c# - iOS C# 将 JSON 数据添加到 tableView

Question

我无法弄清楚这一点。似乎所有数据都进入了 CountryNames 列表，但在函数 GetCountries() 之外，列表显示为空。任何帮助将不胜感激。当我在 GetCountries 函数中将列表打印到控制台时，它会打印出所有数据，但列表之外是空的。

namespace Test
{
  public partial class StartViewController : UIViewController
  {
    private List<String> countryNames;
    //private List<String> countryLanguages;

    public StartViewController() : base("StartViewController", null)
    {
        countryNames = new List<String>();
        GetCountries();

        // function to get country names and languages
        async Task GetCountries()
        {
            List<CountryInfo> countries;
            countries = await RefreshDataAsync();

            foreach (var c in countries)
            {
                countryNames.Add(c.name);
            }
        }

    }

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
        // Perform any additional setup after loading the view, typically from a nib.
        CountriesTableView.Source = new StartViewController.TableSource(countryNames, this);
    }

    public override void DidReceiveMemoryWarning()
    {
        base.DidReceiveMemoryWarning();
        // Release any cached data, images, etc that aren't in use.
    }

    public override void ViewDidAppear(bool animated)
    {
        base.ViewDidAppear(animated);
    }

    #region TableView source stuff
    public class TableSource : UITableViewSource
    {
        StartViewController vc;
        private List<String> countries;
        //private List<String> languages;

        public TableSource(List<String> countriesIn, StartViewController vc_in)
        {
            countries = countriesIn;
            vc = vc_in;
        }

        public override UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
        {
            UITableViewCell cell;
            // try to get a reusable cell
            cell = tableView.DequeueReusableCell("countries");
            if (cell == null)
            {
                cell = new UITableViewCell( UITableViewCellStyle.Subtitle, "countries");
            }

            return cell;
        }

        public override nint NumberOfSections(UITableView tableView)
        {
            return 1;  
        }

        public override string TitleForHeader(UITableView tableView, nint section)
        {
            return " ";
        }

        public override nint RowsInSection(UITableView tableview, nint section)
        {
            return 1;
        }
    }
    #endregion

    // A task that calls other awaitable async methods
    public async Task<List<CountryInfo>> RefreshDataAsync()
    {

        HttpClient client;
        client = new HttpClient();

        var uri = new Uri("https://restcountries.eu/rest/v2/all");


        var response = await client.GetAsync(uri);
        List<CountryInfo> Items = null;
        if (response.IsSuccessStatusCode)
        {
            var content = await response.Content.ReadAsStringAsync();


            Items = JsonConvert.DeserializeObject<List<CountryInfo>>(content);
        }

        return Items;
    }

    #region Json helpers   use http://json2csharp.com/  paste in json or url of service and it generates classes for you
    public class Currency
    {
        public string code { get; set; }
        public string name { get; set; }
        public string symbol { get; set; }
    }

    public class Language
    {
        public string iso639_1 { get; set; }
        public string iso639_2 { get; set; }
        public string name { get; set; }
        public string nativeName { get; set; }
    }

    public class Translations
    {
        public string de { get; set; }
        public string es { get; set; }
        public string fr { get; set; }
        public string ja { get; set; }
        public string it { get; set; }
        public string br { get; set; }
        public string pt { get; set; }
        public string nl { get; set; }
        public string hr { get; set; }
        public string fa { get; set; }
    }

    public class CountryInfo
    {
        public string name { get; set; }
        public List<string> topLevelDomain { get; set; }
        public string alpha2Code { get; set; }
        public string alpha3Code { get; set; }
        public List<string> callingCodes { get; set; }
        public string capital { get; set; }
        public List<object> altSpellings { get; set; }
        public string region { get; set; }
        public string subregion { get; set; }
        public int population { get; set; }
        public List<object> latlng { get; set; }
        public string demonym { get; set; }
        public double? area { get; set; }
        public double? gini { get; set; }
        public List<string> timezones { get; set; }
        public List<object> borders { get; set; }
        public string nativeName { get; set; }
        public string numericCode { get; set; }
        public List<Currency> currencies { get; set; }
        public List<Language> languages { get; set; }
        public Translations translations { get; set; }
        public string flag { get; set; }
        public List<object> regionalBlocs { get; set; }
        public string cioc { get; set; }
    }
    #endregion
}

}

Answer 1

问题可能是 ViewDidLoad在您获得 countryNames 之前被调用从您的网络服务返回。 GetCountries是一个异步方法，因此它返回 Task立即，构造函数完成并立即返回，因此操作系统现在可以自由调用 ViewDidLoad ，它确实如此......但您仍在等待网络请求结果，所以 countryNames当你调用CountriesTableView.Source = new StartViewController.TableSource(countryNames, this);时仍然是一个空列表，因此countries字段也是一个空列表，这是您获取要在列表中显示的数据的位置。所以你需要在 GetCountries 时更新你的源代码完成...但是您无法执行此操作，因为您没有等待 GetCountries 的调用.

namespace Test
{
  public partial class StartViewController : UIViewController
  {
    // No need for this here, since your TableSource will be holding 
    // and providing your data
    //private List<String> countryNames;
    //private List<String> countryLanguages;

    public StartViewController() : base("StartViewController", null)
    {

        // Best Practice not to call async methods from a constructor
        // A constructor should be fast and return, after running all 
        // of its code, right away.
        //GetCountries();
    }


    public override async void ViewDidLoad()
    {
        base.ViewDidLoad();
        // Perform any additional setup after loading the view, typically from a nib.
        // Get your data and then set your source
        var countries = await RefreshDataAsync();

        var countryNames = new List<string>();
        foreach (var c in countries)
        {
            countryNames.Add(c.name);
        }

        // since you awaited the above, countryNames is now populated
        // before you set up your TableSource. 
        CountriesTableView.Source = new StartViewController.TableSource(countryNames, this); 

        // On a side note, by passing and holding onto a reference to
        // this UIViewController, you are creating a circular reference
        // which will cause a memory leak. You should not need a reference
        // to this view controller from your TableSource.             
    }
    ...
  }
  ...
}

就是这样，然后看看你的 TableSource代码，您没有用任何数据填充单元格。您获取并返回单元格，但没有设置任何 UI 控件，例如 UILabel s、文本(或其他)值来显示您的数据。您还需要告诉UITableView通过返回 RowsInSection 中的项目数，您必须显示多少项目方法。事实上，您正在告诉UITableView你只有一件元素。您需要执行以下操作:

    public override UITableViewCell GetCell(UITableView tableView, NSIndexPath indexPath)
    {
        UITableViewCell cell;
        // try to get a reusable cell
        cell = tableView.DequeueReusableCell("countries");
        if (cell == null)
        {
            cell = new UITableViewCell( UITableViewCellStyle.Subtitle, "countries");
        }

         // Get data for row
         string countryName = countries[indexPath.Row];

         // Display data in the cell
         cell.TextLabel.Text = countryName;

         return cell;
    }

    public override nint RowsInSection(UITableView tableview, nint section)
    {
        // return the number of items you need to display
        return countries.Count;
    }

如果我是你，我就会拥有你的TableSource有一个List<CountryInfo>字段而不是 List<string> .这样你的TableSource可以提供 UITableView 的任何和所有国家/地区数据使用它可能需要显示。