public static class NetworkIndicator
{
    static int _counter;

    public static void EnterActivity ()
    {
        Interlocked.Increment (ref _counter);
        RefreshIndicator ();
    }

    public static void LeaveActivity ()
    {
        Interlocked.Decrement (ref _counter);
        RefreshIndicator ();
    }

    public static void AttachToTask (Task task)
    {
        if (task.IsCanceled || task.IsCanceled || task.IsFaulted)
            return;

        EnterActivity ();
        task.ContinueWith (t => {
            LeaveActivity ();
        });
    }

    static void RefreshIndicator ()
    {
        UIApplication.SharedApplication.NetworkActivityIndicatorVisible =
            (_counter > 0);
    }
}

然后我为方便起见添加了两个扩展方法：

public static class TaskExtensions
{
    public static Task WithNetworkIndicator (this Task task)
    {
        NetworkIndicator.AttachToTask (task);
        return task;
    }

    public static Task<TResult> WithNetworkIndicator<TResult> (this Task<TResult> task)
    {
        NetworkIndicator.AttachToTask (task);
        return task;
    }
}

我是如何包装它的：

var task = Api.QueryNotifications (AuthManager.CurrentProfile.Id,NotificationType.All,until,cached)
    .WithNetworkIndicator ();

然后我像往常一样使用任务.