Xamarin Forms 选择器 SelectedIndexChange 空异常

Question

我创建了一个应用程序，要求输入用户名和 returns 该用户的统计信息。当计数器大于 1 时，它会关注选择器并要求选择其中一个选项。之后，将调用 SelectedIndexChanged (Platformselect_SelectedIndexChanged) 方法并获取统计信息。问题是，如果用户输入用户名，则第一次有效。当用户返回并重新输入用户名时，它不会聚焦（它不会显示选择器）并自动调用 SelectedIndexChanged 方法，其中 platformselect.SelectedIndex 为 -1 导致我的程序崩溃。你能帮帮我吗？

我试过将焦点设置在主线程上，我在iOS和Android中都测试过，但出现了同样的问题。

public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
            platformselect.HeightRequest = 0;
            this.search.Clicked += Button_Clicked;
        }



        public async void searchStats(string usernameText)
        {
            search.IsEnabled = false;
            activityIndicator.IsRunning = true;
            var id = await GetDataLogic.GetID(usernameText);
            publicid = id;
            activityIndicator.IsRunning = false;

            if (id.uid == "error")
            {
                await DisplayAlert("Error", "Could not find username: " + usernameText, "OK");
                search.IsEnabled = true;
                return;
            }
            else if (id.uid == "errornointernet")
            {
                await DisplayAlert("Error", "Could not connect to the Internet. Please check your connection and try again.", "OK");
                search.IsEnabled = true;
                return;
            }
            else
            {
                if(id.platforms.Count > 1)
                {
                    platformselect.ItemsSource = publicid.platforms.ToList();
                    //we have to show alert to choose platform and then call get user stats.
                    **Device.BeginInvokeOnMainThread(() =>
                    {
                        if (platformselect.IsFocused)
                            platformselect.Unfocus();
                        platformselect.Focus();
                    });**
                }
                else
                {
                    //we call it without asking because there is only one platform.
                    var userstats = await GetDataLogic.GetStats(id, 0);
                    await Navigation.PushAsync(new Stats(userstats, favorites));
                }

            }

        }
        private async void Platformselect_SelectedIndexChanged(object sender, EventArgs e)
        {
            //the second time it doesnt show the picker and it automatically enters -1... it returns null exception. Has to be investigated.
            var userstats = await GetDataLogic.GetStats(publicid, platformselect.SelectedIndex);
            await Navigation.PushAsync(new Stats(userstats, favorites));
        }
    }

我希望它始终专注于选择器并等待用户的响应，而不是自动调用 SelectedIndexChanged 并将 -1 作为 selectedindex

Answer 1

我建议以这种方式更改您的事件处理程序

 private async void Platformselect_SelectedIndexChanged(object sender, EventArgs e)
 {
    //Check for unselection event
    if(platformselect.SelectedIndex == -1) return;


    var userstats = await GetDataLogic.GetStats(publicid, platformselect.SelectedIndex);
    await Navigation.PushAsync(new Stats(userstats, favorites));
 }

Xamarin Forms 选择器 SelectedIndexChange 空异常

Xamarin Forms picker SelectedIndexChange null exception

picker

xamarin

xamarin.forms