (double)GetValue(ContentViewportHeightProperty);
            }
            set
            {
                SetValue(ContentViewportHeightProperty, value);
            }
        }

        /// <summary>
        /// Set to 'true' to enable the mouse wheel to scroll the zoom and pan control.
        /// This is set to 'false' by default.
        /// </summary>
        public bool IsMouseWheelScrollingEnabled
        {
            get
            {
                return (bool)GetValue(IsMouseWheelScrollingEnabledProperty);
            }
            set
            {
                SetValue(IsMouseWheelScrollingEnabledProperty, value);
            }
        }

        #endregion

        #region Public Methods

        /// <summary>
        /// Do an animated zoom to view a specific scale and rectangle (in content coordinates).
        /// </summary>
        public void AnimatedZoomTo(double newScale, Rect contentRect)
        {
            AnimatedZoomPointToViewportCenter(newScale, new Point(contentRect.X + (contentRect.Width / 2), contentRect.Y + (contentRect.Height / 2)),
                delegate(object sender, EventArgs e)
                {
                    //
                    // At the end of the animation, ensure that we are snapped to the specified content offset.
                    // Due to zooming in on the content focus point and rounding errors, the content offset may
                    // be slightly off what we want at the end of the animation and this bit of code corrects it.
                    //
                    this.ContentOffsetX = contentRect.X;
                    this.ContentOffsetY = contentRect.Y;
                });
        }

        /// <summary>
        /// Do an animated zoom to the specified rectangle (in content coordinates).
        /// </summary>
        public void AnimatedZoomTo(Rect contentRect)
        {
            double scaleX = this.ContentViewportWidth / contentRect.Width;
            double scaleY = this.ContentViewportHeight / contentRect.Height;
            double newScale = this.ContentScale * Math.Min(scaleX, scaleY);

            AnimatedZoomPointToViewportCenter(newScale, new Point(contentRect.X + (contentRect.Width / 2), contentRect.Y + (contentRect.Height / 2)), null);
        }

        /// <summary>
        /// Instantly zoom to the specified rectangle (in content coordinates).
        /// </summary>
        public void ZoomTo(Rect contentRect)
        {
            double scaleX = this.ContentViewportWidth / contentRect.Width;
            double scaleY = this.ContentViewportHeight / contentRect.Height;
            double newScale = this.ContentScale * Math.Min(scaleX, scaleY);

            ZoomPointToViewportCenter(newScale, new Point(contentRect.X + (contentRect.Width / 2), contentRect.Y + (contentRect.Height / 2)));
        }

        /// <summary>
        /// Instantly center the view on the specified point (in content coordinates).
        /// </summary>
        public void SnapTo(Point contentPoint)
        {
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetYProperty);

            this.ContentOffsetX = contentPoint.X - (this.ContentViewportWidth / 2);
            this.ContentOffsetY = contentPoint.Y - (this.ContentViewportHeight / 2);
        }

        /// <summary>
        /// Use animation to center the view on the specified point (in content coordinates).
        /// </summary>
        public void AnimatedSnapTo(Point contentPoint)
        {
            double newX = contentPoint.X - (this.ContentViewportWidth / 2);
            double newY = contentPoint.Y - (this.ContentViewportHeight / 2);

            ZoomPanAnimationHelper.StartAnimation(this, ContentOffsetXProperty, newX, AnimationDuration);
            ZoomPanAnimationHelper.StartAnimation(this, ContentOffsetYProperty, newY, AnimationDuration);
        }

        /// <summary>
        /// Zoom in/out centered on the specified point (in content coordinates).
        /// The focus point is kept locked to it's on screen position (ala google maps).
        /// </summary>
        public void AnimatedZoomAboutPoint(double newContentScale, Point contentZoomFocus)
        {
            newContentScale = Math.Min(Math.Max(newContentScale, MinContentScale), MaxContentScale);

            ZoomPanAnimationHelper.CancelAnimation(this, ContentZoomFocusXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentZoomFocusYProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ViewportZoomFocusXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ViewportZoomFocusYProperty);

            ContentZoomFocusX = contentZoomFocus.X;
            ContentZoomFocusY = contentZoomFocus.Y;
            ViewportZoomFocusX = (ContentZoomFocusX - ContentOffsetX) * ContentScale;
            ViewportZoomFocusY = (ContentZoomFocusY - ContentOffsetY) * ContentScale;

            //
            // When zooming about a point make updates to ContentScale also update content offset.
            //
            enableContentOffsetUpdateFromScale = true;

