更新 ObservableCollection 无法正确更新 Xamarin 表单中的 ListView

Question

我在 XAML 中有一个 ListView，它绑定到 ViewModel 中的 ObservableCollection。在初始化或 OnAppearing() 时，ListView 项目会完美显示。

但是，当我尝试从页面内（通过 ViewModel）更新 ListView 项目时，项目已更新，但旧项目仍然存在。

基本上，新项目被添加到 ListView 但低于之前 ObservableCollection 中的项目。我已经实现了 INotifyPropertyChanged，并且我认为我所做的一切都是正确的（尽管显然不是）。

请告诉我我做错了什么。我在 Collection 上尝试过 Clear() 但无济于事（结果相同）。

BaseViewModel：

public class BaseViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        protected void SetValue<T>(ref T backingField, T value, [CallerMemberName] string propertyName = null)
        {
            if (EqualityComparer<T>.Default.Equals(backingField, value))
                return;

            backingField = value;

            OnPropertyChanged(propertyName);
        }
}

XAML：

<ListView       IsEnabled="{Binding IsLoadingTable, Converter={Helpers:InverseBoolConverter}}"
                        IsVisible="{Binding IsLoadingTable, Converter={Helpers:InverseBoolConverter}}"
                        ItemsSource="{Binding LeagueStandings}"
                        SelectedItem="{Binding SelectedTeam, Mode=TwoWay}"
                        ItemSelected="ListView_ItemSelected"
                        RowHeight="60"
                        SeparatorVisibility="Default"
                        SeparatorColor="{DynamicResource accentColor}">

Page.cs：

protected override void OnAppearing()
        {
            base.OnAppearing();

            ViewModel.LoadLeagueStandingsCommand.Execute(_divisionId);
            ViewModel.LoadPickerItemsCommand.Execute(null);
        }

ViewModel 初始化：

private ObservableCollection<Standing> _leagueStandings;
        public ObservableCollection<Standing> LeagueStandings
        {
            get { return _leagueStandings ?? (_leagueStandings = new ObservableCollection<Standing>()); }
            set { SetValue(ref _leagueStandings, value); }
        }

ViewModel 方法：

private async Task LoadLeagueStandings(string divId)
        {

            if (_hasLoadedStandings)
                return;

            if (IsLoadingTable)
                return;

            _hasLoadedStandings = true;

            _divisionId = divId;

            try
            {
                IsLoadingTable = true;
                await _pageService.DisplayAlert("loading Selected", _divisionId, "ok");
                var v = await GetLeagueTableAsync(_htmlParser, _divisionId);

                LeagueStandings = new ObservableCollection<Standing>(v);

            }
            catch (Exception)
            {
                System.Diagnostics.Debug.WriteLine("Exception caught in DivisionsViewModel.cs.(LoadLeagueStandings).");
            }
            finally
            {
                IsLoadingTable = false;
            }

        }

Picker 项目更改时调用的 ViewModel 方法：

private async Task SelectItem(string item)
        {
            if (item == null)
                return;

            SelectedItem = null;

            var id = await _divisionFinder.GetDivisionIdAsync(item);

            var v = await GetLeagueTableAsync(_htmlParser, id);

            LeagueStandings = new ObservableCollection<Standing>(v);
        }

编辑* - 结果图片。第一个集合在数字 5 处结束，新集合再次从 1 开始追加到列表视图的末尾。

ListView Image

编辑 2：

public async Task<IEnumerable<Standing>> GetLeagueTableAsync(string divisionId)
    {
        // todo: get division Id from picker

        string address = "";
        if (IsOnline)
        {
            if (divisionId != "")
                address = $"{BaseUrls.LeagueStandings}{divisionId}";

            try
            {
                var config = Configuration.Default.WithDefaultLoader();
                var document = await BrowsingContext.New(config).OpenAsync(address);
                var cSelector = "table[class='table table-striped table-hover table-bordered'] tr";

                var table = document.QuerySelectorAll(cSelector).Skip(1);

                int count = 0;
                foreach (var c in table)
                {
                    var cells = c.QuerySelectorAll("td").ToArray();

                    _leagueStandings.Add(new Standing(++count, cells[0].TextContent.Trim(), cells[1].TextContent.Trim(),
                                                               cells[2].TextContent.Trim(), cells[3].TextContent.Trim(),
                                                               cells[4].TextContent.Trim(), cells[5].TextContent.Trim(),
                                                               cells[6].TextContent.Trim(), cells[7].TextContent.Trim()));
                }
            }
            catch(Exception e)
            {
                System.Diagnostics.Debug.WriteLine($"\n\n Exception caught LoadingLeagueTable - {e} \n\n");
            }
        }
        return _leagueStandings;

Answer 1

由于您既没有添加也没有删除项目，并且您正在替换引用，您需要引发事件来告知视图已更改。而不是你的代码，用这个替换它

    private ObservableCollection<Standing> _leagueStandings;
    public ObservableCollection<Standing> LeagueStandings
    {
        get { return _leagueStandings; }
        set { 
              _leagueStandings = value;
              RaisePropertyChanged("LeagueStandings"); 
            }
    }

为了将来的参考，ObservableCollection 已经实现了INotifyPropertyChanged，所以你不需要SetValue(x)..