Sunday, January 13, 2008

Placeholder text using Adorner

In the previous post, I subclassed TextBox in order to introduce a placeholder text. I realised that there is a better way - one that allows me to use only 1 class to add placeholder text to a TextBox, RichTextBox and PasswordBox - and that is using an adorner.



To use it in xaml, simply add
xmlns:src="clr-namespace:Huan.WhiteDwarf.UI"
to the root element, e.g. Window, and add
src:Placeholder.Text="Your Placeholder"
to the TextBox/RichTextBox/PasswordBox.

To use it in code, use this
Huan.WhiteDwarf.UI.Placeholder.SetText(TextBoxName, PlaceholderText);

using System;

using System.Globalization;

using System.Windows;

using System.Windows.Controls;

using System.Windows.Controls.Primitives;

using System.Windows.Documents;

using System.Windows.Media;

 

namespace Huan.WhiteDwarf.UI

{

    /// <summary>

    ///   Represents an adorner that adds placeholder text to a <see cref="T:System.Windows.Controls.TextBox"/>,

    ///   <see cref="T:System.Windows.Controls.RichTextBox"/> or <see cref="T:System.Windows.Controls.PasswordBox"/>.

    /// </summary>

    public class Placeholder : Adorner

    {

        /// <summary>

        ///   Event handler for <see cref="E:System.Windows.Controls.Primitives.TextBoxBase.TextChanged" />.

        /// </summary>

        private readonly TextChangedEventHandler _textChangedHandler;

 

        /// <summary>

        ///   Event handler for <see cref="E:System.Windows.Controls.PasswordBox.PasswordChanged" />.

        /// </summary>

        private readonly RoutedEventHandler _passwordChangedHandler;

 

        /// <summary>

        ///   <see langword="true" /> when the placeholder text is visible, <see langword="false" /> otherwise.

        ///   Used to avoid calling <see cref="M:System.Windows.UIElement.InvalidateVisual"/> unnecessarily.

        /// </summary>

        private bool _isPlaceholderVisible;

 

        #region Dependency Property

        /// <summary>

        ///    Identifies the Huan.WhiteDwarf.UI.Placeholder.Text attached property.

        /// </summary>

        public static readonly DependencyProperty TextProperty = DependencyProperty.RegisterAttached(

            "Text", typeof(string), typeof(Placeholder),

            new FrameworkPropertyMetadata(string.Empty, new PropertyChangedCallback(OnTextChanged)));

        #endregion

 

        #region Constructors

        /// <summary>

        ///   Initializes a new instance of the <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> class.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element to bind the adorner to.

        /// </param>

        /// <exception cref="T:System.ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        protected Placeholder(Control adornedElement)

            : base(adornedElement)

        {

            this.IsHitTestVisible = false;

            _textChangedHandler = new TextChangedEventHandler(AdornedElement_ContentChanged);

            _passwordChangedHandler = new RoutedEventHandler(AdornedElement_ContentChanged);

 

            adornedElement.GotFocus += new RoutedEventHandler(AdornedElement_GotFocus);

            adornedElement.LostFocus += new RoutedEventHandler(AdornedElement_LostFocus);

        }

 

        /// <summary>

        ///   Initializes a new instance of the <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> class.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element to bind the adorner to.

        /// </param>

        /// <exception cref="T:System.ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        public Placeholder(PasswordBox adornedElement)

            : this((Control)adornedElement)

        {

            if (!adornedElement.IsFocused)

                adornedElement.PasswordChanged += _passwordChangedHandler;

        }

 

        /// <summary>

        ///   Initializes a new instance of the <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> class.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element to bind the adorner to.

        /// </param>

        /// <exception cref="T:System.ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        public Placeholder(TextBox adornedElement)

            : this((Control)adornedElement)

        {

            if (!adornedElement.IsFocused)

                adornedElement.TextChanged += _textChangedHandler;

        }

 

        /// <summary>

        ///   Initializes a new instance of the <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> class.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element to bind the adorner to.

        /// </param>

        /// <exception cref="T:System.ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        public Placeholder(RichTextBox adornedElement)

            : this((Control)adornedElement)

        {

            if (!adornedElement.IsFocused)

                adornedElement.TextChanged += _textChangedHandler;

        }

        #endregion

 

        #region Property Changed Callbacks

        /// <summary>

        ///   Invoked whenever Huan.WhiteDwarf.UI.Placeholder.Text attached property is changed.

        /// </summary>

        /// <param name="sender">

        ///   The object where the event handler is attached.

