ZoomAndPanControl.cs

using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Media;

namespace ZoomAndPan
{
    /// <summary>
    /// A class that wraps up zooming and panning of it's content.
    /// </summary>
    public partial class ZoomAndPanControl : ContentControl, IScrollInfo, INotifyPropertyChanged
    {
        #region Fields
        /// <summary>
        /// Reference to the underlying content, which is named PART_Content in the template.
        /// </summary>
        private FrameworkElement _content = null;

        /// <summary>
        /// The transform that is applied to the content to scale it by 'ViewportZoom'.
        /// </summary>
        private ScaleTransform _contentZoomTransform = null;

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

        /// <summary>
        /// The height of the viewport in content coordinates, clamped to the height of the content.
        /// </summary>
        private double _constrainedContentViewportHeight = 0.0;

        /// <summary>
        /// The width of the viewport in content coordinates, clamped to the width of the content.
        /// </summary>
        private double _constrainedContentViewportWidth = 0.0;

        /// <summary>
        /// Normally when content offsets changes the content focus is automatically updated.
        /// This syncronization 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 = false;

        /// <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 = false;

        /// <summary>
        /// Used to disable syncronization between IScrollInfo interface and ContentOffsetX/ContentOffsetY.
        /// </summary>
        private bool _disableScrollOffsetSync = false;
        #endregion

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

        /// <summary>
        /// Need to update zoom values if size changes, and update ViewportZoom if too low
        /// </summary>
        protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
        {
            base.OnRenderSizeChanged(sizeInfo);
            if (sizeInfo.NewSize.Width <= 1 || sizeInfo.NewSize.Height <= 1) return;
            switch (_currentZoomTypeEnum)
            {
                case CurrentZoomTypeEnum.Fit:
                    InternalViewportZoom = ViewportHelpers.FitZoom(sizeInfo.NewSize.Width, sizeInfo.NewSize.Height,
                        _content?.ActualWidth, _content?.ActualHeight);
                    break;
                case CurrentZoomTypeEnum.Fill:
                    InternalViewportZoom = ViewportHelpers.FillZoom(sizeInfo.NewSize.Width, sizeInfo.NewSize.Height,
                        _content?.ActualWidth, _content?.ActualHeight);
                    break;
            }
            if (InternalViewportZoom < MinimumZoomClamped) InternalViewportZoom = MinimumZoomClamped;
            //
            // INotifyPropertyChanged property update
            //
            OnPropertyChanged(nameof(MinimumZoomClamped));
            OnPropertyChanged(nameof(FillZoomValue));
            OnPropertyChanged(nameof(FitZoomValue));
        }

        /// <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 'ViewportZoom'.
                //
                this._contentZoomTransform = new ScaleTransform(this.InternalViewportZoom, this.InternalViewportZoom);

                //
                // 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'.
                //
                var transformGroup = new TransformGroup();
                transformGroup.Children.Add(this._contentOffsetTransform);
                transformGroup.Children.Add(this._contentZoomTransform);
                _content.RenderTransform = transformGroup;
                ZoomAndPanControl_EventHandlers_OnApplyTemplate();
            }
        }

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

            if (childSize != _unScaledExtent)
            {
                //
                // Use the size of the child as the un-scaled extent content.
                //
                _unScaledExtent = childSize;
                ScrollOwner?.InvalidateScrollInfo();
            }

            //
            // Update the size of the viewport onto the content based on the passed in 'constraint'.
            //
            UpdateViewportSize(constraint);
            var width = constraint.Width;
            var height = constraint.Height;
            if (double.IsInfinity(width)) width = childSize.Width;
            if (double.IsInfinity(height)) height = childSize.Height;
            UpdateTranslationX();
            UpdateTranslationY();
            return new Size(width, height);
        }

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

            if (_content.DesiredSize != _unScaledExtent)
            {
                //
                // Use the size of the child as the un-scaled extent content.
                //
                _unScaledExtent = _content.DesiredSize;
                ScrollOwner?.InvalidateScrollInfo();
            }

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

            return size;
        }
        #endregion 

        #region IScrollInfo Data Members
        //
        // These data members are for the implementation of the IScrollInfo interface.
        // This interface works with the ScrollViewer such that when ZoomAndPanControl is 
        // wrapped (in XAML) with a ScrollViewer the IScrollInfo interface allows the ZoomAndPanControl 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>
        /// Records the unscaled extent of the content.
        /// This is calculated during the measure and arrange.
        /// </summary>
        private Size _unScaledExtent = new Size(0, 0);

