How do I bind to a property?

Here's my simplified scenario. I can adjust my inner rectangle width using the mouse. A textblock displays the width which changes as I adjust it. I want a second textblock to display the value of a property which also changes with the width but I cannot figure out how to bind it.

<Grid x:Name="LayoutRoot" Background="White" VerticalAlignment="Center">

    <Rectangle x:Name="aRec" Height="100" Width="100" MinWidth="10" Fill="Blue" />

    <Rectangle x:Name="myRec" Height="100" Width="300" MinWidth="10" Fill="Red" Opacity="0.5" 
               MouseLeftButtonDown="myRec_MouseLeftButtonDown" 
               MouseLeftButtonUp="myRec_MouseLeftButtonUp" 
               MouseMove=开发者_JS百科"myRec_MouseMove"></Rectangle>

    <StackPanel>
        <TextBlock x:Name="myText1" Width="40" Height="20" Foreground="White" Text="{Binding ElementName=aRec, Path=Width}" />
        <TextBlock x:Name="myText2" Width="40" Height="20" Foreground="White" Text="{Binding Value}" />
    </StackPanel>

</Grid>

public partial class MainPage : UserControl
{
    Boolean active = false;

    private Double _value;
    public Double Value
    {
        get { return _value; }
        set { _value = value; }
    }

    public MainPage()
    {
        InitializeComponent();
    }

    private void myRec_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        active = true;
    }

    private void myRec_MouseMove(object sender, MouseEventArgs e)
    {
        if (active == true)
        {
            aRec.Width = e.GetPosition(myRec).X;
            _value = aRec.Width * 10;
        }
    }

    private void myRec_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        active = false;
    }          
}

You MainPage must implement INotifyPropertyChanged interface (example for How to: Implement the INotifyPropertyChanged Interface) and your properties must fire event on set, or you must use Dependency property for Value.
Also on myRec_MouseMove hanlder assign width to Value property, not _value member.

You have to declare the property "Value" as a dependency property.

In your code-behind:

myText2.DataContext = Value;

In your xaml:

<TextBlock x:Name="myText2" Width="40" Height="20" Foreground="White" Text="{Binding Path=.}" />

"Path=." will point to your datacontext.