            ZoomPanAnimationHelper.StartAnimation(this, ContentScaleProperty, newContentScale, AnimationDuration,
                delegate(object sender, EventArgs e)
                {
                    enableContentOffsetUpdateFromScale = false;

                    ResetViewportZoomFocus();
                });
        }

        /// <summary>
        /// Zoom in/out centered on the specified point (in content coordinates).
        /// The focus point is kept locked to it's on screen position (ala google maps).
        /// </summary>
        public void ZoomAboutPoint(double newContentScale, Point contentZoomFocus)
        {
            newContentScale = Math.Min(Math.Max(newContentScale, MinContentScale), MaxContentScale);

            double screenSpaceZoomOffsetX = (contentZoomFocus.X - ContentOffsetX) * ContentScale;
            double screenSpaceZoomOffsetY = (contentZoomFocus.Y - ContentOffsetY) * ContentScale;
            double contentSpaceZoomOffsetX = screenSpaceZoomOffsetX / newContentScale;
            double contentSpaceZoomOffsetY = screenSpaceZoomOffsetY / newContentScale;
            double newContentOffsetX = contentZoomFocus.X - contentSpaceZoomOffsetX;
            double newContentOffsetY = contentZoomFocus.Y - contentSpaceZoomOffsetY;

            ZoomPanAnimationHelper.CancelAnimation(this, ContentScaleProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetYProperty);

            this.ContentScale = newContentScale;
            this.ContentOffsetX = newContentOffsetX;
            this.ContentOffsetY = newContentOffsetY;
        }

        /// <summary>
        /// Zoom in/out centered on the viewport center.
        /// </summary>
        public void AnimatedZoomTo(double contentScale)
        {
            Point zoomCenter = new Point(ContentOffsetX + (ContentViewportWidth / 2), ContentOffsetY + (ContentViewportHeight / 2));
            AnimatedZoomAboutPoint(contentScale, zoomCenter);
        }

        /// <summary>
        /// Zoom in/out centered on the viewport center.
        /// </summary>
        public void ZoomTo(double contentScale)
        {
            Point zoomCenter = new Point(ContentOffsetX + (ContentViewportWidth / 2), ContentOffsetY + (ContentViewportHeight / 2));
            ZoomAboutPoint(contentScale, zoomCenter);
        }

        /// <summary>
        /// Do animation that scales the content so that it fits completely in the control.
        /// </summary>
        public void AnimatedScaleToFit()
        {
            if (content == null)
            {
                throw new ApplicationException("PART_Content was not found in the ZoomPanControl visual template!");
            }

            AnimatedZoomTo(new Rect(0, 0, content.ActualWidth, content.ActualHeight));
        }

        /// <summary>
        /// Instantly scale the content so that it fits completely in the control.
        /// </summary>
        public void ScaleToFit()
        {
            if (content == null)
            {
                throw new ApplicationException("PART_Content was not found in the ZoomPanControl visual template!");
            }

            ZoomTo(new Rect(0, 0, content.ActualWidth, content.ActualHeight));
        }

        /// <summary>
        /// Called when a template has been applied to the control.
        /// </summary>
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();

            content = this.Template.FindName("PART_Content", this) as FrameworkElement;
            if (content != null)
            {
                //
                // Setup the transform on the content so that we can scale it by 'ContentScale'.
                //
                this.contentScaleTransform = new ScaleTransform(this.ContentScale, this.ContentScale);

                //
                // Setup the transform on the content so that we can translate it by 'ContentOffsetX' and 'ContentOffsetY'.
                //
                this.contentOffsetTransform = new TranslateTransform();
                UpdateTranslationX();
                UpdateTranslationY();

                //
                // Setup a transform group to contain the translation and scale transforms, and then
                // assign this to the content's 'RenderTransform'.
                //
                TransformGroup transformGroup = new TransformGroup();
                transformGroup.Children.Add(this.contentOffsetTransform);
                transformGroup.Children.Add(this.contentScaleTransform);
                content.RenderTransform = transformGroup;
            }
        }

        #endregion

        #region Protected Methods

        /// <summary>
        /// Measure the control and it's children.
        /// </summary>
        protected override Size MeasureOverride(Size constraint)
        {
            Size infiniteSize = new Size(double.PositiveInfinity, double.PositiveInfinity);
            Size childSize = base.MeasureOverride(infiniteSize);

            if (childSize != unScaledExtent)
            {
                //
                // Use the size of the child as the un-scaled extent content.
                //
                unScaledExtent = childSize;

                if (scrollOwner != null)
                {
                    scrollOwner.InvalidateScrollInfo();
                }
            }

