How to: Create a Custom Behavior

• Jun 07, 2019
• 7 minutes to read

The DevExpress.Mvvm.UI.Interactivity.Interaction class implements a Behaviors attached property. This property is read-only and contains a collection of Behaviors.

XAML

<TextBox Text="Text">
    <dxmvvm:Interaction.Behaviors>
        <local:ValidationBehavior />
        <!--Other Behaviors-->
    </dxmvvm:Interaction.Behaviors>
</TextBox>

All Behaviors descend from the Behavior<T> class. The T parameter sets the type of the associated control. For instance, the Behavior below can only be applied to the TextBox control (or for a control descended from the TextBox control).

C#

public class ValidationBehavior : Behavior<TextBox> {
    ...
}

VB.NET

Public Class ValidationBehavior
    Inherits Behavior(Of TextBox)
    '...
End Class

The Behavior<T> class provides the AssociatedObject property that is set internally once a Behavior is added to the Behaviors collection. After the AssociatedObject property is set, the OnAttached virtual method is invoked. You can override this method to subscribe to AssociatedObject events and initialize properties. When the Behavior is destroyed, an OnDetaching virtual method is called, where you can unsubscribe from events.

Also, behaviors may provide properties and events that you can set or bind in Xaml.

C#

public class ValidationBehavior : Behavior<TextBox> {
    public static readonly DependencyProperty ValidForegroundProperty =
        DependencyProperty.Register("ValidForeground", typeof(Brush), typeof(ValidationBehavior),
        new PropertyMetadata(new SolidColorBrush(Colors.Black), (d, e) => ((ValidationBehavior)d).Update()));
    public static readonly DependencyProperty InvalidForegroundProperty =
        DependencyProperty.Register("InvalidForeground", typeof(Brush), typeof(ValidationBehavior), 
        new PropertyMetadata(new SolidColorBrush(Colors.Red), (d,e) => ((ValidationBehavior)d).Update()));
    public static readonly DependencyProperty InvalidValueProperty =
        DependencyProperty.Register("InvalidValue", typeof(string), typeof(ValidationBehavior),
        new PropertyMetadata(string.Empty, (d, e) => ((ValidationBehavior)d).Update()));
    public Brush ValidForeground {
        get { return (Brush)GetValue(ValidForegroundProperty); }
        set { SetValue(ValidForegroundProperty, value); }
    }
    public Brush InvalidForeground {
        get { return (Brush)GetValue(InvalidForegroundProperty); }
        set { SetValue(InvalidForegroundProperty, value); }
    }
    public string InvalidValue {
        get { return (string)GetValue(InvalidValueProperty); }
        set { SetValue(InvalidValueProperty, value); }
    }

    protected override void OnAttached() {
        base.OnAttached();
        AssociatedObject.TextChanged += OnAssociatedObjectTextChanged;
        Update();
    }
    protected override void OnDetaching() {
        AssociatedObject.TextChanged -= OnAssociatedObjectTextChanged;
        base.OnDetaching();
    }
    void OnAssociatedObjectTextChanged(object sender, TextChangedEventArgs e) {
        Update();
    }
    void Update() {
        if(AssociatedObject == null) return;
        if(AssociatedObject.Text == InvalidValue)
            AssociatedObject.Foreground = InvalidForeground;
        else AssociatedObject.Foreground = ValidForeground;
    }
}

VB.NET

Public Class ValidationBehavior
    Inherits Behavior(Of TextBox)

    Public Shared ReadOnly ValidForegroundProperty As DependencyProperty = DependencyProperty.Register("ValidForeground", GetType(Brush), GetType(ValidationBehavior), New PropertyMetadata(New SolidColorBrush(Colors.Black), Sub(d, e) CType(d, ValidationBehavior).Update()))
    Public Shared ReadOnly InvalidForegroundProperty As DependencyProperty = DependencyProperty.Register("InvalidForeground", GetType(Brush), GetType(ValidationBehavior), New PropertyMetadata(New SolidColorBrush(Colors.Red), Sub(d,e) CType(d, ValidationBehavior).Update()))
    Public Shared ReadOnly InvalidValueProperty As DependencyProperty = DependencyProperty.Register("InvalidValue", GetType(String), GetType(ValidationBehavior), New PropertyMetadata(String.Empty, Sub(d, e) CType(d, ValidationBehavior).Update()))
    Public Property ValidForeground() As Brush
        Get
            Return CType(GetValue(ValidForegroundProperty), Brush)
        End Get
        Set(ByVal value As Brush)
            SetValue(ValidForegroundProperty, value)
        End Set
    End Property
    Public Property InvalidForeground() As Brush
        Get
            Return CType(GetValue(InvalidForegroundProperty), Brush)
        End Get
        Set(ByVal value As Brush)
            SetValue(InvalidForegroundProperty, value)
        End Set
    End Property
    Public Property InvalidValue() As String
        Get
            Return CStr(GetValue(InvalidValueProperty))
        End Get
        Set(ByVal value As String)
            SetValue(InvalidValueProperty, value)
        End Set
    End Property