        /// </param>

        /// <param name="e">

        ///   Provides data about the event.

        /// </param>

        private static void OnTextChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)

        {

            Control adornedElement = sender as Control;

            if (adornedElement.IsLoaded)

                AddAdorner(adornedElement);

            else

                adornedElement.Loaded += new RoutedEventHandler(AdornedElement_Loaded);

        }

        #endregion

 

        #region Event Handlers

        /// <summary>

        ///   Event handler for AdornedElement.Loaded.

        /// </summary>

        /// <param name="sender">

        ///   The AdornedElement where the event handler is attached.

        /// </param>

        /// <param name="e">

        ///   Provides data about the event.

        /// </param>

        private static void AdornedElement_Loaded(object sender, RoutedEventArgs e)

        {

            Control adornedElement = (Control)sender;

            adornedElement.Loaded -= AdornedElement_Loaded;

            AddAdorner(adornedElement);

        }

 

        /// <summary>

        ///   Event handler for AdornedElement.GotFocus.

        /// </summary>

        /// <param name="sender">

        ///   The AdornedElement where the event handler is attached.

        /// </param>

        /// <param name="e">

        ///   Provides data about the event.

        /// </param>

        private void AdornedElement_GotFocus(object sender, RoutedEventArgs e)

        {

            TextBoxBase textBoxBase = AdornedElement as TextBoxBase;

            if (textBoxBase != null)

                textBoxBase.TextChanged -= AdornedElement_ContentChanged;

            else

            {

                PasswordBox passwordBox = AdornedElement as PasswordBox;

                if (passwordBox != null)

                    passwordBox.PasswordChanged -= AdornedElement_ContentChanged;

            }

 

            if (_isPlaceholderVisible)

                this.InvalidateVisual();

        }

 

        /// <summary>

        ///   Event handler for AdornedElement.LostFocus.

        /// </summary>

        /// <param name="sender">

        ///   The AdornedElement where the event handler is attached.

        /// </param>

        /// <param name="e">

        ///   Provides data about the event.

        /// </param>

        public void AdornedElement_LostFocus(object sender, RoutedEventArgs e)

        {

            TextBoxBase textBoxBase = AdornedElement as TextBoxBase;

            if (textBoxBase != null)

                textBoxBase.TextChanged += _textChangedHandler;

            else

            {

                PasswordBox passwordBox = AdornedElement as PasswordBox;

                if (passwordBox != null)

                    passwordBox.PasswordChanged += _passwordChangedHandler;

            }

 

            if (!_isPlaceholderVisible && IsElementEmpty())

                this.InvalidateVisual();

        }

 

        /// <summary>

        ///   Event handler for AdornedElement.ContentChanged.

        /// </summary>

        /// <param name="sender">

        ///   The AdornedElement where the event handler is attached.

        /// </param>

        /// <param name="e">

        ///   Provides data about the event.

        /// </param>

        private void AdornedElement_ContentChanged(object sender, RoutedEventArgs e)

        {

            if (_isPlaceholderVisible ^ IsElementEmpty())

                this.InvalidateVisual();

        }

        #endregion

 

        #region Attached Property Getters and Setters

        /// <summary>

        ///   Gets the value of the Huan.WhiteDwarf.UI.Placeholder.Text attached property for a specified element.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element from which the property value is read.

        /// </param>

        /// <returns>

        ///   The placeholder text property value for the element.

        /// </returns>

        /// <exception cref="T:ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        public static string GetText(Control adornedElement)

        {

            if (adornedElement == null)

                throw new ArgumentNullException("adornedElement");

 

            return (string)adornedElement.GetValue(TextProperty);

        }

 

        /// <summary>

        ///   Sets the value of the Huan.WhiteDwarf.UI.Placeholder.Text attached property to a specified element.

        /// </summary>

        /// <param name="adornedElement">

        ///   The element to which the attached property is written.

        /// </param>

        /// <param name="placeholderText">

        ///   The needed placeholder text value.

        /// </param>

        /// <exception cref="T:ArgumentNullException">

        ///   Raised when adornedElement is null.

        /// </exception>

        /// <exception cref="T:InvalidOperationException">

        ///   Raised when adornedElement is not a <see cref="T:System.Windows.Controls.TextBox"/>,

        ///   <see cref="T:System.Windows.Controls.RichTextBox"/> or <see cref="T:System.Windows.Controls.PasswordBox"/>.

