如何在设计时获取WPF UserControl更新？

Question

I am making custom WPF control based on UserControl class. It's a simple checkbox, rectangle with two diagonal lines that are scaled correctly when I scale whole control. Here is XAML:

<UserControl x:Name="Container" 
     x:Class="MyProject.Controls.VirmanCheckbox"
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
     xmlns:local="clr-namespace:MyProject.Controls"
     mc:Ignorable="d"
     d:DesignHeight="50" d:DesignWidth="50" BorderThickness="1" BorderBrush="Black" Width="50" Height="50" Cursor="Hand" IsTabStop="True" Focusable="True" KeyUp="Grid_KeyUp" GotFocus="Container_GotFocus" LostFocus="Container_LostFocus">
<Grid MouseUp="Grid_MouseUp" Background="#00000000">
    <Line x:Name="diagonal1" X1="0" Y1="0" X2="{Binding Width, ElementName=Container}" Y2="{Binding Height, ElementName=Container}" Stroke="{Binding ElementName=Container, Path=BorderBrush}" StrokeThickness="{Binding ElementName=Container, Path=BorderThickness}" Visibility="Hidden"/>
    <Line x:Name="diagonal2" X1="0" Y1="{Binding Height, ElementName=Container}" X2="{Binding Width, ElementName=Container}" Y2="0" Stroke="{Binding ElementName=Container, Path=BorderBrush}" StrokeThickness="{Binding ElementName=Container, Path=BorderThickness}" Visibility="Hidden"/>
</Grid>

I added DependencyProperty IsChecked to CustomControl class. If IsChecked is true diagonal lines should be visible. If IsChecked is false diagonal lines should be hidden. It looks like this:

public partial class CustomCheckbox : UserControl
{
    public static readonly DependencyProperty IsCheckedProperty;

    static CustomCheckbox()
    {
        IsCheckedProperty = DependencyProperty.Register(
            name: "IsChecked",
            propertyType: typeof(Boolean),
            ownerType: typeof(VirmanCheckbox),
            typeMetadata: new FrameworkPropertyMetadata(
                defaultValue: false, 
                flags: FrameworkPropertyMetadataOptions.AffectsRender
            )
        );
    }

    public bool IsChecked
    {
        get { return (bool)GetValue(IsCheckedProperty); }
        set
        {
            var old = (bool)GetValue(IsCheckedProperty);
            SetValue(IsCheckedProperty, value);

            if (old != value)
            {
                if (value)
                {
                    diagonal1.Visibility = Visibility.Visible;
                    diagonal2.Visibility = Visibility.Visible;
                }
                else
                {
                    diagonal1.Visibility = Visibility.Hidden;
                    diagonal2.Visibility = Visibility.Hidden;
                }
            }
        }
    }
 }

When I use this control in my project I get this XAML:

<Controls:CustomCheckbox HorizontalAlignment="Left" VerticalAlignment="Top"/>

During runtime it works perfectly, but during design time if I change IsChecked property in XAML my CustomCheckbox won't graphically update. For example, if I set IsChecked to true , diagonal lines won't show:

<Controls:CustomCheckbox HorizontalAlignment="Left" VerticalAlignment="Top" IsChecked="True"/>
<Controls:CustomCheckbox HorizontalAlignment="Left" VerticalAlignment="Top" IsChecked="False"/>

These two controls are visually same during design time. What am I missing?

Answer 1

Quoting MSDN :

Because the current WPF implementation of the XAML processor behavior for property setting bypasses the wrappers entirely, you should not put any additional logic into the set definitions of the wrapper for your custom dependency property. If you put such logic in the set definition, then the logic will not be executed when the property is set in XAML rather than in code.

You should register a PropertyChangedCallback instead.

public partial class CustomCheckbox : UserControl
{
    public CustomCheckbox()
    {
        InitializeComponent();
    }

    #region IsChecked
    public static readonly DependencyProperty IsCheckedProperty =
        DependencyProperty.Register("IsChecked",
            typeof(bool), typeof(CustomCheckbox),
            new FrameworkPropertyMetadata(
                false, 
                FrameworkPropertyMetadataOptions.AffectsRender,
                IsCheckedPropertyChanged));

    public bool IsChecked
    {
        get { return (bool)GetValue(IsCheckedProperty); }
        set { SetValue(IsCheckedProperty, value); }
    }
    #endregion

    #region IsCheckedPropertyChanged
    private static void IsCheckedPropertyChanged
        (DependencyObject source, DependencyPropertyChangedEventArgs e)
    {
        if (source is CustomCheckbox)
        {
            CustomCheckbox control = source as CustomCheckbox;
            bool value = (bool)e.NewValue;

            if (value)
            {
                control.diagonal1.Visibility = Visibility.Visible;
                control.diagonal2.Visibility = Visibility.Visible;
            }
            else
            {
                control.diagonal1.Visibility = Visibility.Hidden;
                control.diagonal2.Visibility = Visibility.Hidden;
            }
        }
    }
    #endregion
}

As a side note, in WPF you would usually swap the template of a regular CheckBox to change it's appearance.