列表视图页面上的无限滚动

Question

I have a listview whose data is fetched from json on the server. Code:

ObservableCollection<QuizHome> quizhomedatasource = new ObservableCollection<QuizHome>();
private bool incall = false, endoflist = false;
int offset = 1, halaman = 1;
private void MainGrid_Loaded(object sender, RoutedEventArgs e)
        {
            quizhomedatasource.Clear();
            QuizList.ItemsSource = quizhomedatasource;
            ProgressQuiz(offset);
        }
private void QuizList_Loaded(object sender, RoutedEventArgs e)
        {
            ScrollViewer viewer = GetScrollViewer(this.QuizList);
            viewer.ViewChanged += Viewer_ViewChanged;
        }

        private void Viewer_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
        {
            ScrollViewer view = (ScrollViewer)sender;
            double progress = view.VerticalOffset / view.ScrollableHeight;
            if (progress > 0.7 && !endoflist)
            {
                incall = true;
                while (offset < halaman)
                {
                    ProgressQuiz(++offset);
                }
            }
            else
            {
                incall = false;
            }
        }

        public static ScrollViewer GetScrollViewer(DependencyObject depObj)
        {
            if (depObj is ScrollViewer) return depObj as ScrollViewer;

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
            {
                var child = VisualTreeHelper.GetChild(depObj, i);
                var result = GetScrollViewer(child);
                if (result != null) return result;
            }
            return null;
        }
private async void ProgressQuiz(int offset)
        {
                        try
                        {
                                urlPath = "https://mhnkp2.com/school/api-v3/fetch/tryout_paket_perkelas";
                                var values = new List<KeyValuePair<string, string>>
                                {
                                    new KeyValuePair<string, string>("halaman", offset.ToString()),
                                    new KeyValuePair<string, string>("limit", "10"),
                                    new KeyValuePair<string, string>("kelas", "2")
                                };
                                var httpClient = new HttpClient(new HttpClientHandler());
                                httpClient.DefaultRequestHeaders.TryAddWithoutValidation("SCH-API-KEY", "SCH_KEnaBiDeplebt");
                                httpClient.DefaultRequestHeaders.TryAddWithoutValidation("token", token.Token);
                                var response = await httpClient.PostAsync(urlPath, new FormUrlEncodedContent(values));
                                response.EnsureSuccessStatusCode();
                                string jsonText = await response.Content.ReadAsStringAsync();
                            try
                            {
                                JsonObject jsonObject = JsonObject.Parse(jsonText);
                                JsonObject groupObject1 = jsonObject.GetObject();                                        
                                double pages = groupObject1["total_page"].GetNumber();
                                    double page = groupObject1["current_page"].GetNumber();
                                    Buku file = new Buku();
                                    file.PageNo = Convert.ToInt32(page);
                                    file.Pages = Convert.ToInt32(pages);
                                    halaman = file.Pages;
                                    JsonArray jsonData1 = jsonObject["data"].GetArray();
                                    foreach (JsonValue groupValue1 in jsonData1)
                                    {
                                        JsonObject groupObject2 = groupValue1.GetObject();
                                        string title = groupObject2["judul"].GetString();

                                        QuizHome quiz = new QuizHome();
                                        quiz.Title = title;
                                        quizhomedatasource.Add(quiz);
                                    }
                                    if (quizhomedatasource.Count < 0)
                                    {
                                        QuizList.Visibility = Visibility.Collapsed;
                                        statusKosong.Visibility = Visibility.Visible;
                                    }
                                }
                            }

I have a problem that if the loaded page exceeds the number of pages, then "statuskosong" is displayed. How can I prevent "statuskosong" from being displayed and data not being loaded again?

Note:

• "statuskosong" is the text that will be displayed when data = 0

Answer 1

How can I prevent "statuskosong" from being displayed and data not being loaded again?

You can set statusKosong.Visibility to Collapsed every time you call the ProgressQuiz method, and then display it when it detects that quizhomedatasource.Count is 0.

By the way, your judgment condition at the end of the ProgressQuiz method is quizhomedatasource.Count < 0 , this will not happen, you can change it to quizhomedatasource.Count == 0

Regarding avoiding repeated requests, we can add new judgment conditions in the Viewer_ViewChanged callback and set the return value of the ProgressQuiz method to Task :

private async void Viewer_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
     ScrollViewer view = (ScrollViewer)sender;
     double progress = view.VerticalOffset / view.ScrollableHeight;
     if (progress > 0.7 && !endoflist && !incall)
     {
         incall = true;
         while (offset < halaman)
         {
             await ProgressQuiz(++offset);
         }
     }
     else
     {
         incall = false;
     }
}

private async Task ProgressQuiz(int offset)
{
    //code
}

By judging the incall and waiting for the ProgressQuiz method to execute, repeated requests can be avoided.

---

From the code you provided, you are creating an incrementally loaded list.

Obtaining the ScrollViewer in ListView through VisualTreeHelper and attaching events is a workaround, but UWP provides another way to solve the incremental loading problem more elegantly (From Windows Community Toolkit):

• Incremental Loading Collection Helpers