        /// </exception>

        public static void SetText(Control adornedElement, string placeholderText)

        {

            if (adornedElement == null)

                throw new ArgumentNullException("adornedElement");

 

            if (!(adornedElement is TextBox || adornedElement is RichTextBox || adornedElement is PasswordBox))

                throw new InvalidOperationException();

 

            adornedElement.SetValue(TextProperty, placeholderText);

        }

        #endregion

 

        /// <summary>

        ///   Adds a <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> to the adorner layer.

        /// </summary>

        /// <param name="adornedElement">

        ///   The adorned element.

        /// </param>

        private static void AddAdorner(Control adornedElement)

        {

            AdornerLayer adornerLayer = AdornerLayer.GetAdornerLayer(adornedElement);

            if (adornerLayer == null)

                return;

 

            Adorner[] adorners = adornerLayer.GetAdorners(adornedElement);

            if (adorners != null)

                foreach (Adorner adorner in adorners)

                    if (adorner is Placeholder)

                        return;

 

            TextBox textBox = adornedElement as TextBox;

            if (textBox != null)

            {

                adornerLayer.Add(new Placeholder(textBox));

                return;

            }

 

            RichTextBox richTextBox = adornedElement as RichTextBox;

            if (richTextBox != null)

            {

                adornerLayer.Add(new Placeholder(richTextBox));

                return;

            }

 

            PasswordBox passwordBox = adornedElement as PasswordBox;

            if (passwordBox != null)

            {

                adornerLayer.Add(new Placeholder(passwordBox));

                return;

            }

        }

 

        /// <summary>

        ///   Checks if the content of the adorned element is empty.

        /// </summary>

        /// <returns>

        ///   Returns <see langword="true" /> if the content is empty, <see langword="false" /> otherwise.

        /// </returns>

        private bool IsElementEmpty()

        {

            UIElement adornedElement = AdornedElement;

            TextBox textBox = adornedElement as TextBox;

            if (textBox != null)

                return string.IsNullOrEmpty(textBox.Text);

 

            PasswordBox passwordBox = adornedElement as PasswordBox;

            if (passwordBox != null)

                return string.IsNullOrEmpty(passwordBox.Password);

 

            RichTextBox richTextBox = adornedElement as RichTextBox;

            if (richTextBox != null)

            {

                BlockCollection blocks = richTextBox.Document.Blocks;

                if (blocks.Count == 0)

                    return true;

                if (blocks.Count == 1)

                {

                    Paragraph paragraph = blocks.FirstBlock as Paragraph;

                    if (paragraph == null)

                        return false;

 

                    if (paragraph.Inlines.Count == 0)

                        return true;

 

                    if (paragraph.Inlines.Count == 1)

                    {

                        Run run = paragraph.Inlines.FirstInline as Run;

                        return (run != null && string.IsNullOrEmpty(run.Text));

                    }

                }

 

                return false;

            }

 

            return false;

        }

 

        /// <summary>

        ///    Computes the text alignment of the adorned element.

        /// </summary>

        /// <returns>

        ///   Returns the computed text alignment.

        /// </returns>

        private TextAlignment ComputedTextAlignment()

        {

            Control adornedElement = AdornedElement as Control;

            TextBox textBox = adornedElement as TextBox;

            if (textBox != null)

            {

                if (DependencyPropertyHelper.GetValueSource(textBox, TextBox.HorizontalContentAlignmentProperty)

                    .BaseValueSource != BaseValueSource.Local ||

                    DependencyPropertyHelper.GetValueSource(textBox, TextBox.TextAlignmentProperty)

                    .BaseValueSource == BaseValueSource.Local)

 

                    // TextAlignment dominates

                    return textBox.TextAlignment;

            }

 

            RichTextBox richTextBox = adornedElement as RichTextBox;

            if (richTextBox != null)

            {

                BlockCollection blocks = richTextBox.Document.Blocks;

                TextAlignment textAlignment = richTextBox.Document.TextAlignment;

                if (blocks.Count == 0)

                    return textAlignment;

 

                if (blocks.Count == 1)

                {

                    Paragraph paragraph = blocks.FirstBlock as Paragraph;

                    if (paragraph == null)

                        return textAlignment;

 

                    return paragraph.TextAlignment;

                }

 

                return textAlignment;

            }

 

            switch (adornedElement.HorizontalContentAlignment)

            {

                case HorizontalAlignment.Left:

                    return TextAlignment.Left;

                case HorizontalAlignment.Right:

                    return TextAlignment.Right;

                case HorizontalAlignment.Center:

                    return TextAlignment.Center;

                case HorizontalAlignment.Stretch:

                    return TextAlignment.Justify;

            }

 

            return TextAlignment.Left;

        }

 

        /// <summary>

        ///   Draws the content of a <see cref="T:System.Windows.Media.DrawingContext" /> object during the render pass of a <see cref="T:Huan.WhiteDwarf.UI.Placeholder"/> element.

