Question

I attached behavior (Behavior in Blend SDK) to a UIElement as I generated it in run time. I also had two buttons to trigger Detach() and Attach() – basically to enable or disable the behavior.

The problem is: after I Detach() the behavior, Attach() failed to restore the behavior to the dependency object, which then remained behavior-less:

// Declare the dependency object and behavior at class scope 
MyControl c = new MyControl();
MyBehavior b = new MyBehavior();

// Function that generate UI Controls with behavior attached
b.Attach(c);

// Function that detach behavior from dependency object
b.Detach();

// Function that re-attach behavior to the same object after it has been detached
b.Attach(c); // <-- Do not see the behavior...

Why is the behavior not re-attaching? And is there a solution or work-around to switch on or off a behavior?

Answer 1

It seems the problem is specific of your behavior’s logic. In the following test the behavior reattaches without problems:

public class ColorBehavior : Behavior<Border>
{
    public Brush OriginalBrush { get; set; }

    protected override void OnAttached()
    {
        base.OnAttached();

        this.OriginalBrush = this.AssociatedObject.Background;
        this.AssociatedObject.Background = Brushes.CadetBlue;
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();
        this.AssociatedObject.Background = this.OriginalBrush;
    }
}

public partial class MainWindow : Window
{
    private ColorBehavior behavior = new ColorBehavior();
    private bool isAttached;

    public MainWindow()
    {
        InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (!this.isAttached)
        {
            this.behavior.Attach(this.Border);
            this.isAttached = true;
        }
        else
        {
            this.behavior.Detach();
            this.isAttached = false;
        }
    }
}

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
    xmlns:local="clr-namespace:WpfApplication1"
    Title="MainWindow"
    Width="525"
    Height="350">
<Grid>
    <Border x:Name="Border" Background="Red" />
    <Button Width="50"
            Height="20"
            Click="Button_Click"
            Content="Hey" />
</Grid>