WPF知识点全攻略11- 命令(Command)

先看一下命令的简单使用：

<Window.CommandBindings>
    <CommandBinding Command="ApplicationCommands.Cut" CanExecute="CutCommand_CanExecute" Executed="CutCommand_Executed" />
    <CommandBinding Command="ApplicationCommands.Paste" CanExecute="PasteCommand_CanExecute" Executed="PasteCommand_Executed" />
Window.CommandBindings>

<Window.InputBindings>
    <KeyBinding Gesture="Alt+X" Command="ApplicationCommands.Cut">KeyBinding>
    <KeyBinding Modifiers="Alt" Key="V" Command="ApplicationCommands.Paste">KeyBinding>
Window.InputBindings>

<DockPanel>
    <WrapPanel DockPanel.Dock="Top" Margin="3">
        <Button Command="ApplicationCommands.Cut" Width="60">_CutButton>
        <Button Command="ApplicationCommands.Paste" Width="60" Margin="3,0">_PasteButton>
    WrapPanel>
    <TextBox AcceptsReturn="True" Name="txtEditor" />
DockPanel>

private void CutCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = (txtEditor != null) && (txtEditor.SelectionLength > 0);
}
private void CutCommand_Executed(object sender, ExecutedRoutedEventArgs e)
{
    txtEditor.Cut();
}
private void PasteCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = Clipboard.ContainsText();
}
private void PasteCommand_Executed(object sender, ExecutedRoutedEventArgs e)
{
    txtEditor.Paste();
}

对比一下类似WinForm事件写法：

<DockPanel>
    <WrapPanel Margin="3" DockPanel.Dock="Top">
        <Button Width="60" Click="Cut_Click">_CutButton>
        <Button Width="60" Margin="3,0" Click="Paste_Click">_PasteButton>
    WrapPanel>
    <TextBox Name="txtEditor" AcceptsReturn="True" />
DockPanel>

private void Cut_Click(object sender, RoutedEventArgs e)
{
    txtEditor.Cut();
}

private void Paste_Click(object sender, RoutedEventArgs e)
{
    txtEditor.Paste();
}

命令模式对比普通事件模式，可以通过CanExecute设置操作是否可用，可以比较方便的把操作绑定到点击按钮、快捷键等多种类型。

命令继承自ICommand接口，ICommand 公开了两种方法 Execute 和 CanExecute，以及一个事件 CanExecuteChanged。

命令的两个重要辅助属性，CommandTarget指定的命令的目标元素，CommandParameter设置命令参数，CommandManager在元素中添加和删除 PreviewExecuted、Executed、PreviewCanExecute 和 CanExecute 事件处理程序。

命令的优点不止以上的简单使用，最关键的是在使用MVVM模式中，命令和依赖属性配合通过Binding可以很好的做到UI和数据分离，降低View和Model的耦合。

命令直接绑定到元素控件上，一般会触发最常用属性，比如Button的Click，如果想绑定其他属性MoseMove的动作怎么办呢？

可以引入System.Windows.Interactivity.dll

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

<Button Name="btn" Content="Button" Height="33" HorizontalAlignment="Left" Margin="40,24,0,0" VerticalAlignment="Top" Width="109">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Click">
            <i:InvokeCommandAction Command="{Binding Command1}" CommandParameter="10" />
        i:EventTrigger>
        <i:EventTrigger EventName="MouseMove">
            <i:InvokeCommandAction Command="{Binding Command2}" CommandParameter="{Binding ElementName=btn}" />
        i:EventTrigger>
    i:Interaction.Triggers>
Button>

使用System.Windows.Interactivity.dll不仅可以给Button、CheckBox等添加命令绑定，也可以给Image、Grid、Border等添加各种事件绑定。