Windows Phone ISupportIncrementalLoading - 始终列出更新

Question

我使用从API加载的对象实现listview。我的问题是我的列表一直加载对象，不会停止。列表不会等到用户到达其结尾。

这是我的模特：

public class AdModel
{
    public string Title { get; set; 
}

这是接口的实现：

public class IncrementalCollection<T> : ObservableCollection<T>, ISupportIncrementalLoading
{
    private Func<uint, Task<LoadDataResult>> loadDataTask = null;

    public IncrementalCollection(Func<uint, Task<LoadDataResult>> loadDataTask)
        : base()
    {
        this.loadDataTask = loadDataTask;
        this.HasMoreItems = true;
    }

    public bool HasMoreItems
    {
        get;
        private set;
    }

    public Windows.Foundation.IAsyncOperation<LoadMoreItemsResult> LoadMoreItemsAsync(uint count)
    {
        return Task.Run<LoadMoreItemsResult>(async () => {
            return await this.LoadMoreItemsInternalAsync(count);
        }).AsAsyncOperation<LoadMoreItemsResult>();
    }

    private async Task<LoadMoreItemsResult> LoadMoreItemsInternalAsync(uint count)
    {
        var loadDataResult = await this.loadDataTask(count);
        this.HasMoreItems = loadDataResult.HasMoreItems;

        return new LoadMoreItemsResult { Count = loadDataResult.LoadedItemsCount };
    }
}

这是viewmodel：

public class MainPageViewModel
{
    private int alreadyGet = 0;
    private int collectionSize = 20;
    private int PageNumber = 0;
    private IEnumerable<int> sourceData = null;

    public MainPageViewModel(/*int collectionSize*/)
    {
        //this.collectionSize = collectionSize;
        this.sourceData = GetData(this.collectionSize);
        this.Collection = new IncrementalCollection<AdModel>(this.LoadData);
    }

    public IncrementalCollection<AdModel> Collection { get; private set; }

    public Visibility LoaderVisibility { get; private set; }

    public async Task<LoadDataResult> LoadData(uint count)
    {
        var items = this.sourceData.Skip(this.alreadyGet).Take((int)count).ToList();
        int itemsNo = items.Count();
        this.alreadyGet += itemsNo;

        var dispatcher = CoreApplication.MainView.CoreWindow.Dispatcher;
        await dispatcher.RunAsync(
            CoreDispatcherPriority.Normal,
            async () =>
            {
                this.LoaderVisibility = Visibility.Visible;


                string url = "https://" + (++PageNumber).ToString();

                var abc = new MainModel();

                using (HttpClient client = new HttpClient())
                {
                    client.DefaultRequestHeaders.IfModifiedSince = DateTimeOffset.Now;

                    HttpResponseMessage response = await client.GetAsync(url);

                    if (response.IsSuccessStatusCode)
                    {
                        var stream = await response.Content.ReadAsStringAsync();

                        abc = JsonConvert.DeserializeObject<MainModel>(stream);
                    }
                }
                foreach (AdModel n in abc.ads)
                {
                    this.Collection.Add(n);
                }

                this.LoaderVisibility = Visibility.Collapsed;
            });

        return new LoadDataResult
        {
            HasMoreItems = this.alreadyGet < this.collectionSize,
            LoadedItemsCount = (uint)itemsNo
        };

    }

    private static IEnumerable<int> GetData(int collectionSize)
    {
        return Enumerable.Range(1, collectionSize).OrderBy(c => c).ToList();
    }
}

简单的观点：

<Grid Margin="19">
    <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <ListView ItemsSource="{Binding Collection}">
        <ListView.ItemContainerStyle>
            <Style TargetType="ListViewItem">
                <Setter Property="HorizontalContentAlignment" Value="Stretch" />
            </Style>
        </ListView.ItemContainerStyle>
        <ListView.ItemTemplate>
            <DataTemplate>
                    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center"
                               Text="{Binding}" FontSize="20" />
                </Grid>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
    <ProgressRing Visibility="{Binding LoaderVisibility}" 
                  Margin="19" Grid.Row="1" IsActive="True"/>
</Grid>