        /// <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 = new Size(0, 0);
        #endregion IScrollInfo Data Members

        #region Dependency Property Definitions
        //
        // Definitions for dependency properties.
        //
        /// <summary>
        /// This allows the same property name be used for direct and indirect access to this control.
        /// </summary>
        public ZoomAndPanControl ZoomAndPanContent => this;

        /// <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); }
        }
        public static readonly DependencyProperty AnimationDurationProperty = DependencyProperty.Register("AnimationDuration",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.4));

        /// <summary>
        /// The duration of the animations (in seconds) started by calling AnimatedZoomTo and the other animation methods.
        /// </summary>
        public ZoomAndPanInitialPositionEnum ZoomAndPanInitialPosition
        {
            get { return (ZoomAndPanInitialPositionEnum)GetValue(ZoomAndPanInitialPositionProperty); }
            set { SetValue(ZoomAndPanInitialPositionProperty, value); }
        }
        public static readonly DependencyProperty ZoomAndPanInitialPositionProperty = DependencyProperty.Register("ZoomAndPanInitialPosition",
            typeof(ZoomAndPanInitialPositionEnum), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(ZoomAndPanInitialPositionEnum.Default, ZoomAndPanInitialPositionChanged));

        private static void ZoomAndPanInitialPositionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var zoomAndPanControl = (ZoomAndPanControl)d;
            // zoomAndPanControl.SetZoomAndPanInitialPosition();
        }

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

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

        /// <summary>
        /// Get the viewport height, in content coordinates.
        /// </summary>
        public double ContentViewportHeight
        {
            get { return (double)GetValue(ContentViewportHeightProperty); }
            set { SetValue(ContentViewportHeightProperty, value); }
        }
        public static readonly DependencyProperty ContentViewportHeightProperty = DependencyProperty.Register("ContentViewportHeight",
             typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        /// <summary>
        /// Get the viewport width, in content coordinates.
        /// </summary>
        public double ContentViewportWidth
        {
            get { return (double)GetValue(ContentViewportWidthProperty); }
            set { SetValue(ContentViewportWidthProperty, value); }
        }
        public static readonly DependencyProperty ContentViewportWidthProperty = DependencyProperty.Register("ContentViewportWidth",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        /// <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); }
        }
        public static readonly DependencyProperty ContentZoomFocusXProperty = DependencyProperty.Register("ContentZoomFocusX",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        /// <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); }
        }
        public static readonly DependencyProperty ContentZoomFocusYProperty = DependencyProperty.Register("ContentZoomFocusY",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        /// <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); }
        }
        public static readonly DependencyProperty IsMouseWheelScrollingEnabledProperty = DependencyProperty.Register("IsMouseWheelScrollingEnabled",
            typeof(bool), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(false));

        /// <summary>
        /// Get/set the maximum value for 'ViewportZoom'.
        /// </summary>
        public double MaximumZoom
        {
            get { return (double)GetValue(MaximumZoomProperty); }
            set { SetValue(MaximumZoomProperty, value); }
        }
        public static readonly DependencyProperty MaximumZoomProperty = DependencyProperty.Register("MaximumZoom",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(10.0, MinimumOrMaximumZoom_PropertyChanged));

        /// <summary>
        /// Get/set the maximum value for 'ViewportZoom'.
        /// </summary>
        public MinimumZoomTypeEnum MinimumZoomType
        {
            get { return (MinimumZoomTypeEnum)GetValue(MinimumZoomTypeProperty); }
            set { SetValue(MinimumZoomTypeProperty, value); }
        }
        public static readonly DependencyProperty MinimumZoomTypeProperty = DependencyProperty.Register("MinimumZoomType",
            typeof(MinimumZoomTypeEnum), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(MinimumZoomTypeEnum.MinimumZoom));

        /// <summary>
        /// Get/set the MinimumZoom value for 'ViewportZoom'.
        /// </summary>
        public double MinimumZoom
        {
            get { return (double)GetValue(MinimumZoomProperty); }
            set { SetValue(MinimumZoomProperty, value); }
        }
        public static readonly DependencyProperty MinimumZoomProperty = DependencyProperty.Register("MinimumZoom",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.1, MinimumOrMaximumZoom_PropertyChanged));