            //
            // Update the size of the viewport onto the content based on the passed in 'constraint'.
            //
            UpdateViewportSize(constraint);

            double width = constraint.Width;
            double height = constraint.Height;

            if (double.IsInfinity(width))
            {
                //
                // Make sure we don't return infinity!
                //
                width = childSize.Width;
            }

            if (double.IsInfinity(height))
            {
                //
                // Make sure we don't return infinity!
                //
                height = childSize.Height;
            }
            
            return new Size(width, height);
        }

        /// <summary>
        /// Arrange the control and it's children.
        /// </summary>
        protected override Size ArrangeOverride(Size arrangeBounds)
        {
            Size size = base.ArrangeOverride(this.DesiredSize);

            if (content.DesiredSize != unScaledExtent)
            {
                //
                // Use the size of the child as the un-scaled extent content.
                //
                unScaledExtent = content.DesiredSize;

                if (scrollOwner != null)
                {
                    scrollOwner.InvalidateScrollInfo();
                }
            }

            //
            // Update the size of the viewport onto the content based on the passed in 'arrangeBounds'.
            //
            UpdateViewportSize(arrangeBounds);

            return size;
        }

        #endregion

        #region Private Methods

        /// <summary>
        /// Zoom to the specified scale and move the specified focus point to the center of the viewport.
        /// </summary>
        private void AnimatedZoomPointToViewportCenter(double newContentScale, Point contentZoomFocus, EventHandler callback)
        {
            newContentScale = Math.Min(Math.Max(newContentScale, MinContentScale), MaxContentScale);

            ZoomPanAnimationHelper.CancelAnimation(this, ContentZoomFocusXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentZoomFocusYProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ViewportZoomFocusXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ViewportZoomFocusYProperty);

            ContentZoomFocusX = contentZoomFocus.X;
            ContentZoomFocusY = contentZoomFocus.Y;
            ViewportZoomFocusX = (ContentZoomFocusX - ContentOffsetX) * ContentScale;
            ViewportZoomFocusY = (ContentZoomFocusY - ContentOffsetY) * ContentScale;

            //
            // When zooming about a point make updates to ContentScale also update content offset.
            //
            enableContentOffsetUpdateFromScale = true;

            ZoomPanAnimationHelper.StartAnimation(this, ContentScaleProperty, newContentScale, AnimationDuration,
                delegate(object sender, EventArgs e)
                {
                    enableContentOffsetUpdateFromScale = false;

                    if (callback != null)
                    {
                        callback(this, EventArgs.Empty);
                    }
                });

            ZoomPanAnimationHelper.StartAnimation(this, ViewportZoomFocusXProperty, ViewportWidth / 2, AnimationDuration);
            ZoomPanAnimationHelper.StartAnimation(this, ViewportZoomFocusYProperty, ViewportHeight / 2, AnimationDuration);
        }

        /// <summary>
        /// Zoom to the specified scale and move the specified focus point to the center of the viewport.
        /// </summary>
        private void ZoomPointToViewportCenter(double newContentScale, Point contentZoomFocus)
        {
            newContentScale = Math.Min(Math.Max(newContentScale, MinContentScale), MaxContentScale);

            ZoomPanAnimationHelper.CancelAnimation(this, ContentScaleProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetXProperty);
            ZoomPanAnimationHelper.CancelAnimation(this, ContentOffsetYProperty);

            this.ContentScale = newContentScale;
            this.ContentOffsetX = contentZoomFocus.X - (ContentViewportWidth / 2);
            this.ContentOffsetY = contentZoomFocus.Y - (ContentViewportHeight / 2);
        }

        /// <summary>
        /// Event raised when the 'ContentScale' property has changed value.
        /// </summary>
        private static void ContentScale_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            ZoomPanControl c = (ZoomPanControl)o;

            if (c.contentScaleTransform != null)
            {
                //
                // Update the content scale transform whenever 'ContentScale' changes.
                //
                c.contentScaleTransform.ScaleX = c.ContentScale;
                c.contentScaleTransform.ScaleY = c.ContentScale;
            }

            //
            // Update the size of the viewport in content coordinates.
            //
            c.UpdateContentViewportSize();

            if (c.enableContentOffsetUpdateFromScale)
            {
                try
                {
                    // 
                    // Disable content focus synchronization.  We are about to update content offset whilst zooming
                    // to ensure that the viewport is focused on our desired content focus point.  Setting this
                    // to 'true' stops the automatic update of the content focus when content offset changes.
                    //
                    c.disableContentFocusSync = true;

