Rxを使ってファイルが変更されたタイミングでWPF画面に反映する

public partial class MainWindow : Window
{
    private FileSystemWatcher watcher;

    private IDisposable obs;

    public MainWindow()
    {
        InitializeComponent();

        var f = new FileInfo(@"C:\temp\Sample1.txt");

        this.watcher = new FileSystemWatcher();
        this.watcher.Path = f.Directory.FullName;
        this.watcher.Filter = f.Name;
        this.watcher.NotifyFilter = NotifyFilters.LastWrite;
        var o = Observable.FromEvent<FileSystemEventHandler, FileSystemEventArgs>(
            a => (obj, e) => a(e),
            h => this.watcher.Changed += h,
            h => this.watcher.Changed -= h,
            DispatcherScheduler.Current);
        this.obs = o.ObserveOnDispatcher()
            .Select(x => GetContents(f))
            .Subscribe(x => {
                this.mainText.Text = x;
            });

        this.watcher.EnableRaisingEvents = true;
    }

    private string GetContents(FileInfo f)
    {
        using (var st = f.Open(FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
        using (var r = new StreamReader(st)) {
            return r.ReadToEnd();
        }
    }
}

DispatcherScheduler.Current + ObserveOnDispatcher() メソッドがポイントかしら。