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));
}
}
}
}
