silverlight - UserControl 公开多个内容属性!那将是多么令人兴奋!

Question

我正在尝试创建一个有望公开多个内容属性的 UserControl。然而，我却被失败所困扰!

我的想法是创建一个公开两个内容属性的优秀用户控件(我们称之为 MultiContent)，以便我可以执行以下操作:

    <local:MultiContent>
        <local:MultiContent.ListContent>
            <ListBox x:Name="lstListOfStuff" Width="50" Height="50" />                
        </local:MultiContent.ListContent>
        <local:MultiContent.ItemContent>
            <TextBox x:Name="txtItemName" Width="50" />
        </local:MultiContent.ItemContent>
    </local:MultiContent>

这将非常有用，现在我可以根据情况更改 ListContent 和 ItemContent，将常用功能分解到 MultiContent 用户控件中。

但是，按照我目前的实现方式，我无法访问 MultiContent 控件的这些内容属性内的 UI 元素。例如，lstListOfStuff 和 txtItemName 在我尝试访问它们时都是 null:

public MainPage() {
    InitializeComponent();
    this.txtItemName.Text = "Item 1"; // <-- txtItemName is null, so this throws an exception
}

下面是我如何实现 MultiContent 用户控件:

XAML:多内容.xaml

<UserControl x:Class="Example.MultiContent"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100" />
            <ColumnDefinition Width="100" />
        </Grid.ColumnDefinitions>
        <ContentControl x:Name="pnlList" Grid.Column="0" />
        <ContentControl x:Name="pnlItem" Grid.Column="1" />
    </Grid>
</UserControl>

代码隐藏:MultiContent.xaml.cs

// Namespaces Removed
namespace Example
{
    public partial class MultiContent : UserControl
    {
        public UIElement ListContent
        {
            get { return (UIElement)GetValue(ListContentProperty); }
            set 
            {
                this.pnlList.Content = value;
                SetValue(ListContentProperty, value); 
            }
        }
        public static readonly DependencyProperty ListContentProperty =
            DependencyProperty.Register("ListContent", typeof(UIElement), typeof(MultiContent), new PropertyMetadata(null));

        public UIElement ItemContent
        {
            get { return (UIElement)GetValue(ItemContentProperty); }
            set 
            {
                this.pnlItem.Content = value;
                SetValue(ItemContentProperty, value); 
            }
        }
        public static readonly DependencyProperty ItemContentProperty =
            DependencyProperty.Register("ItemContent", typeof(UIElement), typeof(MultiContent), new PropertyMetadata(null));


        public MultiContent()
        {
            InitializeComponent();
        }
    }
}

我可能完全错误地实现了这个。有谁知道我怎样才能让它正常工作？如何从父控件按名称访问这些 UI 元素？关于如何更好地做到这一点的任何建议？谢谢!

Answer 1

你当然可以实现你的目标，但你需要采取不同的方法。

在您的解决方案中，您试图为 UIElement 设置一个依赖属性 - 由于它永远不会被设置并且默认值为 null，这就是您获得 NullReference 异常的原因。您可能会继续将默认值从 null 更改为 new TextBox 或类似的东西，但即使它确实有效，它仍然感觉像是 hack。

在 Silverlight 中，您必须自己实现依赖属性。但是您没有按应有的方式实现它们 - 我倾向于使用此 dependency property generator这样做。

DP 的一大优点是它们支持更改通知。因此，考虑到这一点，让您的示例工作所需要做的就是为 DP 定义:ItemContent 和 ListContent，其类型与 Content(对象)相同，当框架通知您其中任何一个已更改时，只需更新您的文本框!所以这是执行此操作的代码:

多内容.xaml:

   <Grid x:Name="LayoutRoot" Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="100" />
            <ColumnDefinition Width="100" />
        </Grid.ColumnDefinitions>
        <ContentControl x:Name="pnlList" Grid.Column="0" />
        <ContentControl x:Name="pnlItem" Grid.Column="1" />
    </Grid>    

MultiContent.xaml.cs:

namespace MultiContent
{
    public partial class MultiContent : UserControl
    {
        #region ListContent

        /// <summary>
        /// ListContent Dependency Property
        /// </summary>
        public object ListContent
        {
            get { return (object)GetValue(ListContentProperty); }
            set { SetValue(ListContentProperty, value); }
        }
        /// <summary>
        /// Identifies the ListContent Dependency Property.
        /// </summary>
        public static readonly DependencyProperty ListContentProperty =
            DependencyProperty.Register("ListContent", typeof(object),
            typeof(MultiContent), new PropertyMetadata(null, OnListContentPropertyChanged));

        private static void OnListContentPropertyChanged
          (object sender, DependencyPropertyChangedEventArgs e)
        {
            MultiContent m = sender as MultiContent;
            m.OnPropertyChanged("ListContent");
        }

        #endregion

        #region ItemContent

        /// <summary>
        /// ItemContent Dependency Property
        /// </summary>
        public object ItemContent
        {
            get { return (object)GetValue(ItemContentProperty); }
            set { SetValue(ItemContentProperty, value); }
        }
        /// <summary>
        /// Identifies the ItemContent Dependency Property.
        /// </summary>
        public static readonly DependencyProperty ItemContentProperty =
            DependencyProperty.Register("ItemContent", typeof(object),
            typeof(MultiContent), new PropertyMetadata(null, OnItemContentPropertyChanged));

        private static void OnItemContentPropertyChanged
          (object sender, DependencyPropertyChangedEventArgs e)
        {
            MultiContent m = sender as MultiContent;
            m.OnPropertyChanged("ItemContent");
        }

        #endregion

        /// <summary>
        ///  Event called when any chart property changes
        ///  Note that this property is not used in the example but is good to have if you plan to extend the class!
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        ///  Called to invoke the property changed event
        /// </summary>
        /// <param name="propertyName">The property that has changed</param>
        protected void OnPropertyChanged(string propertyName)
        {
            if (propertyName == "ListContent")
            {
                // The ListContent property has been changed, let's update the control!
                this.pnlList.Content = this.ListContent;
            }
            if (propertyName == "ItemContent")
            {
                // The ListContent property has been changed, let's update the control!
                this.pnlItem.Content = this.ItemContent;
            }
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public MultiContent()
        {
            InitializeComponent();
        }
    }
}

主页.xaml:

    <Grid x:Name="LayoutRoot" Background="White">
        <local:MultiContent>
            <local:MultiContent.ListContent>
                <ListBox x:Name="lstListOfStuff" Width="50" Height="50" />
            </local:MultiContent.ListContent>
            <local:MultiContent.ItemContent>
                <TextBox x:Name="txtItemName" Width="50" />
            </local:MultiContent.ItemContent>
        </local:MultiContent>
    </Grid>

这应该可以解决问题!