WPF中实现进度条

XAML:

<Window x:Class="MyWPF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="DownLoad Demo" Height="350" Width="525" Loaded="Window_Loaded">
    <Window.Background>
        <LinearGradientBrush>
            <GradientStop Color="Red" Offset="0"></GradientStop>
            <GradientStop Color="Yellow" Offset="0.5"></GradientStop>
        </LinearGradientBrush>
    </Window.Background>
    <Grid>
        <Label x:Name="txtWords"></Label>
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" VerticalAlignment="Bottom">
            <Button Content="start download" Width="120" Height="23" Margin="8" Click="Button_Click">
                <Button.Effect>
                    <DropShadowEffect Color="Blue" BlurRadius="1"></DropShadowEffect>
                </Button.Effect>