如何：實作 ICommandSource
此範例示範如何藉由實作 ICommandSource 以建立命令來源。 命令來源是知道如何叫用命令的物件。 ICommandSource 介面會公開三個成員：
- Command：將會叫用的命令。
- CommandParameter：使用者定義資料類型，會從命令來源傳遞至處理命令的方法。
- CommandTarget：正在其上執行命令的物件。
在此範例中，會建立繼承自 Slider 控制項並實作 ICommandSource 介面的類別。
範例
WPF 提供實作 ICommandSource 的一些類別，例如 Button、MenuItem 和 Hyperlink。 命令來源會定義其叫用命令的方式。 這些類別會在按一下時叫用命令，而且只有在設定其 Command 屬性時才會成為命令來源。
在此範例中，您會在移動滑桿時，或更準確來說當 Value 屬性變更時，叫用命令。
以下是類別定義：
public class CommandSlider : Slider, ICommandSource
{
public CommandSlider() : base()
{
}
Public Class CommandSlider
Inherits Slider
Implements ICommandSource
Public Sub New()
MyBase.New()
End Sub
下一個步驟是實作 ICommandSource 成員。 在此範例中，屬性會實作為 DependencyProperty 物件。 這可讓屬性使用資料繫結。 如需 DependencyProperty 類別的詳細資訊，請參閱相依性屬性概觀。 如需資料繫結的詳細資訊，請參閱資料繫結概觀。
這裡只會顯示 Command 屬性。
// Make Command a dependency property so it can use databinding.
public static readonly DependencyProperty CommandProperty =
DependencyProperty.Register(
"Command",
typeof(ICommand),
typeof(CommandSlider),
new PropertyMetadata((ICommand)null,
new PropertyChangedCallback(CommandChanged)));
public ICommand Command
{
get
{
return (ICommand)GetValue(CommandProperty);
}
set
{
SetValue(CommandProperty, value);
}
}
' 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
以下是 DependencyProperty 變更回呼：
// Command dependency property change callback.
private static void CommandChanged(DependencyObject d,
DependencyPropertyChangedEventArgs e)
{
CommandSlider cs = (CommandSlider)d;
cs.HookUpCommand((ICommand)e.OldValue,(ICommand)e.NewValue);
}
' 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
下一個步驟是新增和移除與命令來源相關聯的命令。 新增命令時，無法直接覆寫 Command 屬性，因為如果有與上一個命令相關聯的事件處理常式，必須先移除。
// Add a new command to the Command Property.
private void HookUpCommand(ICommand oldCommand, ICommand newCommand)
{
// If oldCommand is not null, then we need to remove the handlers.
if (oldCommand != null)
{
RemoveCommand(oldCommand, newCommand);
}
AddCommand(oldCommand, newCommand);
}
// Remove an old command from the Command Property.
private void RemoveCommand(ICommand oldCommand, ICommand newCommand)
{
EventHandler handler = CanExecuteChanged;
oldCommand.CanExecuteChanged -= handler;
}
// Add the command.
private void AddCommand(ICommand oldCommand, ICommand newCommand)
{
EventHandler handler = new EventHandler(CanExecuteChanged);
canExecuteChangedHandler = handler;
if (newCommand != null)
{
newCommand.CanExecuteChanged += canExecuteChangedHandler;
}
}
' 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
下一個步驟是建立 CanExecuteChanged 處理常式的邏輯。
CanExecuteChanged 事件會通知命令來源，命令在目前的命令目標上執行的能力可能已變更。 當命令來源收到此事件時，通常會在命令上呼叫 CanExecute 方法。 如果命令無法在目前的命令目標上執行，命令來源通常會自行停用。 如果命令可在目前的命令目標上執行，命令來源通常會自行啟用。
private void CanExecuteChanged(object sender, EventArgs e)
{
if (this.Command != null)
{
RoutedCommand command = this.Command as RoutedCommand;
// If a RoutedCommand.
if (command != null)
{
if (command.CanExecute(CommandParameter, CommandTarget))
{
this.IsEnabled = true;
}
else
{
this.IsEnabled = false;
}
}
// If a not RoutedCommand.
else
{
if (Command.CanExecute(CommandParameter))
{
this.IsEnabled = true;
}
else
{
this.IsEnabled = false;
}
}
}
}
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
最後一個步驟是 Execute 方法。 如果命令是 RoutedCommand，則會呼叫 RoutedCommand Execute 方法；否則，會呼叫 ICommand Execute 方法。
// If Command is defined, moving the slider will invoke the command;
// Otherwise, the slider will behave normally.
protected override void OnValueChanged(double oldValue, double newValue)
{
base.OnValueChanged(oldValue, newValue);
if (this.Command != null)
{
RoutedCommand command = Command as RoutedCommand;
if (command != null)
{
command.Execute(CommandParameter, CommandTarget);
}
else
{
((ICommand)Command).Execute(CommandParameter);
}
}
}
' 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
