Xamarin.Forms HttpClient.GetAsync阻止UI线程

Question

使用await client.GetAsync(uri)阻止我的UI线程。

这是我的按钮点击事件处理程序中的代码：

private async void Button_Clicked(object sender, EventArgs e)
    {
        var vm = new MainPageViewModel();

        HttpClient client = new HttpClient();
        client.MaxResponseContentBufferSize = 256000;

        var uri = new Uri(string.Format("http://localhost:50715/api/values"));

        var response = await client.GetAsync(uri); // This is where UI gets blocked
        SongView.ItemsSource = new List<Song>();
        if (response.IsSuccessStatusCode)
        {
            var content = await response.Content.ReadAsStringAsync();
            vm.Songs = await Task.Factory.StartNew(() =>
            {
                return JsonConvert.DeserializeObject<List<Song>>(content);
            });
        }

        SongView.ItemsSource = vm.Songs;
    }

即使只使用此代码，它仍然会阻止同一行的UI。

private async void Button_Clicked(object sender, EventArgs e)
    {
        HttpClient client = new HttpClient();
        client.MaxResponseContentBufferSize = 256000;

        var uri = new Uri(string.Format("http://localhost:50715/api/values"));

        var response = await client.GetAsync(uri); // This is where UI gets blocked
    }

你能指出我做错了吗？