    Protected Overrides Sub OnAttached()
        MyBase.OnAttached()
        AddHandler AssociatedObject.TextChanged, AddressOf OnAssociatedObjectTextChanged
        Update()
    End Sub
    Protected Overrides Sub OnDetaching()
        RemoveHandler AssociatedObject.TextChanged, AddressOf OnAssociatedObjectTextChanged
        MyBase.OnDetaching()
    End Sub
    Private Sub OnAssociatedObjectTextChanged(ByVal sender As Object, ByVal e As TextChangedEventArgs)
        Update()
    End Sub
    Private Sub Update()
        If AssociatedObject Is Nothing Then
            Return
        End If
        If AssociatedObject.Text = InvalidValue Then
            AssociatedObject.Foreground = InvalidForeground
        Else
            AssociatedObject.Foreground = ValidForeground
        End If
    End Sub
End Class

XAML

<TextBox Text="Text">
    <dxmvvm:Interaction.Behaviors>
        <Behaviors:ValidationBehavior ValidForeground="{Binding ValidBrush}"
                                      InvalidForeground="{Binding InvalidBrush}"
                                      InvalidValue="{Binding InvalidValue}"/>
    </dxmvvm:Interaction.Behaviors>
</TextBox>

Example

The below example demonstrates a more complex example of how to create a custom Behavior.

Note

A complete sample project is available at https://github.com/DevExpress-Examples/how-to-create-a-custom-attached-behavior-e5161.

MainViewModel.vb

Imports DevExpress.Mvvm.DataAnnotations
Imports System.Windows.Media

Namespace Example.ViewModel
    <POCOViewModel> _
    Public Class MainViewModel
        Public Overridable Property InvalidValue() As String
        Public Overridable Property ValidBrush() As Brush
        Public Overridable Property InvalidBrush() As Brush

        Public Sub SetRedValidBrush()
            ValidBrush = New SolidColorBrush(Colors.Red)
        End Sub
        Public Sub SetBlueValidBrush()
            ValidBrush = New SolidColorBrush(Colors.Blue)
        End Sub
        Public Sub SetBlackValidBrush()
            ValidBrush = New SolidColorBrush(Colors.Black)
        End Sub

        Public Sub SetRedInvalidBrush()
            InvalidBrush = New SolidColorBrush(Colors.Red)
        End Sub
        Public Sub SetBlueInvalidBrush()
            InvalidBrush = New SolidColorBrush(Colors.Blue)
        End Sub
        Public Sub SetBlackInvalidBrush()
            InvalidBrush = New SolidColorBrush(Colors.Black)
        End Sub

        Public Sub New()
            InvalidValue = "Error"
            SetBlackValidBrush()
            SetRedInvalidBrush()
        End Sub
    End Class
End Namespace

MainWindow.xaml.vb

Imports System.Windows

Namespace Example
    Partial Public Class MainWindow
        Inherits Window

        Public Sub New()
            InitializeComponent()
        End Sub
    End Class
End Namespace

MainView.xaml.vb

Imports System.Windows.Controls

Namespace Example.View
    Partial Public Class MainView
        Inherits UserControl

        Public Sub New()
            InitializeComponent()
        End Sub
    End Class
End Namespace

ValidationBehavior.vb

Imports DevExpress.Mvvm.UI.Interactivity
Imports System.Windows
Imports System.Windows.Controls
Imports System.Windows.Media

Namespace Example.Behaviors
    Public Class ValidationBehavior
        Inherits Behavior(Of TextBox)

