How To Scan Files and Show Current Scanning Without Making My Application Non-Responding?

public MainWindow()
 {
     InitializeComponent();
     ItemsSource = new ObservableCollection<string>() { "pme", "two", 3.ToString() };
     DataContext = this;
     var bw = new BackgroundWorker();
     bw.DoWork += new DoWorkEventHandler(bw_DoWork);
     bw.WorkerReportsProgress = true;
     bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);
     bw.RunWorkerAsync();

 }

 void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
 {
     ItemsSource.Add(e.UserState.ToString());
}

 void bw_DoWork(object sender, DoWorkEventArgs e)
 {
     var bw = (BackgroundWorker) sender;
     for (int i = 1; i < 10000; i++)
         bw.ReportProgress(i / 10000, i);
 }

 public ObservableCollection<string> ItemsSource { get; set; }