        /// <summary>
        /// Get/set the MinimumZoom value for 'ViewportZoom'.
        /// </summary>
        public Point? MousePosition
        {
            get { return (Point?)GetValue(MousePositionProperty); }
            set { SetValue(MousePositionProperty, value); }
        }
        public static readonly DependencyProperty MousePositionProperty = DependencyProperty.Register("MousePosition",
            typeof(Point?), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(null, MinimumOrMaximumZoom_PropertyChanged));

        /// <summary>
        /// This is used for binding a slider to control the zoom. Cannot use the InternalUseAnimations because of all the 
        /// assumptions in when the this property is changed. THIS IS NOT USED FOR THE ANIMATIONS
        /// </summary>
        public bool UseAnimations
        {
            get { return (bool)GetValue(UseAnimationsProperty); }
            set { SetValue(UseAnimationsProperty, value); }
        }
        public static readonly DependencyProperty UseAnimationsProperty = DependencyProperty.Register("UseAnimations",
            typeof(bool), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(true));

        /// <summary>
        /// This is used for binding a slider to control the zoom. Cannot use the InternalViewportZoom because of all the 
        /// assumptions in when the this property is changed. THIS IS NOT USED FOR THE ANIMATIONS
        /// </summary>
        public double ViewportZoom
        {
            get { return (double)GetValue(ViewportZoomProperty); }
            set { SetValue(ViewportZoomProperty, value); }
        }
        public static readonly DependencyProperty ViewportZoomProperty = DependencyProperty.Register("ViewportZoom",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(1.0, ViewportZoom_PropertyChanged));

        /// <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); }
        }
        public static readonly DependencyProperty ViewportZoomFocusXProperty = DependencyProperty.Register("ViewportZoomFocusX",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        /// <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); }
        }
        public static readonly DependencyProperty ViewportZoomFocusYProperty = DependencyProperty.Register("ViewportZoomFocusY",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(0.0));

        #endregion Dependency Property Definitions

        #region 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 ViewportZoom property has changed.
        /// </summary>
        public event EventHandler ContentZoomChanged;
        #endregion

        #region Event Handlers

        /// <summary>
        /// This is required for the animations, but has issues if set by something like a slider.
        /// </summary>
        private double InternalViewportZoom
        {
            get { return (double)GetValue(InternalViewportZoomProperty); }
            set { SetValue(InternalViewportZoomProperty, value); }
        }
        private static readonly DependencyProperty InternalViewportZoomProperty = DependencyProperty.Register("InternalViewportZoom",
            typeof(double), typeof(ZoomAndPanControl), new FrameworkPropertyMetadata(1.0, InternalViewportZoom_PropertyChanged, InternalViewportZoom_Coerce));

        /// <summary>
        /// Event raised when the 'ViewportZoom' property has changed value.
        /// </summary>
        private static void InternalViewportZoom_PropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
        {
            var c = (ZoomAndPanControl)dependencyObject;

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

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

            if (c._enableContentOffsetUpdateFromScale)
            {
                try
                {
                    // 
                    // Disable content focus syncronization.  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).
                    //
                    var viewportOffsetX = c.ViewportZoomFocusX - (c.ViewportWidth / 2);
                    var viewportOffsetY = c.ViewportZoomFocusY - (c.ViewportHeight / 2);
                    var contentOffsetX = viewportOffsetX / c.InternalViewportZoom;
                    var contentOffsetY = viewportOffsetY / c.InternalViewportZoom;
                    c.ContentOffsetX = (c.ContentZoomFocusX - (c.ContentViewportWidth / 2)) - contentOffsetX;
                    c.ContentOffsetY = (c.ContentZoomFocusY - (c.ContentViewportHeight / 2)) - contentOffsetY;
                }
                finally
                {
                    c._disableContentFocusSync = false;
                }
            }
            c.ContentZoomChanged?.Invoke(c, EventArgs.Empty);
            c.ViewportZoom = c.InternalViewportZoom;
            c.OnPropertyChanged(new DependencyPropertyChangedEventArgs(ViewportZoomProperty, c.ViewportZoom, c.InternalViewportZoom));
            c.ScrollOwner?.InvalidateScrollInfo();
            c.SetCurrentZoomTypeEnum();
            c.RaiseCanExecuteChanged();
        }