        Public Shared ReadOnly ValidForegroundProperty As DependencyProperty = DependencyProperty.Register("ValidForeground", GetType(Brush), GetType(ValidationBehavior), New PropertyMetadata(New SolidColorBrush(Colors.Black), Sub(d, e) CType(d, ValidationBehavior).Update()))
        Public Shared ReadOnly InvalidForegroundProperty As DependencyProperty = DependencyProperty.Register("InvalidForeground", GetType(Brush), GetType(ValidationBehavior), New PropertyMetadata(New SolidColorBrush(Colors.Red), Sub(d,e) CType(d, ValidationBehavior).Update()))
        Public Shared ReadOnly InvalidValueProperty As DependencyProperty = DependencyProperty.Register("InvalidValue", GetType(String), GetType(ValidationBehavior), New PropertyMetadata(String.Empty, Sub(d, e) CType(d, ValidationBehavior).Update()))
        Public Property ValidForeground() As Brush
            Get
                Return CType(GetValue(ValidForegroundProperty), Brush)
            End Get
            Set(ByVal value As Brush)
                SetValue(ValidForegroundProperty, value)
            End Set
        End Property
        Public Property InvalidForeground() As Brush
            Get
                Return CType(GetValue(InvalidForegroundProperty), Brush)
            End Get
            Set(ByVal value As Brush)
                SetValue(InvalidForegroundProperty, value)
            End Set
        End Property
        Public Property InvalidValue() As String
            Get
                Return CStr(GetValue(InvalidValueProperty))
            End Get
            Set(ByVal value As String)
                SetValue(InvalidValueProperty, value)
            End Set
        End Property

        Protected Overrides Sub OnAttached()
            MyBase.OnAttached()
            AddHandler AssociatedObject.TextChanged, AddressOf OnAssociatedObjectTextChanged
            Update()
        End Sub
        Protected Overrides Sub OnDetaching()
            RemoveHandler AssociatedObject.TextChanged, AddressOf OnAssociatedObjectTextChanged
            MyBase.OnDetaching()
        End Sub
        Private Sub OnAssociatedObjectTextChanged(ByVal sender As Object, ByVal e As TextChangedEventArgs)
            Update()
        End Sub
        Private Sub Update()
            If AssociatedObject Is Nothing Then
                Return
            End If
            If AssociatedObject.Text = InvalidValue Then
                AssociatedObject.Foreground = InvalidForeground
            Else
                AssociatedObject.Foreground = ValidForeground
            End If
        End Sub
    End Class
End Namespace

Application.xaml.vb

Imports System.Windows

Namespace Example
    Partial Public Class App
        Inherits Application

        Public Sub New()
            AddHandler Startup, AddressOf OnStartup
#If SILVERLIGHT Then
            InitializeComponent()
#End If
        End Sub
        Private Overloads Sub OnStartup(ByVal sender As Object, ByVal e As StartupEventArgs)
#If SILVERLIGHT Then
            Me.RootVisual = New MainPage()
#Else
            MainWindow = New MainWindow()
            MainWindow.Show()
#End If
        End Sub
    End Class
End Namespace

MainView.xaml (C#)

<UserControl x:Class="Example.View.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ViewModel="clr-namespace:Example.ViewModel"
    xmlns:Behaviors="clr-namespace:Example.Behaviors"
    xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignHeight="500" d:DesignWidth="600"
    DataContext="{dxmvvm:ViewModelSource Type=ViewModel:MainViewModel}">

    <Grid x:Name="LayoutRoot" Background="White">
        <StackPanel Orientation="Vertical">
            <StackPanel Orientation="Horizontal" Margin="10">
                <TextBlock Text="InputBox: " VerticalAlignment="Center" Margin="5"/>
                <TextBox Text="Error" Margin="5" Width="80">
                    <dxmvvm:Interaction.Behaviors>
                        <Behaviors:ValidationBehavior ValidForeground="{Binding ValidBrush}"
                                                      InvalidForeground="{Binding InvalidBrush}"
                                                      InvalidValue="{Binding InvalidValue}"/>
                    </dxmvvm:Interaction.Behaviors>
                </TextBox>
            </StackPanel>
            <StackPanel Orientation="Horizontal" Margin="10">
                <TextBlock Text="InvalidValue: " VerticalAlignment="Center" Margin="5"/>
                <TextBox Text="{Binding InvalidValue, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Margin="5"  Width="80"/>
            </StackPanel>
            <StackPanel Orientation="Horizontal" Margin="10">
                <TextBlock Text="ValidBrush" VerticalAlignment="Center" Margin="5"/>
                <Button Command="{Binding SetRedValidBrushCommand}">
                    <Border Width="20" Height="20" Background="Red" Margin="5" VerticalAlignment="Center"/>
                </Button>
                <Button Command="{Binding SetBlueValidBrushCommand}">
                    <Border Width="20" Height="20" Background="Blue" Margin="5" VerticalAlignment="Center"/>
                </Button>
                <Button Command="{Binding SetBlackValidBrushCommand}">
                    <Border Width="20" Height="20" Background="Black" Margin="5" VerticalAlignment="Center"/>
                </Button>
            </StackPanel>
            <StackPanel Orientation="Horizontal" Margin="10">
                <TextBlock Text="InvalidBrush" VerticalAlignment="Center" Margin="5"/>
                <Button Command="{Binding SetRedInvalidBrushCommand}">
                    <Border Width="20" Height="20" Background="Red" Margin="5" VerticalAlignment="Center"/>
                </Button>
                <Button Command="{Binding SetBlueInvalidBrushCommand}">
                    <Border Width="20" Height="20" Background="Blue" Margin="5" VerticalAlignment="Center"/>
                </Button>
                <Button Command="{Binding SetBlackInvalidBrushCommand}">
                    <Border Width="20" Height="20" Background="Black" Margin="5" VerticalAlignment="Center"/>
                </Button>
            </StackPanel>