                    //
                    // Whilst zooming in or out keep the content offset up-to-date so that the viewport is always
                    // focused on the content focus point (and also so that the content focus is locked to the 
                    // viewport focus point - this is how the Google maps style zooming works).
                    //
                    double viewportOffsetX = c.ViewportZoomFocusX - (c.ViewportWidth / 2);
                    double viewportOffsetY = c.ViewportZoomFocusY - (c.ViewportHeight / 2);
                    double contentOffsetX = viewportOffsetX / c.ContentScale;
                    double contentOffsetY = viewportOffsetY / c.ContentScale;
                    c.ContentOffsetX = (c.ContentZoomFocusX - (c.ContentViewportWidth / 2)) - contentOffsetX;
                    c.ContentOffsetY = (c.ContentZoomFocusY - (c.ContentViewportHeight / 2)) - contentOffsetY;
                }
                finally
                {
                    c.disableContentFocusSync = false;
                }
            }

            if (c.ContentScaleChanged != null)
            {
                c.ContentScaleChanged(c, EventArgs.Empty);
            }

            if (c.scrollOwner != null)
            {
                c.scrollOwner.InvalidateScrollInfo();
            }
        }

        /// <summary>
        /// Method called to clamp the 'ContentScale' value to its valid range.
        /// </summary>
        private static object ContentScale_Coerce(DependencyObject d, object baseValue)
        {
            ZoomPanControl c = (ZoomPanControl)d;
            double value = (double) baseValue;
            value = Math.Min(Math.Max(value, c.MinContentScale), c.MaxContentScale);
            return value;
        }

        /// <summary>
        /// Event raised 'MinContentScale' or 'MaxContentScale' has changed.
        /// </summary>
        private static void MinOrMaxContentScale_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            ZoomPanControl c = (ZoomPanControl)o;
            c.ContentScale = Math.Min(Math.Max(c.ContentScale, c.MinContentScale), c.MaxContentScale);
        }

        /// <summary>
        /// Event raised when the 'ContentOffsetX' property has changed value.
        /// </summary>
        private static void ContentOffsetX_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            ZoomPanControl c = (ZoomPanControl)o;

            c.UpdateTranslationX();

            if (!c.disableContentFocusSync)
            {
                //
                // Normally want to automatically update content focus when content offset changes.
                // Although this is disabled using 'disableContentFocusSync' when content offset changes due to in-progress zooming.
                //
                c.UpdateContentZoomFocusX();
            }

            if (c.ContentOffsetXChanged != null)
            {
                //
                // Raise an event to let users of the control know that the content offset has changed.
                //
                c.ContentOffsetXChanged(c, EventArgs.Empty);
            }

            if (!c.disableScrollOffsetSync && c.scrollOwner != null)
            {
                //
                // Notify the owning ScrollViewer that the scrollbar offsets should be updated.
                //
                c.scrollOwner.InvalidateScrollInfo();
            }
        }

        /// <summary>
        /// Method called to clamp the 'ContentOffsetX' value to its valid range.
        /// </summary>
        private static object ContentOffsetX_Coerce(DependencyObject d, object baseValue)
        {
            ZoomPanControl c = (ZoomPanControl)d;
            double value = (double)baseValue;
            double minOffsetX = 0.0;
            double maxOffsetX = Math.Max(0.0, c.unScaledExtent.Width - c.constrainedContentViewportWidth);
            value = Math.Min(Math.Max(value, minOffsetX), maxOffsetX);
            return value;
        }

        /// <summary>
        /// Event raised when the 'ContentOffsetY' property has changed value.
        /// </summary>
        private static void ContentOffsetY_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            ZoomPanControl c = (ZoomPanControl)o;

            c.UpdateTranslationY();

            if (!c.disableContentFocusSync)
            {
                //
                // Normally want to automatically update content focus when content offset changes.
                // Although this is disabled using 'disableContentFocusSync' when content offset changes due to in-progress zooming.
                //
                c.UpdateContentZoomFocusY();
            }

            if (c.ContentOffsetYChanged != null)
            {
                //
                // Raise an event to let users of the control know that the content offset has changed.
                //
                c.ContentOffsetYChanged(c, EventArgs.Empty);
            }

            if (!c.disableScrollOffsetSync && c.scrollOwner != null)
            {
                //
                // Notify the owning ScrollViewer that the scrollbar offsets should be updated.
                //
                c.scrollOwner.InvalidateScrollInfo();
            }

