Custom Control Part 2

In "Custom Control Part 1" we created a raw custom control that does nothing. In this post we'll change the control so it only does "one thing" - stores its original value and when the current value differs it sets the HasChanged property to true.

This needs a couple of private member vars (OldText & Initialised) to hold the state, a new DependencyProperty (HasChangedProperty), a standard CLR property (HasChanged) and some logic added to the TextPropertyChanged event handler.

public class DemoTextBox : TextBox
{
   static DemoTextBox()
   {
      // Create a callback for when the TextProperty changes
      PropertyChangedCallback callback = new PropertyChangedCallback(TextPropertyChanged);

      // Override the TextProperty metadata in order to link up the callback routine
      TextProperty.OverrideMetadata(
         typeof(DemoTextBox),
         new FrameworkPropertyMetadata(callback));
   }
   static void TextPropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
   {
      // Cast the Sender to an object of this class
      DemoTextBox textBox = (DemoTextBox)sender;
 

      // Store the OldText value when this event is
      // fired for the first time
 
      if (!textBox.Initialised)
      {
         textBox.Initialised = true;
         textBox.OldText = textBox.Text;
      }
      
      // Set the HasChangedProperty to indicate whether the text has changed

      textBox.SetValue(HasChangedProperty, (textBox.OldText != textBox.Text));

 
   }

 
   private bool Initialised = false;
   private string OldText = "";
 
   // The new DependencyProperty
 
   static readonly DependencyProperty HasChangedProperty = DependencyProperty.Register(
      "HasChanged",
      typeof(bool),
      typeof(DemoTextBox)
      );
 
   // The new CLR Property
 
   public bool HasChanged
   {
      get { return (bool)GetValue(HasChangedProperty); }
   }
}

In the Generic.xaml we'll add a Trigger to turn the textbox background Red when the Text value has changed.

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:DepProp">
   <Style TargetType="{x:Type local:DemoTextBox}">      
      <Style.Triggers>

         <Trigger  Property="HasChanged" Value="true">
            <Setter Property="Background" Value="Red"/>
         </Trigger>

      </Style.Triggers>
   </Style>
</ResourceDictionary>