MainWindow.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using WpfApplication30.Annotations;

namespace WpfApplication30
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : INotifyPropertyChanged 
    {
        private bool _doingWork;

        public MainWindow()
        {
            InitializeComponent();

            DataContext = this;
        }

        public bool DoingWork
        {
            get { return _doingWork; }
            set
            {
                _doingWork = value;
                OnPropertyChanged("DoingWork");
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }

        private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            DoingWork = true;
            Task.Factory.StartNew(delegate
            {
                //Look at me doing some work, going to take about 15 seconds
                Thread.Sleep(15000);
                DoingWork = false;
            });
        }
    }
}

MainWindow.xaml

<Window x:Class="WpfApplication30.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:wpfApplication30="clr-namespace:WpfApplication30"
        Title="MainWindow" Height="350" Width="525">
    
    <Grid>
        
        <Grid.Resources>
            <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
        </Grid.Resources>

        <wpfApplication30:CircularProgressBar Height="30" Width="30" Visibility="{Binding DoingWork, Converter={StaticResource BooleanToVisibilityConverter}}" />
        
        <Button VerticalAlignment="Bottom" Click="ButtonBase_OnClick" Content="Do Work" Height="35" Width="100" Margin="0,0,0,25"></Button>
        
    </Grid>
</Window>