Introduction
I'll highlight here, no pun intended, how you can go about highlighting the text in a TextBlock using attached properties, which enable setting of the text to highlight and its background and foreground color.
TextBlockHighlighter
TextBlockHighlighter is a class containing definitions of attached properties for highlighting text in a TextBlock. The first property, Selection, is used to specify which text in the TextBlock will be highlighted. Highlighting is done in the property's callback method by making use of Inlines.
public static readonly DependencyProperty SelectionProperty =
DependencyProperty.RegisterAttached("Selection", typeof(string), typeof(TextBlockHighlighter),
new PropertyMetadata(new PropertyChangedCallback(SelectText)));
private static void SelectText(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (d == null) return;
if (!(d is TextBlock)) throw new InvalidOperationException("Only valid for TextBlock");
TextBlock txtBlock = d as TextBlock;
string text = txtBlock.Text;
if (string.IsNullOrEmpty(text)) return;
string highlightText = (string)d.GetValue(SelectionProperty);
if (string.IsNullOrEmpty(highlightText)) return;
int index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase);
if (index < 0) return;
Brush selectionColor = (Brush)d.GetValue(HighlightColorProperty);
Brush forecolor = (Brush)d.GetValue(ForecolorProperty);
txtBlock.Inlines.Clear();
while (true)
{
txtBlock.Inlines.AddRange(new Inline[] {
new Run(text.Substring(0, index)),
new Run(text.Substring(index, highlightText.Length)) {Background = selectionColor,
Foreground = forecolor}
});
text = text.Substring(index + highlightText.Length);
index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase);
if (index < 0)
{
txtBlock.Inlines.Add(new Run(text));
break;
}
}
}
Public Shared ReadOnly SelectionProperty As DependencyProperty =
DependencyProperty.RegisterAttached("Selection", GetType(String), GetType(TextBlockHighlighter),
New PropertyMetadata(New PropertyChangedCallback
(AddressOf SelectText)))
Private Shared Sub SelectText(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
If d Is Nothing Then Exit Sub
If TypeOf d IsNot TextBlock Then Throw New InvalidOperationException("Only valid for textBlock")
Dim txtBlock As TextBlock = CType(d, TextBlock)
Dim text As String = txtBlock.Text
If String.IsNullOrEmpty(text) Then Exit Sub
Dim highlightText As String = CStr(d.GetValue(SelectionProperty))
If String.IsNullOrEmpty(highlightText) Then Exit Sub
Dim index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase)
If index < 0 Then Exit Sub
Dim selectionColor As Brush = CType(d.GetValue(HighlightColorProperty), Brush)
Dim forecolor As Brush = CType(d.GetValue(ForecolorProperty), Brush)
txtBlock.Inlines.Clear()
While True
txtBlock.Inlines.AddRange(New Inline() {
New Run(text.Substring(0, index)),
New Run(text.Substring(index, highlightText.Length)) _
With {.Background = selectionColor, .Foreground = forecolor}
})
text = text.Substring(index + highlightText.Length)
index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase)
If index < 0 Then
txtBlock.Inlines.Add(New Run(text))
Exit While
End If
End While
End Sub
Because the attached property is meant for exclusive use on a TextBlock, an exception is thrown if you attempt to use it on any other element.
Error message displayed in XAML designer
There are two other attached properties which can be used for setting the background and foreground color of the highlighted text.
public static readonly DependencyProperty HighlightColorProperty =
DependencyProperty.RegisterAttached("HighlightColor", typeof(Brush), typeof(TextBlockHighlighter),
new PropertyMetadata(Brushes.Yellow, new PropertyChangedCallback(HighlightText)));
...
public static readonly DependencyProperty ForecolorProperty =
DependencyProperty.RegisterAttached("Forecolor", typeof(Brush), typeof(TextBlockHighlighter),
new PropertyMetadata(Brushes.Black, new PropertyChangedCallback(HighlightText)));
Public Shared ReadOnly HighlightColorProperty As DependencyProperty =
DependencyProperty.RegisterAttached("HighlightColor", GetType(Brush), GetType(TextBlockHighlighter),
New PropertyMetadata(Brushes.Yellow,
New PropertyChangedCallback
(AddressOf SelectText)))
...
Public Shared ReadOnly ForecolorProperty As DependencyProperty =
DependencyProperty.RegisterAttached("Forecolor", GetType(Brush), GetType(TextBlockHighlighter),
New PropertyMetadata(Brushes.Black,
New PropertyChangedCallback
(AddressOf SelectText)))
Usage
To use the attached properties just add a reference to the namespace containing the class with the properties and apply them to a TextBlock element.
<TextBlock Text="Awesome"
local:TextBlockHighlighter.Selection="awe"
local:TextBlockHighlighter.HighlightColor="Aquamarine"
local:TextBlockHighlighter.Forecolor="Red"/>
You can also bind the attached properties: In the downloadable sample project, the TextBlockHighlighter.Selection property is bound to a property of type string which acts as a filter for the elements displayed in an ItemsControl. The TextBlock on which the property is applied is the content of the ItemsControl's data template.
<ItemsControl Margin="10,10,10,0" Background="White"
ItemsSource="{Binding Words}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBlock Margin="10,2" FontWeight="Bold"
Text="{Binding}"
local:TextBlockHighlighter.Selection="{Binding DataContext.Filter,
Mode=OneWay, RelativeSource={RelativeSource AncestorType=Window}}"
local:TextBlockHighlighter.HighlightColor="LightGreen"
local:TextBlockHighlighter.Forecolor="Teal"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
Conclusion
Hopefully, the information provided in this tip was useful, or will be useful to you at some point. The approach I have taken here, using Inlines and a string to define the selection, was inspired by a suggestion on an article on the same subject. Kudos to Bruce Greene on the nice suggestion.
History
- 9th February, 2018: Initial post