        }

        /// <summary>
        /// Method called to clamp the 'ContentOffsetY' value to its valid range.
        /// </summary>
        private static object ContentOffsetY_Coerce(DependencyObject d, object baseValue)
        {
            ZoomPanControl c = (ZoomPanControl)d;
            double value = (double)baseValue;
            double minOffsetY = 0.0;
            double maxOffsetY = Math.Max(0.0, c.unScaledExtent.Height - c.constrainedContentViewportHeight);
            value = Math.Min(Math.Max(value, minOffsetY), maxOffsetY);
            return value;
        }

        /// <summary>
        /// Reset the viewport zoom focus to the center of the viewport.
        /// </summary>
        private void ResetViewportZoomFocus()
        {
            ViewportZoomFocusX = ViewportWidth / 2;
            ViewportZoomFocusY = ViewportHeight / 2;
        }

        /// <summary>
        /// Update the viewport size from the specified size.
        /// </summary>
        private void UpdateViewportSize(Size newSize)
        {
            if (viewport == newSize)
            {
                //
                // The viewport is already the specified size.
                //
                return;
            }

            viewport = newSize;

            //
            // Update the viewport size in content coordiates.
            //
            UpdateContentViewportSize();

            //
            // Initialise the content zoom focus point.
            //
            UpdateContentZoomFocusX();
            UpdateContentZoomFocusY();

            //
            // Reset the viewport zoom focus to the center of the viewport.
            //
            ResetViewportZoomFocus();

            //
            // Update content offset from itself when the size of the viewport changes.
            // This ensures that the content offset remains properly clamped to its valid range.
            //
            this.ContentOffsetX = this.ContentOffsetX;
            this.ContentOffsetY = this.ContentOffsetY;

            if (scrollOwner != null)
            {
                //
                // Tell that owning ScrollViewer that scrollbar data has changed.
                //
                scrollOwner.InvalidateScrollInfo();
            }
        }

        /// <summary>
        /// Update the size of the viewport in content coordinates after the viewport size or 'ContentScale' has changed.
        /// </summary>
        private void UpdateContentViewportSize()
        {
            ContentViewportWidth  = ViewportWidth / ContentScale;
            ContentViewportHeight = ViewportHeight / ContentScale;

            constrainedContentViewportWidth  = Math.Min(ContentViewportWidth, unScaledExtent.Width);
            constrainedContentViewportHeight = Math.Min(ContentViewportHeight, unScaledExtent.Height);

            UpdateTranslationX();
            UpdateTranslationY();
        }

        /// <summary>
        /// Update the X coordinate of the translation transformation.
        /// </summary>
        private void UpdateTranslationX()
        {
            if (this.contentOffsetTransform != null)
            {
                double scaledContentWidth = this.unScaledExtent.Width * this.ContentScale;
                if (scaledContentWidth < this.ViewportWidth)
                {
                    //
                    // When the content can fit entirely within the viewport, center it.
                    //
                    this.contentOffsetTransform.X = (this.ContentViewportWidth - this.unScaledExtent.Width) / 2;
                }
                else
                {
                    this.contentOffsetTransform.X = -this.ContentOffsetX;
                }
            }
        }

        /// <summary>
        /// Update the Y coordinate of the translation transformation.
        /// </summary>
        private void UpdateTranslationY()
        {
            if (this.contentOffsetTransform != null)
            {
                double scaledContentHeight = this.unScaledExtent.Height * this.ContentScale;
                if (scaledContentHeight < this.ViewportHeight)
                {
                    //
                    // When the content can fit entirely within the viewport, center it.
                    //
                    this.contentOffsetTransform.Y = (this.ContentViewportHeight - this.unScaledExtent.Height) / 2;
                }
                else
                {
                    this.contentOffsetTransform.Y = -this.ContentOffsetY;
                }
            }
        }

        /// <summary>
        /// Update the X coordinate of the zoom focus point in content coordinates.
        /// </summary>
        private void UpdateContentZoomFocusX()
        {
            ContentZoomFocusX = ContentOffsetX + (constrainedContentViewportWidth / 2);
        }

        /// <summary>
        /// Update the Y coordinate of the zoom focus point in content coordinates.
        /// </summary>
        private void UpdateContentZoomFocusY()
        {
            ContentZoomFocusY = ContentOffsetY + (constrainedContentViewportHeight / 2);
        }

        #endregion Private Methods
    }
}