[UWP 自定义控件]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