How to detect a change in the Text property of a TextBlock?

Question

Is there any way to detect a change in the Text property of a TextBlock element using events?

(I'm trying to provide an animation for highlighting the TextBlocks whose Text property change within a DataGrid)

Sandhog

Take a look at this: https://mcmap.net/q/454072/-wpf-textblock-text-changed-notify – Tumbling

Answer 1

As far as I can understand there isn't any textchanged event in TextBlock. Looking at your requirement, I feel that re-templating a textbox will also not be a viable solution. From my preliminary searching around, this seems to be a possible solution.

<TextBlock x:Name="tbMessage" Text="{Binding Path=StatusBarText, NotifyOnTargetUpdated=True}">
    <TextBlock.Triggers>
        <EventTrigger RoutedEvent="Binding.TargetUpdated">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation Storyboard.TargetProperty="Opacity" Duration="0:0:0″
To="1.0″ />
                    <DoubleAnimation Storyboard.TargetProperty="Opacity" Duration="0:0:2″
From="1.0″ To="0.0″ BeginTime="0:0:5″ />
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </TextBlock.Triggers>
</TextBlock>

Guan

That's exactly what I wanted to do. Thank you very much. – Sandhog

Answer 2

It's easier than that! Late answer, but much simpler.

// assume textBlock is your TextBlock
var dp = DependencyPropertyDescriptor.FromProperty(
             TextBlock.TextProperty,
             typeof(TextBlock));
dp.AddValueChanged(textBlock, (sender, args) =>
{
    MessageBox.Show("text changed");
});

Analgesic

Where are you putting dp.AddValueChanged? – Leenaleeper

you can put that in the code behind. for example below InitializeComponent() – Nierman

Keep in mind this will introduce a memory leak as well! You need to make sure to unsubscribe as well – Brobdingnagian

Answer 3

This is like the code from the link in bioskope's answer, but simplified. You need the TargetUpdated event and add NotifyOnTargetUpdated=True to the binding.

<TextBlock Text="{Binding YourTextProperty, NotifyOnTargetUpdated=True}" 
           TargetUpdated="YourTextEventHandler"/>

Moire

By using the code behind event, in my opinion, this should be the correct answer. – Adamsen

Disagree. Bioscope's answer has the advantage of not needing code behind. It's nice to know about this answer too, but there is no "the correct answer" here. – Ruscio

Answer 4

Bind the Text property to a DependencyProperty, which has an event trigger:

public static string GetTextBoxText(DependencyObject obj)
{
    return (string)obj.GetValue(TextBoxTextProperty);
}

public static void SetTextBoxText(DependencyObject obj, string value)
{
    obj.SetValue(TextBoxTextProperty, value);
}

public static readonly DependencyProperty TextBoxTextProperty =
    DependencyProperty.RegisterAttached(
    "TextBoxText",
    typeof(string),
    typeof(TextBlockToolTipBehavior),
    new FrameworkPropertyMetadata(string.Empty, TextBoxTextChangedCallback)
    );

private static void TextBoxTextChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    TextBlock textBlock = d as TextBlock;
    HandleTextChange(textBlock);
}

In the XAML Bind to the TextBlock text Property:

<TextBlock  
 Text="{Binding SomeProperty, UpdateSourceTrigger=PropertyChanged}"  
     th:TextBlockBehavior.TextBoxText="{Binding Text, 
     RelativeSource={RelativeSource Self}}" />

Kindhearted

Answer 5

Here is a similar example on MSDN using code-behind: http://msdn.microsoft.com/en-us/library/system.windows.data.binding.targetupdated.aspx

Totalizer

Answer 6

Here's something you can use I picked up from Jerry Nixon and Daren May at the Microsoft Virtual Academy "Developing Universal Windows Apps with C# and XAML" and the code that contains the DependencyObject logic is here "(W8.1-WP8.1) UNIVERSAL APP FOR MVA".

namespace App1.Behaviors
{
// <summary>
/// Helper class that allows you to monitor a property corresponding to a dependency property 
/// on some object for changes and have an event raised from
/// the instance of this helper that you can handle.
/// Usage: Construct an instance, passing in the object and the name of the normal .NET property that
/// wraps a DependencyProperty, then subscribe to the PropertyChanged event on this helper instance. 
/// Your subscriber will be called whenever the source DependencyProperty changes.
/// </summary>
public class DependencyPropertyChangedHelper : DependencyObject
{
    /// <summary>
    /// Constructor for the helper. 
    /// </summary>
    /// <param name="source">Source object that exposes the DependencyProperty you wish to monitor.</param>
    /// <param name="propertyPath">The name of the property on that object that you want to monitor.</param>
    public DependencyPropertyChangedHelper(DependencyObject source, string propertyPath)
    {
        // Set up a binding that flows changes from the source DependencyProperty through to a DP contained by this helper 
        Binding binding = new Binding
        {
            Source = source,
            Path = new PropertyPath(propertyPath)
        };
        BindingOperations.SetBinding(this, HelperProperty, binding);
    }

    /// <summary>
    /// Dependency property that is used to hook property change events when an internal binding causes its value to change.
    /// This is only public because the DependencyProperty syntax requires it to be, do not use this property directly in your code.
    /// </summary>
    public static DependencyProperty HelperProperty =
        DependencyProperty.Register("Helper", typeof(object), typeof(DependencyPropertyChangedHelper), new PropertyMetadata(null, OnPropertyChanged));

    /// <summary>
    /// Wrapper property for a helper DependencyProperty used by this class. Only public because the DependencyProperty syntax requires it.
    /// DO NOT use this property directly.
    /// </summary>
    public object Helper
    {
        get { return (object)GetValue(HelperProperty); }
        set { SetValue(HelperProperty, value); }
    }

    // When our dependency property gets set by the binding, trigger the property changed event that the user of this helper can subscribe to
    private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var helper = (DependencyPropertyChangedHelper)d;
        helper.PropertyChanged(d, e);
    }

    /// <summary>
    /// This event will be raised whenever the source object property changes, and carries along the before and after values
    /// </summary>
    public event EventHandler<DependencyPropertyChangedEventArgs> PropertyChanged = delegate { };
}
}

Usage XAML:

<TextBlock Grid.Row="0"
       x:Name="WritingMenuTitle"
       HorizontalAlignment="Left"
       FontSize="32"
       FontWeight="SemiBold"
       Text="{Binding WritingMenu.Title}"
       TextAlignment="Left"
       TextWrapping="Wrap"/>

Usage xaml.cs:

Behaviors.DependencyPropertyChangedHelper helper = new Behaviors.DependencyPropertyChangedHelper(this.WritingMenuTitle, Models.CommonNames.TextBlockText);
helper.PropertyChanged += viewModel.OnSenarioTextBlockTextChangedEvent;

Usage viewmodel.cs:

public async void OnSenarioTextBlockTextChangedEvent(object sender, DependencyPropertyChangedEventArgs args)
{
StringBuilder sMsg = new StringBuilder();

try
{
    Debug.WriteLine(String.Format(".....WritingMenuTitle : New ({0}), Old ({1})", args.NewValue, args.OldValue));
}
catch (Exception msg)
{
    #region Exception
    .....
    #endregion
}
}

Mindamindanao