原文:
[UWP]了解模板化控件(7)：支持Command
以我的经验来说，要让TemplatedControl支持Command的需求不会很多，大部分情况用附加属性解决这个需求会更便利些，譬如UWPCommunityToolkit的HyperlinkExtensions。
如果正在从头设计自定义控件并真的需要提供命令支持，可以参考这篇文章。支持Command的步骤比较简单，所以这篇文章比较简短。
要实现Command支持，控件中要执行如下步骤：
- 定义Command和CommandParameter属性。
- 监视Command的CanExecuteChanged事件。
- 在CanExecuteChanged的事件处理函数及CommandParameter的PropertyChangedCallback中，根据Command.CanExecute(CommandParameter)的结果设置控件的IsEnabled属性。
- 在某个事件（Click或者ValueChanged）中执行Command。
MenuItem是实现了Command支持的示例，重载了OnPointerPressed并且在其中执行Command：
public class MenuItem : Control
{
/// <summary>
/// 标识 Command 依赖属性。
/// </summary>
public static readonly DependencyProperty CommandProperty =
DependencyProperty.Register("Command", typeof(ICommand), typeof(MenuItem), new PropertyMetadata(null, OnCommandChanged));
private static void OnCommandChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
MenuItem target = obj as MenuItem;
ICommand oldValue = (ICommand)args.OldValue;
ICommand newValue = (ICommand)args.NewValue;
if (oldValue != newValue)
target.OnCommandChanged(oldValue, newValue);
}
/// <summary>
/// 标识 CommandParameter 依赖属性。
/// </summary>
public static readonly DependencyProperty CommandParameterProperty =
DependencyProperty.Register("CommandParameter", typeof(object), typeof(MenuItem), new PropertyMetadata(null, OnCommandParameterChanged));
private static void OnCommandParameterChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
MenuItem target = obj as MenuItem;
object oldValue = (object)args.OldValue;
object newValue = (object)args.NewValue;
if (oldValue != newValue)
target.OnCommandParameterChanged(oldValue, newValue);
}
public MenuItem()
{
this.DefaultStyleKey = typeof(MenuItem);
}
public event RoutedEventHandler Click;
/// <summary>
/// 获取或设置Command的值
/// </summary>
public ICommand Command
{
get { return (ICommand)GetValue(CommandProperty); }
set { SetValue(CommandProperty, value); }
}
/// <summary>
/// 获取或设置CommandParameter的值
/// </summary>
public object CommandParameter
{
get { return (object)GetValue(CommandParameterProperty); }
set { SetValue(CommandParameterProperty, value); }
}
protected virtual void OnCommandParameterChanged(object oldValue, object newValue)
{
UpdateIsEnabled();
}
protected virtual void OnCommandChanged(ICommand oldValue, ICommand newValue)
{
if (oldValue != null)
oldValue.CanExecuteChanged -= OnCanExecuteChanged;
if (newValue != null)
newValue.CanExecuteChanged += OnCanExecuteChanged;
UpdateIsEnabled();
}
protected virtual void UpdateVisualState(bool useTransitions)
{
if (IsEnabled)
{
VisualStateManager.GoToState(this, "Normal", useTransitions);
}
else
{
VisualStateManager.GoToState(this, "Disabled", useTransitions);
}
}
protected override void OnPointerPressed(PointerRoutedEventArgs e)
{
base.OnPointerPressed(e);
Click?.Invoke(this, new RoutedEventArgs());
if ((null != Command) && Command.CanExecute(CommandParameter))
{
Command.Execute(CommandParameter);
}
}
private void OnCanExecuteChanged(object sender, EventArgs e)
{
UpdateIsEnabled();
}
private void UpdateIsEnabled()
{
IsEnabled = (null == Command) || Command.CanExecute(CommandParameter);
UpdateVisualState(true);
}
}以下是使用示例，作用是当TextBox的Text不为空时可以点击MenuItem，并且将Text作为MessageDialog的内容输出：
<StackPanel>
<TextBox x:Name="TextElement"/>
<local:MenuItem Command="{Binding}" CommandParameter="{Binding ElementName=TextElement,Path=Text}"/>
</StackPanel>public MenuItemSamplePage()
{
this.InitializeComponent();
var command = new DelegateCommand<object>(Click, CanExecute);
this.DataContext = command;
}
private void Click(object parameter)
{
MessageDialog dialog = new MessageDialog(parameter.ToString());
dialog.ShowAsync();
}
private bool CanExecute(object parameter)
{
string text = parameter as string;
return string.IsNullOrWhiteSpace(text) == false;
}这里用到的DelegateCommand也是UWPCommunityToolkit中的类 ：DelegateCommand
