WPF UserControl公开用于绑定的子控件的属性

Question

I'm working on developing a custom user control for my application. This control is very simple. It's just a grid, with a checkbox in [0,0] and a TextBlock in [0,1]. I've had no issues getting it designed how I'd like in XAML.

However, the second step is giving me some trouble. I'm trying to expose the IsChecked bool? of my sub-control that is a Checkbox for binding on my mainform, and the same idea with the Text property of TextBlock.

I've tried a few different ways of going about this, but to no avail.

Here's the general code I have:

public partial class CDCheckBox : UserControl
{
    public bool? IsChecked
    {
        get { return chk.IsChecked; }
        set { chk.IsChecked = value; }
    }

    public string Text
    {
        get { return lbl.Text; }
        set { lbl.Text = value; }
    }

    public static readonly DependencyProperty IsCheckedProperty =
    DependencyProperty.Register(
        "IsChecked",
        typeof(bool?),
        typeof(CDCheckBox),
        new PropertyMetadata(default(bool?), OnItemsPropertyChanged));

    public static readonly DependencyProperty TextProperty =
    DependencyProperty.Register(
        "Text",
        typeof(string),
        typeof(CDCheckBox),
        new PropertyMetadata(default(string), OnItemsPropertyChanged));

    /*
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnNotify(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }
    */
    private static void OnItemsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        // AutocompleteTextBox source = d as AutocompleteTextBox;
        // Do something...
        //lbl.Text = e.NewValue.ToString();
    }

    /*
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnNotify(string propName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }
    */

    public CDCheckBox()
    {
        InitializeComponent();
    }
}

When I run the code above, I get no errors, but my binded data doesn't show up in my TextBlock control. When I tried before I wrote the depenency properties, it gave me an error in my XAML saying "A 'Binding' cannot be set on the 'IsChecked' property of type 'CDCheckBox'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject."

Interestingly however, this error does not appear in the constructor, but instead in the window_loaded method I've written. This appears to be a red herring however, as if I comment out that code, it still fails before the form can display with XAMLParse Error.

Answer 1

Further to my comment, you could try styling an existing control that has the property types that you need. For example, in your custom control you have a nullable Boolean property and a string property. If you repurpose a CheckBox control, it already has a nullable Boolean property ( IsChecked ) and an object property ( Content ) which can be used to hold a string.

Here's how you might restyle a CheckBox control and change its template to achieve the result you're after:

<Window x:Class="..."
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Window.Resources>
        <Style x:Key="MySuperCheckboxStyle"
               TargetType="{x:Type CheckBox}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type CheckBox}">
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition />
                                <ColumnDefinition />
                            </Grid.ColumnDefinitions>
                            <CheckBox Grid.Column="0"
                                      IsChecked="{TemplateBinding IsChecked}"
                                      Content="Whatever you need here" />
                            <TextBlock Grid.Column="1"
                                       Text="{TemplateBinding Content}" />
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

    <StackPanel>
        <CheckBox IsChecked="True"
                  Content="Unstyled check box"
                  Margin="10" />
        <CheckBox Style="{StaticResource MySuperCheckboxStyle}"
                  IsChecked="True"
                  Content="Styled check box"
                  Margin="10" />
    </StackPanel>
</Window>

Key here are the TemplateBinding bindings used in the control template. These bind not to a data context like in normal data binding, but rather to properties of the control being templated.

Whenever you find yourself wanting to create a custom control in WPF it is worth exploring whether you can take an existing control and change its appearance to suit what you need, as this is often less work than creating a new control (on the flipside it's not always possible to repurpose an existing control, particularly if you need different behaviour).