C＃事件触发很多次

Question

我有问题。我使用的CollectionView可以从我的网页接收自定义ViewModel中的数据，只要它返回包含数据的JSON。一旦呼叫中的[C0

= Offset，该网页就会打印num_of_rows。如果发生这种情况，我将设置一个布尔值"Nothing"。现在，每次要进行网络通话时，它都会检查HitBottomOfList = true;。

完整代码

ViewModel：

HitBottomOfList == false
XAML：

public class TemplateListViewModel
{
    public double WidthHeight { get; set; }

    public ICommand LoadTemplates => new Command(LoadTemplateList);
    public int CurrentTemplateCountReceived;
    public bool HitBottomOfList = false;
    public ObservableCollection<TemplateSource> sourceList { 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;

        WidthHeight = (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)
    {
        if (HitBottomOfList == false)
        {
            List<Template> templateList = await App.RestService.GetTemplates(App.User, CurrentTemplateCountReceived);

            if (templateList != null)
            {
                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 = WidthHeight, FileName = template.FileName };
                    sourceList.Add(templateSource);
                }

                CurrentTemplateCountReceived = sourceList.Count;
            }
            else
            {
                HitBottomOfList = true;
            }
        }
    }
}
最后是我做的WebCall：

<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>
现在的问题是，它确实触发了public async Task<List<Template>> GetTemplates(User user, int offset)
{
    var postData = new List<KeyValuePair<string, string>>();
    postData.Add(new KeyValuePair<string, string>("un", user.Username));
    postData.Add(new KeyValuePair<string, string>("pw", user.Password));
    postData.Add(new KeyValuePair<string, string>("offset", offset.ToString()));
    var content = new FormUrlEncodedContent(postData);
    var weburl = "mysite.org/myapp/get_templates.php";
    List<Template> response = await PostResponseTemplates(weburl, content);

    return response;
}

public async Task<List<Template>> PostResponseTemplates(string weburl, FormUrlEncodedContent content)
{
    var response = await client.PostAsync(weburl, content);
    var json = await response.Content.ReadAsStringAsync();

    if (json != "Nothing")
    {
        var jObject = JObject.Parse(json);
        var templatePropery = jObject["Templates"] as JArray;
        List<Template> templateList = new List<Template>();

        foreach (var property in templatePropery)
        {
            List<Template> propertyList = new List<Template>();
            propertyList = JsonConvert.DeserializeObject<List<Template>>(property.ToString());
            templateList.AddRange(propertyList);
        }

        var sourcePropery = (JObject)jObject["Source"];
        foreach (var property in sourcePropery)
        {
            string tempplateSource = property.Value.Value<string>();
            App.TemplateSource = tempplateSource;
        }

        return templateList;
    }
    else
    {
        ErrorMessage = json;
        return default(List<Template>);
    }
}
它认为需要更多的数据，却又一次又一次地执行命令，而已经有一条命令可以获取新数据。这会导致该应用多次获得具有相同RemainingItemsThresholdReachedCommand="{Binding LoadTemplates}"的新数据，因此该应用将多次在CollectionView中获得相同的数据。
我希望该应用调用该网页1次，以接收更多图像，然后直接加载它，而无需再次询问新数据，因此列表中的重复项将消失。因此，当几乎触底时，我如何确保它只询问一次数据？
更新
使用@Jason的代码，以下操作出错：当代码通过offset时，它会触发MyHandler，但在完成之前会跳转到LoadTemplateList();，因此允许启动下一个命令，而无需完成另一个命令。知道如何等待方法完成吗？

Answer 1

使用布尔值来跟踪您是否已经在处理该事件并忽略任何新事件

handling = false;