Binding string

Question

MainWindow.xaml:

    <Grid Initialized="MainWindow_Initialized">
        <TextBlock Height="23" HorizontalAlignment="Left" Margin="324,159,0,0" Name="ProcessingBlock" Text="TextBlock" VerticalAlignment="Top" Width="150" Background="#FFEFDFDF" DataContext="{Binding}"></TextBlock>
        <Button Content="Refresh" Height="23" HorizontalAlignment="Left" Margin="301,235,0,0" Name="Refresh" VerticalAlignment="Top" Width="75" Click="Refresh_Click" />
    </Grid>

MainWindow.xaml.cs

    public partial class MainWindow : Window
    {
        string sProcessing = "abc";
        public MainWindow()
        {
            InitializeComponent();
            this.ProcessingBlock.SetBinding(TextBlock.TextProperty, new Binding(".") { Source = sProcessing });
        }

        private void Refresh_Click(object sender, RoutedEventArgs e)
        {
            sProcessing = "opq";
        }
    }

我只是想简单地让TextBlock显示的数据随着String值的改变而改变。可是，点击Refresh后，sProcessing的值改变了，而TextBlock显示值没有变。为什么呢？