c# – WPF自定义控件DependencyProperty不会数据绑定

<UserControl x:Class="AGWPFControls.SetSpeed"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             MinHeight="50" MinWidth="110">
    <Canvas>
        <Slider Name="sldSetSpeed" MinWidth="100" Canvas.Top="5" Canvas.Left="5" />
        <TextBox Name="txtSpeed" MinWidth="100" Canvas.Bottom="5" Canvas.Right="5" 
                Text="{Binding ElementName=sldSetSpeed,Path=Value}" />
    </Canvas>
</UserControl>

它有一个名为Speed的DependencyProperty：

public partial class SetSpeed : UserControl
{
    public SetSpeed()
    {
        InitializeComponent();
    }
    public static readonly DependencyProperty SpeedProperty;
    static SetSpeed()
    {
        var md = new FrameworkPropertyMetadata(0.0);
        SetSpeed.SpeedProperty = DependencyProperty.Register(
            "Speed",typeof(double),typeof(SetSpeed),md);
    }
    public double Speed
    {
        get { return (double)GetValue(SetSpeed.SpeedProperty); }
        set { SetValue(SetSpeed.SpeedProperty,value); }
    }
}

我已将控件放在Window中并将元素(任何元素)绑定到它：

<Window x:Class="WpfApplication3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" xmlns:my="clr-namespace:AGWPFControls;assembly=AGWPFControls">
    <StackPanel>
        <my:SetSpeed Name="setSpeed1" />
        <TextBlock Text="{Binding ElementName=setSpeed1,Path=Speed}" />
    </StackPanel>
</Window>

它很简单.不过没有骰子.当我移动滑块时,TextBlock中的值永远不会改变.我错过了什么,这里？