        /// <summary>
        /// Method called to clamp the 'ViewportZoom' value to its valid range.
        /// </summary>
        private static object InternalViewportZoom_Coerce(DependencyObject dependencyObject, object baseValue)
        {
            var c = (ZoomAndPanControl)dependencyObject;
            var value = Math.Max((double)baseValue, c.MinimumZoomClamped);
            switch (c.MinimumZoomType)
            {
                case MinimumZoomTypeEnum.FitScreen:
                    value = Math.Min(Math.Max(value, c.FitZoomValue), c.MaximumZoom);
                    break;
                case MinimumZoomTypeEnum.FillScreen:
                    value = Math.Min(Math.Max(value, c.FillZoomValue), c.MaximumZoom);
                    break;
                case MinimumZoomTypeEnum.MinimumZoom:
                    value = Math.Min(Math.Max(value, c.MinimumZoom), c.MaximumZoom);
                    break;
                default:
                    throw new ArgumentOutOfRangeException();
            }
            return value;
        }
        #endregion

        #region DependencyProperty Event Handlers
        /// <summary>
        /// Event raised 'MinimumZoom' or 'MaximumZoom' has changed.
        /// </summary>
        private static void MinimumOrMaximumZoom_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            var c = (ZoomAndPanControl)o;
            c.InternalViewportZoom = Math.Min(Math.Max(c.InternalViewportZoom, c.MinimumZoomClamped), c.MaximumZoom);
        }

        /// <summary>
        /// Event raised when the 'ContentOffsetX' property has changed value.
        /// </summary>
        private static void ContentOffsetX_PropertyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {
            var c = (ZoomAndPanControl)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();
            //
            // Raise an event to let users of the control know that the content offset has changed.
            //
            c.ContentOffsetXChanged?.Invoke(c, EventArgs.Empty);

            if (!c._disableScrollOffsetSync)
                //
                // 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)
        {
            var c = (ZoomAndPanControl)d;
            var value = (double)baseValue;
            var minOffsetX = 0.0;
            var 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)
        {
            var c = (ZoomAndPanControl)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._disableScrollOffsetSync)
                //
                // Notify the owning ScrollViewer that the scrollbar offsets should be updated.
                //
                c.ScrollOwner?.InvalidateScrollInfo();
            //
            // Raise an event to let users of the control know that the content offset has changed.
            //
            c.ContentOffsetYChanged?.Invoke(c, EventArgs.Empty);
        }

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

        private static void ViewportZoom_PropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
        {
            var c = (ZoomAndPanControl)dependencyObject;
            var newZoom = (double)e.NewValue;
            if (c.InternalViewportZoom != newZoom)
            {
                var centerPoint = new Point(c.ContentOffsetX + (c._constrainedContentViewportWidth / 2), c.ContentOffsetY + (c._constrainedContentViewportHeight / 2));
                c.ZoomAboutPoint(newZoom, centerPoint);
            }
        }
        #endregion

        /// <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)
                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;

            //
            // 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 'ViewportZoom' has changed.
        /// </summary>
        private void UpdateContentViewportSize()
        {
            ContentViewportWidth = ViewportWidth / InternalViewportZoom;
            ContentViewportHeight = ViewportHeight / InternalViewportZoom;

            _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)
            {
                var scaledContentWidth = this._unScaledExtent.Width * this.InternalViewportZoom;
                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)
            {
                var scaledContentHeight = this._unScaledExtent.Height * this.InternalViewportZoom;
                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);
        }

        public double FitZoomValue => ViewportHelpers.FitZoom(ActualWidth, ActualHeight, _content?.ActualWidth, _content?.ActualHeight);
        public double FillZoomValue => ViewportHelpers.FillZoom(ActualWidth, ActualHeight, _content?.ActualWidth, _content?.ActualHeight);
        public double MinimumZoomClamped => ((MinimumZoomType == MinimumZoomTypeEnum.FillScreen) ? FillZoomValue
                                      : (MinimumZoomType == MinimumZoomTypeEnum.FitScreen) ? FitZoomValue
                                      : MinimumZoom).ToRealNumber();

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private enum CurrentZoomTypeEnum { Fill, Fit, Other}

        private CurrentZoomTypeEnum _currentZoomTypeEnum;

        private void SetCurrentZoomTypeEnum()
        {
            if (ViewportZoom.IsWithinOnePercent(FitZoomValue))
                _currentZoomTypeEnum = CurrentZoomTypeEnum.Fit;
            else if (ViewportZoom.IsWithinOnePercent(FillZoomValue))
                _currentZoomTypeEnum = CurrentZoomTypeEnum.Fill;
            else
                _currentZoomTypeEnum = CurrentZoomTypeEnum.Other;
        }
    }
}