Xamarin.Forms with Net.Http

Question

伙计们和女士们，您能否告诉我此代码中存在什么问题，或者我可以以哪种方式更改 Xamarin 设置以成功执行代码？ Xam.Plugin.Connectivity.CrossConnectivity说："设备已连接到互联网"，但在任何实现中DownloadCountriesListAsync()卡住了（UWP 不起作用，清单中也有选定的 INTERNET 参数的 Android）。此代码在 c# 控制台应用中工作。

using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Threading.Tasks;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
namespace radacodeTestApp
{
    public class ListsDownloader
    {
        public List<Country> Countries { get; private set; }
        public ListsDownloader()
        {
            Countries = new List<Country>();
            var task = DownloadCountriesListAsync();
        }
        public async Task<bool> DownloadCountriesListAsync()
        {
            try
            {
                var vkjsonResponse = await GetResponse(@"https://api.vk.com/api.php?oauth=1&method=database.getCountries&need_all=1&v=5.60");
                var jsonObject = JObject.Parse(vkjsonResponse);
                foreach (var jO in jsonObject["response"]["items"])
                    Countries.Add(JsonConvert.DeserializeObject<Country>(jO.ToString()));
            }
            catch (OperationCanceledException)
            {
                return false;
            }
            return true;
        }
        public async Task<string> GetResponse(string url)
        {
            using (var httpClient = new HttpClient())
                return await httpClient.GetStringAsync(url);
        }

    }
    public class Country
    {
        public int Cid { get; set; }
        public string Title { get; set; }
        public override string ToString()
        {
            return Title;
        }
    }
}

Answer 1

从后台线程调用 DownloadCountriesListAsync 方法;代码当前正在从主线程（也称为 UIThread）调用DownloadCountriesListAsync，这可能会导致 UIThread 冻结。

我已经更新了下面的代码，以显示如何从后台线程调用 DownloadCountriesListAsync 方法。

Async/Await是一个棘手的野兽。将方法标记为async并不意味着它将自动在后台线程上运行;它只是意味着异步方法能够将其进程交给其父线程，直到其进程完成。@Clancey最近的Xamarin会议上做了一个关于Async/Await的精彩演讲。我强烈推荐它！

https://www.youtube.com/watch?v=jgxJbshvCXQ

using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Threading.Tasks;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
namespace radacodeTestApp
{
    public class ListsDownloader
    {
        public List<Country> Countries { get; private set; }
        public ListsDownloader()
        {
            Countries = new List<Country>();
            Task.Run(async () => await DownloadCountriesListAsync());
        }
        public async Task<bool> DownloadCountriesListAsync()
        {
            try
            {
                var vkjsonResponse = await GetResponse(@"https://api.vk.com/api.php?oauth=1&method=database.getCountries&need_all=1&v=5.60");
                var jsonObject = JObject.Parse(vkjsonResponse);
                foreach (var jO in jsonObject["response"]["items"])
                    Countries.Add(JsonConvert.DeserializeObject<Country>(jO.ToString()));
            }
            catch (OperationCanceledException)
            {
                return false;
            }
            return true;
        }
        public async Task<string> GetResponse(string url)
        {
            using (var httpClient = new HttpClient())
                return await httpClient.GetStringAsync(url);
        }
    }
    public class Country
    {
        public int Cid { get; set; }
        public string Title { get; set; }
        public override string ToString()
        {
            return Title;
        }
    }
}