在运行异步操作之前更新UI

Question

我尝试在异步操作之前更新我的UI组件，但是在执行异步任务后UI更新。按下按钮时执行以下代码：

  Activity.RunOnUiThread(() => { changeButtonToUnFollow(); });

  var friends = dbr.GetFollowers(user);
  var friendship = friends.Where(f => f.UserName == loggedUser && f.Friend_UserName == user).FirstOrDefault();
  dbr.DeleteFriend(friendship);
  followerscount.Text = (Convert.ToInt16(followerscount.Text) - 1).ToString();
  Task.Factory.StartNew(async () =>
                    {
                        await client.Follow(loggedUser, user, "true");
                    });

方法changeButtonToUnFollow()是更新UI组件的方法。类＆＃34;客户端＆＃34;中的异步方法定义如下：

    public async Task<string> Follow(string username, string friend_username, string unfriend)
{
    var client = new HttpClient();

    var response = await client.GetAsync(GlobalVariables.host + "api/member/AddFriend/" + username + "/" + friend_username + "/" + unfriend).Result;

    return response.ToString();
}

如何在进行异步调用之前更新UI？