        /// </summary>

        /// <param name="drawingContext">

        ///   The <see cref="T:System.Windows.Media.DrawingContext" /> object to draw. This context is provided to the layout system.

        /// </param>

        protected override void OnRender(DrawingContext drawingContext)

        {

            Control adornedElement = this.AdornedElement as Control;

            string placeholderText;

 

            if (adornedElement == null ||

                adornedElement.IsFocused ||

                !IsElementEmpty() ||

                string.IsNullOrEmpty(placeholderText = (string)adornedElement.GetValue(TextProperty)))

 

                _isPlaceholderVisible = false;

            else

            {

                _isPlaceholderVisible = true;

                Size size = adornedElement.RenderSize;

                TextAlignment computedTextAlignment = ComputedTextAlignment();

                // foreground brush does not need to be dynamic. OnRender called when SystemColors changes.

                Brush foreground = SystemColors.GrayTextBrush.Clone();

                foreground.Opacity = adornedElement.Foreground.Opacity;

                Typeface typeface = new Typeface(adornedElement.FontFamily, FontStyles.Italic, adornedElement.FontWeight, adornedElement.FontStretch);

                FormattedText formattedText = new FormattedText(placeholderText,

                                                  CultureInfo.CurrentCulture,

                                                  adornedElement.FlowDirection,

                                                  typeface,

                                                  adornedElement.FontSize,

                                                  foreground);

                formattedText.TextAlignment = computedTextAlignment;

                formattedText.MaxTextHeight = size.Height - adornedElement.BorderThickness.Top - adornedElement.BorderThickness.Bottom - adornedElement.Padding.Top - adornedElement.Padding.Bottom;

                formattedText.MaxTextWidth = size.Width - adornedElement.BorderThickness.Left - adornedElement.BorderThickness.Right - adornedElement.Padding.Left - adornedElement.Padding.Right - 4.0;

 

                double left;

                double top = 0.0;

                if (adornedElement.FlowDirection == FlowDirection.RightToLeft)

                    left = adornedElement.BorderThickness.Right + adornedElement.Padding.Right + 2.0;

                else

                    left = adornedElement.BorderThickness.Left + adornedElement.Padding.Left + 2.0;

 

                switch (adornedElement.VerticalContentAlignment)

                {

                    case VerticalAlignment.Top:

                    case VerticalAlignment.Stretch:

                        top = adornedElement.BorderThickness.Top + adornedElement.Padding.Top;

                        break;

                    case VerticalAlignment.Bottom:

                        top = size.Height - adornedElement.BorderThickness.Bottom - adornedElement.Padding.Bottom - formattedText.Height;

                        break;

                    case VerticalAlignment.Center:

                        top = (size.Height + adornedElement.BorderThickness.Top - adornedElement.BorderThickness.Bottom + adornedElement.Padding.Top - adornedElement.Padding.Bottom - formattedText.Height) / 2.0;

                        break;

                }

 

                if (adornedElement.FlowDirection == FlowDirection.RightToLeft)

                {

                    // Somehow everything got drawn reflected. Add a transform to correct.

                    drawingContext.PushTransform(new ScaleTransform(-1.0, 1.0, RenderSize.Width / 2.0, 0.0));

                    drawingContext.DrawText(formattedText, new Point(left, top));

                    drawingContext.Pop();

                }

                else

                    drawingContext.DrawText(formattedText, new Point(left, top));

            }

        }

    }

}

4 comments:

Dima said...

Nice.
I was about to start writing my own watermark textbox when I found yours. I like the idea of attached property to make this class more universal.
Good job :)

Anonymous said...

Hi. I love this solution. Thanks!

One constructive criticism:
I recommend keeping the onloaded event permanently assigned (i.e. assign it whether or not the element has already loaded in OnTextChanged and don't remove the event in AdornedElementLoaded). This is so that it the adornment survives when you switch tabs or similar. That the element will only be loaded once is not a safe assumption.

Thanks again!

Anonymous said...

if the you used the placeholder in a usercontrol and added it to window and tried to hide the usercontrol the placeholder will stay there how to fix that ?

Jeow Li Huan said...

On top of registering for DependencyProperty.RegisterAttached("Text", ..., you will also need to register for Visibility