windows phone 7 silverlight user control: data binding not working on custom property

问题

I have a rather simple user control (RatingControl) that has a dependency property defined as follows:

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

    public static readonly DependencyProperty RatingValueProperty = DependencyProperty.Register("RatingValue", typeof(double), typeof(RatingControl), new PropertyMetadata(0.0));

    public double RatingValue
    {
        set 
        { 
            double normalizeValue = 0.0;

            if (value > 10.0)
            {
                normalizeValue = 10.0;
            }
            else if (value > 0.0)
            {
                normalizeValue = value;
            }

            SetValue(RatingValueProperty, normalizeValue);
            RenderRatingValue();
        }
        get { return (double)GetValue(RatingValueProperty); }
    }

...

This control receives properly the RatingValue if I assign it directly:

<gtcontrols:RatingControl RatingValue="2.0" />

However, if I try to assign it with data binding, it does not work. The "set" code for RatingValue is never called, nor I see data binding errors in the debug output window. Note below that I tried to assign the same value to a standard property (Width), and in that case the value is properly passed to it.

<StackPanel>
                <TextBox Name="Test" Text="200.0" />

                <gtcontrols:RatingControl Width="{Binding ElementName=Test, Path=Text}" RatingValue="{Binding ElementName=Test, Path=Text}" />
                <TextBlock Text="{Binding ElementName=Test, Path=Text}" />
            </StackPanel>

Not only the TextBlock receives the value correctly. also RatingControl receives is for the width, properly set at 200 pixels; however, the RatingValue is not set (method set not even called)

What am I missing? Thanks in advance.

回答1:

The thing is that the binding system does not use the CLR property wrapper (getter and setter) to assign the value of a dependency property. Those are there just for convenience, so you could use the property just like a normal property in your code. Internally it uses SetValue()/GetValue() methods.

So, the proper place for the value normalization would be the property changed callback for the dependency property:

public static readonly DependencyProperty RatingValueProperty =
    DependencyProperty.Register("RatingValue", typeof(double), typeof(RatingControl), 
    new PropertyMetadata(0.0, new PropertyChangedCallback(RatingValuePropertyChanged))));

static void RatingValuePropertyChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
{
    var ratingControl = (RatingControl)sender;
    var val = (double)e.NewValue;

    double normalizeValue = 0.0;

    if (val > 10.0)
    {
        normalizeValue = 10.0;
    }
    else if (val > 0.0)
    {
        normalizeValue = val;
    }      

    ratingControl.RatingValue = normalizeValue;  
}

来源：https://stackoverflow.com/questions/6808670/windows-phone-7-silverlight-user-control-data-binding-not-working-on-custom-pro