windows phone 7 silverlight用户控件：数据绑定不起作用的自定义属性 [英] windows phone 7 silverlight user control: data binding not working on custom property

问题描述

我有一个相当简单的用户控件（RatingControl），它具有如下定义的依赖属性：

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); }
    }

...

如果我直接分配了此控件，则此控件会正确收到RatingValue：

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

<gtcontrols:RatingControl RatingValue="2.0" />

但是，如果我尝试分配数据绑定，它不起作用。 RatingValue的set代码从未被调用，也不会在调试输出窗口中看到数据绑定错误。
请注意，我试图为标准属性（宽度）分配相同的值，在这种情况下，该值正确传递给它。

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>

不仅TextBlock正确接收值。还有RatingControl接收的是宽度，正确设置为200像素;然而，RatingValue未设置（方法设置甚至不调用）

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)

我缺少什么？
提前感谢

What am I missing? Thanks in advance.

推荐答案

事情是绑定系统不使用CLR属性包装器（getter和setter）来分配依赖属性的值。那些只是为了方便，所以你可以使用属性，就像代码中的普通属性一样。在内部它使用SetValue（）/ GetValue（）方法。

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.

所以，值归一化的适当位置是依赖属性的属性更改的回调：

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;  
}

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