        </StackPanel>

    </Grid>
</UserControl>

Application.xaml

<Application x:Class="Example.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Application.Resources>

    </Application.Resources>
</Application>

MainWindow.xaml (VB.NET)

<Window x:Class="Example.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:View="clr-namespace:Example.View"
        Title="MainWindow" Height="600" Width="800">
    <Grid>
        <View:MainView/>
    </Grid>
</Window>

MainViewModel.cs

using DevExpress.Mvvm.DataAnnotations;
using System.Windows.Media;

namespace Example.ViewModel {
    [POCOViewModel]
    public class MainViewModel {
        public virtual string InvalidValue { get; set; }
        public virtual Brush ValidBrush { get; set; }
        public virtual Brush InvalidBrush { get; set; }

        public void SetRedValidBrush() {
            ValidBrush = new SolidColorBrush(Colors.Red);
        }
        public void SetBlueValidBrush() {
            ValidBrush = new SolidColorBrush(Colors.Blue);
        }
        public void SetBlackValidBrush() {
            ValidBrush = new SolidColorBrush(Colors.Black);
        }

        public void SetRedInvalidBrush() {
            InvalidBrush = new SolidColorBrush(Colors.Red);
        }
        public void SetBlueInvalidBrush() {
            InvalidBrush = new SolidColorBrush(Colors.Blue);
        }
        public void SetBlackInvalidBrush() {
            InvalidBrush = new SolidColorBrush(Colors.Black);
        }

        public MainViewModel() {
            InvalidValue = "Error";
            SetBlackValidBrush();
            SetRedInvalidBrush();
        }
    }
}

MainView.xaml.cs

using System.Windows.Controls;

namespace Example.View {
    public partial class MainView : UserControl {
        public MainView() {
            InitializeComponent();
        }
    }
}

MainWindow.xaml.cs

using System.Windows;

namespace Example {
    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
        }
    }
}

ValidationBehavior.cs

using DevExpress.Mvvm.UI.Interactivity;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace Example.Behaviors {
    public class ValidationBehavior : Behavior<TextBox> {
        public static readonly DependencyProperty ValidForegroundProperty =
            DependencyProperty.Register("ValidForeground", typeof(Brush), typeof(ValidationBehavior),
            new PropertyMetadata(new SolidColorBrush(Colors.Black), (d, e) => ((ValidationBehavior)d).Update()));
        public static readonly DependencyProperty InvalidForegroundProperty =
            DependencyProperty.Register("InvalidForeground", typeof(Brush), typeof(ValidationBehavior), 
            new PropertyMetadata(new SolidColorBrush(Colors.Red), (d,e) => ((ValidationBehavior)d).Update()));
        public static readonly DependencyProperty InvalidValueProperty =
            DependencyProperty.Register("InvalidValue", typeof(string), typeof(ValidationBehavior),
            new PropertyMetadata(string.Empty, (d, e) => ((ValidationBehavior)d).Update()));
        public Brush ValidForeground {
            get { return (Brush)GetValue(ValidForegroundProperty); }
            set { SetValue(ValidForegroundProperty, value); }
        }
        public Brush InvalidForeground {
            get { return (Brush)GetValue(InvalidForegroundProperty); }
            set { SetValue(InvalidForegroundProperty, value); }
        }
        public string InvalidValue {
            get { return (string)GetValue(InvalidValueProperty); }
            set { SetValue(InvalidValueProperty, value); }
        }

        protected override void OnAttached() {
            base.OnAttached();
            AssociatedObject.TextChanged += OnAssociatedObjectTextChanged;
            Update();
        }
        protected override void OnDetaching() {
            AssociatedObject.TextChanged -= OnAssociatedObjectTextChanged;
            base.OnDetaching();
        }
        void OnAssociatedObjectTextChanged(object sender, TextChangedEventArgs e) {
            Update();
        }
        void Update() {
            if(AssociatedObject == null) return;
            if(AssociatedObject.Text == InvalidValue)
                AssociatedObject.Foreground = InvalidForeground;
            else AssociatedObject.Foreground = ValidForeground;
        }
    }
}