Xamarin Forms 停止在 CollectionView 中加載數據

Question

我有個問題。 我創建了一個使用自定義ViewModel的CollectionView 。 在該ViewModel我對我的網頁進行了網絡調用以獲取 20 個圖像文件名。 得到結果后，我會調用 foreach filename 來獲取該文件名的ImageSource 。 現在我創建了一個Load data incrementally代碼，以 20 個為一組加載CollectionView數據。這是我的 xaml：

<ContentPage.Content>
    <StackLayout HorizontalOptions="Fill" Padding="15">
        <Frame IsClippedToBounds="True" HeightRequest="45" CornerRadius="5" Padding="0" BackgroundColor="Transparent">
            <Entry Placeholder="Search" ReturnType="Done" PlaceholderColor="Gray" x:Name="txtSearch" Margin="5,0,0,0" TextColor="White" />
        </Frame>

        <CollectionView ItemsSource="{Binding sourceList}" RemainingItemsThreshold="6"
            RemainingItemsThresholdReachedCommand="{Binding LoadTemplates}">
            <CollectionView.ItemsLayout>
                <GridItemsLayout Orientation="Vertical"
                Span="2" />
            </CollectionView.ItemsLayout>
            <CollectionView.ItemTemplate>
                <DataTemplate>

                    <ff:CachedImage
                Source="{Binding Source}"
                VerticalOptions="Center"
                HorizontalOptions="Center"
                WidthRequest="{Binding WidthHeight}"
                HeightRequest="{Binding WidthHeight}">
                        <ff:CachedImage.GestureRecognizers>
                            <TapGestureRecognizer Tapped="imgTemplate_Clicked" />
                        </ff:CachedImage.GestureRecognizers>
                    </ff:CachedImage>

                </DataTemplate>
            </CollectionView.ItemTemplate>
        </CollectionView>
    </StackLayout>
</ContentPage.Content>

這是頁面構造函數：

public TemplateList()
{
    InitializeComponent();

    TemplateListViewModel vm = new TemplateListViewModel();
    BindingContext = vm;
}

這是視圖模型：

public class TemplateListViewModel
{
    public ICommand LoadTemplates => new Command(LoadTemplateList);

    public int CurrentTemplateCountReceived;
    public ObservableCollection<TemplateSource> sourceList { get; set; }
    public double MemeWidthHeight { get; set; }

    public TemplateListViewModel()
    {
        CurrentTemplateCountReceived = 0;
        sourceList = new ObservableCollection<TemplateSource>();

        var mainDisplayInfo = DeviceDisplay.MainDisplayInfo;
        var width = mainDisplayInfo.Width;
        var density = mainDisplayInfo.Density;
        var ScaledWidth = width / density;

        MemeWidthHeight = (ScaledWidth / 2);

        loadingTemplates += onLoadingTemplates;
        LoadTemplateList();
    }

    private event EventHandler loadingTemplates = delegate { };

    private void LoadTemplateList()
    {
        loadingTemplates(this, EventArgs.Empty);
    }

    private async void onLoadingTemplates(object sender, EventArgs args)
    {
        List<Template> templateList = await App.RestService.GetTemplates(App.User, CurrentTemplateCountReceived);

        foreach (var template in templateList)
        {
            ImageSource source = ImageSource.FromUri(new Uri("mysite.org/myapp/" + template.FileName));
            TemplateSource templateSource = new TemplateSource { Id = template.Id, Source = source, WidthHeight= MemeWidthHeight, FileName = template.FileName };
            sourceList.Add(templateSource);
        }

        CurrentTemplateCountReceived = sourceList.Count;
    }
}

現在App.RestService.GetTemplates(App.User, CurrentTemplateCountReceived); 只是返回一個包含文件名的列表，但問題是當我不再接收任何信息時，它會繼續進行網絡呼叫。 在我的服務器上，我有 38 張圖像，因此在 2 次網絡調用后，應用程序得到了一切。 之后，應用程序從網絡電話收到的結果是"Nothing" 。

所以我的問題是：
當我處於 CollectionView 的底部時，如何停止進行網絡調用？

Answer 1

bool moreData = true;

private async void onLoadingTemplates(object sender, EventArgs args)
    {
        if (!moreData) return;

        List<Template> templateList = await App.RestService.GetTemplates(App.User, CurrentTemplateCountReceived);

        if (templateList is null or templateList.Count == 0) {
          moreData = false;
          return;
        }

        foreach (var template in templateList)
        {
            ImageSource source = ImageSource.FromUri(new Uri("mysite.org/myapp/" + template.FileName));
            TemplateSource templateSource = new TemplateSource { Id = template.Id, Source = source, WidthHeight= MemeWidthHeight, FileName = template.FileName };
            sourceList.Add(templateSource);
        }

        CurrentTemplateCountReceived = sourceList.Count;
    }