WPF笔记汇总之命令的使用

WPF命令的使用

接上一篇《WPF笔记汇总之数据绑定及依赖属性》，这篇主要总结WPF特有的关于命令的用法，包括系统命令使用及自定义命令的创建及使用。

1. 命令概述

在WPF中，允许在一个地方定义命令，并且在所有的用户接口控件之中调用这些命令，它们由ICommand接口组成，该接口仅定义一个事件和两个方法：Execute()和CanExecute()。第一个用于执行实际操作，而第二个用于确定操作当前是否可用。要执行命令的实际操作，您需要在命令和代码之间使用CommandBinding作为链接，CommandBinding通常在Window或UserControl上定义，并保存对它处理的Command的引用，以及用于处理Command的Execute()和CanExecute()事件的实际事件处理程序。

2. 使用命令

2.1 使用系统自带命令

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

2.2 自定义系统命令

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

<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();
}

3. 自定义命令

3.1 使用ICommand创建

• 第一步：创建命令

class CustomCommand : ICommand
{
    public event EventHandler CanExecuteChanged;
    public void Execute(object param)  
    {
        ExecuteAction?.Invoke(param);
    }
    public bool CanExecute(object param)  
    {
        if (CanExecuteAction != null)
            return CanExecuteAction(param);
        return false;
    }
    public Action<object> ExecuteAction { get; set; }
    public Func<object, bool> CanExecuteAction { get; set; }
}

• 第二步：ViewModel中使用命令

public CustomCommand MyCommand { get; set; }

public void DoSomething(object param){
}
public bool CanDoSomething(object param){
    return true;  
}
public MyViewModel(){
    MyCommand = new CustomCommand();
    MyCommand.ExecuteAction = new Action<object>(this.DoSomething);
    MyCommand.CanExecuteAction = new Func<object, bool>(this.CanDoSomething);
}

• 第二步：View中调用

<Window xmlns:vm="clr-namespace:MyApp.ViewModel" ... />
    <Grid>
        <Grid.DataContext>
            <vm:MyViewModel> 
        Grid.DataContext>
        <Button Content="Click here" Command="{Binding MyCommand}" />
    Grid>
Window>

3.2 使用RoutedUICommand创建

• 第一步：创建一个命令

public static class CustomCommands
{
    public static readonly RoutedUICommand ExitCommand = new RoutedUICommand(
        "quit app", 
        "ExitCommand",
        typeof(CustomCommands),
        new InputGestureCollection() { 
            new KeyGesture(Key.W, ModifierKeys.Control)  
        });
}

• 第二步：完成两个事件

public void ExitCommand_Execute(object sender, ExecutedRoutedEventArgs e)
{
    e.Handled = true;
}
public void ExitCommand_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = true; 
    e.Handled = true;
}

• 第三步：调用

<Window.CommandBindings>
    <CommandBinding Command="local:CustomCommands.ExitCommand" 
        CanExecute="ExitCommand_CanExecute" 
        Executed="ExitCommand_Execute"/>
Window.CommandBindings>

<Button Content="Exit" Command="local:CustomCommands.ExitCommand"/>