MainWindow.cs

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Input;
using WpfApplication30.Annotations;

namespace WpfApplication30
{
    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 void ButtonClicked(object o)
        {
            DoingWork = true;

            Task.Factory.StartNew(delegate
            {
                //Look at me doing some work, going to take about 15 seconds
                Thread.Sleep(15000);
            }).ContinueWith(t =>
            {
                DoingWork = false;

            });
        }

        private RelayCommand _buttonClicked;
        public ICommand CmdButtonClicked { get { return _buttonClicked ?? (_buttonClicked = new RelayCommand(ButtonClicked)); } }

        public event PropertyChangedEventHandler PropertyChanged;

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