How to: Implement ICommandSource

 

This example shows how to create a command source by implementing ICommandSource. A command source is an object that knows how to invoke a command. The ICommandSource interface exposes three members: Command, CommandParameter, and CommandTarget. Command is the command which will be invoked. The CommandParameter is a user-defined data type which is passed from the command source to the method which handles the command. The CommandTarget is the object that the command is being executed on.

In this example, a class is created which subclasses the Slider control and implements ICommandSource.

WPF provides a number of classes which implement ICommandSource, such as Button, MenuItem, and ListBoxItem. A command source defines how it invokes a command. Button and MenuItem invoke a command when they are clicked. A ListBoxItem invokes a command when it is double clicked. These classes only become a command source when their Command property is set.

For this example we will invoke the command when the slider is moved, or more accurately, when the Value property is changed.

The following is the class definition.

    Public Class CommandSlider
        Inherits Slider
        Implements ICommandSource
        Public Sub New()
            MyBase.New()

        End Sub

The next step is to implement the ICommandSource members. In this example, the properties are implemented as DependencyProperty objects. This enables the properties to use data binding. For more information about the DependencyProperty class, see the Dependency Properties Overview. For more information about data binding, see the Data Binding Overview.

Only the Command property is shown here.

        ' Make Command a dependency property so it can use databinding.
        Public Shared ReadOnly CommandProperty As DependencyProperty =
            DependencyProperty.Register("Command", GetType(ICommand),
                GetType(CommandSlider),
                New PropertyMetadata(CType(Nothing, ICommand),
                    New PropertyChangedCallback(AddressOf CommandChanged)))

        Public ReadOnly Property Command1() As ICommand Implements ICommandSource.Command
            Get
                Return CType(GetValue(CommandProperty), ICommand)
            End Get
        End Property

        Public Property Command() As ICommand
            Get
                Return CType(GetValue(CommandProperty), ICommand)
            End Get
            Set(ByVal value As ICommand)
                SetValue(CommandProperty, value)
            End Set
        End Property

The following is the DependencyProperty change callback.

        ' Command dependency property change callback.
        Private Shared Sub CommandChanged(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
            Dim cs As CommandSlider = CType(d, CommandSlider)
            cs.HookUpCommand(CType(e.OldValue, ICommand), CType(e.NewValue, ICommand))
        End Sub

The next step is to add and remove the command which is associated with the command source. The Command property cannot simply be overwritten when a new command is added, because the event handlers associated with the previous command, if there was one, must be removed first.

        ' Add a new command to the Command Property.
        Private Sub HookUpCommand(ByVal oldCommand As ICommand, ByVal newCommand As ICommand)
            ' If oldCommand is not null, then we need to remove the handlers.
            If oldCommand IsNot Nothing Then
                RemoveCommand(oldCommand, newCommand)
            End If
            AddCommand(oldCommand, newCommand)
        End Sub

        ' Remove an old command from the Command Property.
        Private Sub RemoveCommand(ByVal oldCommand As ICommand, ByVal newCommand As ICommand)
            Dim handler As EventHandler = AddressOf CanExecuteChanged
            RemoveHandler oldCommand.CanExecuteChanged, handler
        End Sub

        ' Add the command.
        Private Sub AddCommand(ByVal oldCommand As ICommand, ByVal newCommand As ICommand)
            Dim handler As New EventHandler(AddressOf CanExecuteChanged)
            canExecuteChangedHandler = handler
            If newCommand IsNot Nothing Then
                AddHandler newCommand.CanExecuteChanged, canExecuteChangedHandler
            End If
        End Sub

The last step is to create logic for the CanExecuteChanged handler and the Execute method.

The CanExecuteChanged event notifies the command source that the ability of the command to execute on the current command target may have changed. When a command source receives this event, it typically calls the CanExecute method on the command. If the command cannot execute on the current command target, the command source will typically disable itself. If the command can execute on the current command target, the command source will typically enable itself.

        Private Sub CanExecuteChanged(ByVal sender As Object, ByVal e As EventArgs)

            If Me.Command IsNot Nothing Then
                Dim command As RoutedCommand = TryCast(Me.Command, RoutedCommand)

                ' If a RoutedCommand.
                If command IsNot Nothing Then
                    If command.CanExecute(CommandParameter, CommandTarget) Then
                        Me.IsEnabled = True
                    Else
                        Me.IsEnabled = False
                    End If
                    ' If a not RoutedCommand.
                Else
                    If Me.Command.CanExecute(CommandParameter) Then
                        Me.IsEnabled = True
                    Else
                        Me.IsEnabled = False
                    End If
                End If
            End If
        End Sub

The last step is the Execute method. If the command is a RoutedCommand, the RoutedCommand Execute method is called; otherwise, the ICommand Execute method is called.

        ' If Command is defined, moving the slider will invoke the command;
        ' Otherwise, the slider will behave normally.
        Protected Overrides Sub OnValueChanged(ByVal oldValue As Double, ByVal newValue As Double)
            MyBase.OnValueChanged(oldValue, newValue)

            If Me.Command IsNot Nothing Then
                Dim command As RoutedCommand = TryCast(Me.Command, RoutedCommand)

                If command IsNot Nothing Then
                    command.Execute(CommandParameter, CommandTarget)
                Else
                    CType(Me.Command, ICommand).Execute(CommandParameter)
                End If
            End If
        End Sub

ICommandSource
ICommand
RoutedCommand
Commanding Overview

Show: