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.
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.
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.
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 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
' 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