如何：启用命令

以下示例演示如何在 Windows Presentation Foundation (WPF) 中使用命令。 示例演示如何将 RoutedCommand 关联到 Button、创建 CommandBinding 以及创建实现 RoutedCommand 的事件处理程序。 有关命令的详细信息，请参阅命令概述。

示例

第一部分代码创建了用户界面 (UI)，界面由 Button 和 StackPanel 组成，并且创建了将命令处理程序与 CommandBinding 关联的 RoutedCommand。

Command 的 Button 属性与 Close 命令相关联。

将 CommandBinding 添加到根 CommandBindingCollection 的 Window。 Executed 和 CanExecute 事件处理程序附加到此绑定，并与 Close 命令相关联。

没有 CommandBinding 就没有命令逻辑，只有调用命令的机制。 单击 Button 时，将对命令目标引发 PreviewExecutedRoutedEvent，随后引发 ExecutedRoutedEvent。 这些事件会遍历元素树，为该特定命令寻找 CommandBinding。 值得注意的是，由于 RoutedEvent 通过元素树使用 tunnel 和 bubble 机制，因此必须注意放置 CommandBinding 的位置。 如果 CommandBinding 位于命令目标的同级节点或位于不在 RoutedEvent 路由上的另一个节点上，则不会访问 CommandBinding。

<Window x:Class="WCSamples.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="CloseCommand"
    Name="RootWindow"
    >
  <Window.CommandBindings>
    <CommandBinding Command="ApplicationCommands.Close"
                    Executed="CloseCommandHandler"
                    CanExecute="CanExecuteHandler"
                    />
  </Window.CommandBindings>
  <StackPanel Name="MainStackPanel">
    <Button Command="ApplicationCommands.Close" 
            Content="Close File" />
  </StackPanel>
</Window>

// Create ui elements.
StackPanel CloseCmdStackPanel = new StackPanel();
Button CloseCmdButton = new Button();
CloseCmdStackPanel.Children.Add(CloseCmdButton);

// Set Button's properties.
CloseCmdButton.Content = "Close File";
CloseCmdButton.Command = ApplicationCommands.Close;

// Create the CommandBinding.
CommandBinding CloseCommandBinding = new CommandBinding(
    ApplicationCommands.Close, CloseCommandHandler, CanExecuteHandler);

// Add the CommandBinding to the root Window.
RootWindow.CommandBindings.Add(CloseCommandBinding);

' Create ui elements.
Dim CloseCmdStackPanel As New StackPanel()
Dim CloseCmdButton As New Button()
CloseCmdStackPanel.Children.Add(CloseCmdButton)

' Set Button's properties.
CloseCmdButton.Content = "Close File"
CloseCmdButton.Command = ApplicationCommands.Close

' Create the CommandBinding.
Dim CloseCommandBinding As New CommandBinding(ApplicationCommands.Close, AddressOf CloseCommandHandler, AddressOf CanExecuteHandler)

' Add the CommandBinding to the root Window.
RootWindow.CommandBindings.Add(CloseCommandBinding)

下一部代码分实现了 Executed 和 CanExecute 事件处理程序。

Executed 处理程序调用了一个方法来关闭打开的文件。 CanExecute 处理程序调用了一个方法来确定文件是否打开。 如果文件处于打开状态，则将 CanExecute 设置为 true，否则，设置为 false。

// Executed event handler.
private void CloseCommandHandler(object sender, ExecutedRoutedEventArgs e)
{
    // Calls a method to close the file and release resources.
    CloseFile();
}

// CanExecute event handler.
private void CanExecuteHandler(object sender, CanExecuteRoutedEventArgs e)
{
    // Call a method to determine if there is a file open.
    // If there is a file open, then set CanExecute to true.
    if (IsFileOpened())
    {
        e.CanExecute = true;
    }
    // if there is not a file open, then set CanExecute to false.
    else
    {
        e.CanExecute = false;
    }
}

' Executed event handler.
Private Sub CloseCommandHandler(ByVal sender As Object, ByVal e As ExecutedRoutedEventArgs)
    ' Calls a method to close the file and release resources.
    CloseFile()
End Sub

' CanExecute event handler.
Private Sub CanExecuteHandler(ByVal sender As Object, ByVal e As CanExecuteRoutedEventArgs)
    ' Call a method to determine if there is a file open.
    ' If there is a file open, then set CanExecute to true.
    If IsFileOpened() Then
        e.CanExecute = True
    ' if there is not a file open, then set CanExecute to false.
    Else
        e.CanExecute = False
    End If
End Sub

另请参阅

• 命令概述