如何在 ListView Xamarin 上显示 System.Collection

Question

我有一个系统集合，想在 ListView 上显示两列 dateForecast 和 levelForecast来自 IEnumerable<AlertLevel>.

这是我的 ListView 和 GoogleFormMap:

 <Grid>
  
  <Label Text="Hydro Forecast Maritsa-Tundzha"
         HorizontalOptions="CenterAndExpand"
         FontSize="Large"
         FontAttributes="Bold"
         Margin="0,40,0,0"/>
   <StackLayout>
       <ListView x:Name="lstLevel">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <StackLayout Orientation="Horizontal">
                                <Label HorizontalOptions="StartAndExpand"
                                       Text="{Binding DateLevel}"
                                       Padding="10,0,0,0"/>
                                 <Label HorizontalOptions="EndAndExpand"
                                       Text="{Binding AlertLevel}"
                                       Padding="10,0,0,0"/>
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
   </StackLayout>
    <Grid Margin="0,70,0,10">
        <Grid.RowDefinitions>
            <RowDefinition Height="250" />
        </Grid.RowDefinitions>
        <StackLayout>
          <local:CustomMap x:Name="customMap"
                           MapType="Street"
                           HasZoomEnabled="True"
                           HasScrollEnabled="False"/>
      </StackLayout>
    </Grid>     
</Grid>

而我想用这个方法显示数据：

 public void MarkerPressed()
    {
        MessagingCenter.Subscribe<object, IEnumerable<AlertLevel>>(this, "PinSelected", (sender, arg) =>
        {
            ListView lstLevel = (sender as ListView);
            lstLevel.BindingContext = arg;
        });
    }

当我单击地图中的标记时，我收到此错误消息：

System.Reflection.TargetInvocationException has been thrown: 

Exception has been thrown by the target of an invocation.

并且在 class CustomMapRenderer.cs 的 android 项目中将此方法发送给我 CustomMapRenderer.cs:

CustomPin GetCustomPin(Marker annotation)
    {
        string id = annotation.Id.Substring(1);

        int mapCode = int.Parse(id);

        var result = DataBaseConnection(mapCode);

        MessagingCenter.Send<object, IEnumerable<AlertLevel>>(this, "PinSelected", result);

        var position = new Position(annotation.Position.Latitude, annotation.Position.Longitude);

        foreach (var pin in customPins)
        {
            if (pin.Position == position)
            {
                return pin;
            }
        }
        return null;
    }

这是我的 android 项目中 CustomMapRenderer.cs 中的所有代码：

    using System;
using System.Collections.Generic;
using System.Linq;
using Android.Content;
using Android.Gms.Maps;
using Android.Gms.Maps.Model;
using Android.Widget;
using MaritsaTundzhaForecast;
using MaritsaTundzhaForecast.Models;
using MaritsaTundzhaForecast.Droid;
using MySqlConnector;
using Xamarin.Forms;
using Xamarin.Forms.Maps;
using Xamarin.Forms.Maps.Android;

[assembly: ExportRenderer(typeof(CustomMap), typeof(CustomMapRenderer))]
namespace MaritsaTundzhaForecast.Droid
{
    public class CustomMapRenderer : MapRenderer, GoogleMap.IInfoWindowAdapter
    {
        List<CustomPin> customPins;

        public CustomMapRenderer(Context context) : base(context)
        {
        }

        protected override void OnElementChanged(Xamarin.Forms.Platform.Android.ElementChangedEventArgs<Map> e)
        {
            base.OnElementChanged(e);

            if (e.OldElement != null)
            {
                NativeMap.InfoWindowClick -= OnInfoWindowClick;
            }

            if (e.NewElement != null)
            {
                var formsMap = (CustomMap)e.NewElement;
                customPins = formsMap.CustomPins;
            }
        }

        protected override void OnMapReady(GoogleMap map)
        {
            base.OnMapReady(map);

            NativeMap.InfoWindowClick += OnInfoWindowClick;
            NativeMap.SetInfoWindowAdapter(this);
        }

        protected override MarkerOptions CreateMarker(Pin pin)
        {
            var marker = new MarkerOptions();
            marker.SetPosition(new LatLng(pin.Position.Latitude, pin.Position.Longitude));
            marker.SetTitle(pin.Label);
            marker.SetSnippet(pin.Address);
            //marker.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.green));

            var custom = customPins.Where(x => x.Label == pin.Label && x.Address == pin.Address).FirstOrDefault();

            if (custom != null)
            {
                if (custom.AlertLevel == 1)
                {
                    marker.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.green));
                }
                if (custom.AlertLevel == 2)
                {
                    marker.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.yellow));
                }
                if (custom.AlertLevel == 3)
                {
                    marker.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.orange));
                }
                if (custom.AlertLevel == 4)
                {
                    marker.SetIcon(BitmapDescriptorFactory.FromResource(Resource.Drawable.red));
                }
            }
            return marker;
        }

        void OnInfoWindowClick(object sender, GoogleMap.InfoWindowClickEventArgs e)
        {
            var customPin = GetCustomPin(e.Marker);
            if (customPin == null)
            {
                throw new Exception("Custom pin not found");
            }

            if (!string.IsNullOrWhiteSpace(customPin.Url))
            {
                var url = Android.Net.Uri.Parse(customPin.Url);
                var intent = new Intent(Intent.ActionView, url);
                intent.AddFlags(ActivityFlags.NewTask);
                Android.App.Application.Context.StartActivity(intent);
            }
        }

        public Android.Views.View GetInfoContents(Marker marker)
        {
            var inflater = Android.App.Application.Context.GetSystemService(Context.LayoutInflaterService) as Android.Views.LayoutInflater;
            if (inflater != null)
            {
                Android.Views.View view;

                var customPin = GetCustomPin(marker);
                if (customPin == null)
                {
                    throw new Exception("Custom pin not found");
                }

                if (customPin.Name.Equals("Xamarin"))
                {
                    view = inflater.Inflate(Resource.Layout.XamarinMapInfoWindow, null);
                }
                else
                {
                    view = inflater.Inflate(Resource.Layout.MapInfoWindow, null);
                }

                CustomPin pin = GetCustomPin(marker);

                int CodeNum = pin.CodeNum;
                int AlertLevel = pin.AlertLevel;

                var infoTitle = view.FindViewById<TextView>(Resource.Id.InfoWindowTitle);
                var infoSubtitle = view.FindViewById<TextView>(Resource.Id.InfoWindowSubtitle);
                var infoSubtitle2 = view.FindViewById<TextView>(Resource.Id.InfoWindowSubtitle2);
                var infoSubtitle3 = view.FindViewById<TextView>(Resource.Id.InfoWindowSubtitle3);

                if (infoTitle != null)
                {
                    infoTitle.Text = marker.Title;
                }
                if (infoSubtitle != null)
                {
                    infoSubtitle.Text = marker.Snippet;
                }
                if (infoSubtitle2 != null)
                {
                    infoSubtitle2.Text = "Тревога: (1-4): " + AlertLevel;
                }
                if (infoSubtitle3 != null)
                {
                    infoSubtitle3.Text = "Код на станция: " + CodeNum;
                }

                return view;
            }
            return null;
        }

        public Android.Views.View GetInfoWindow(Marker marker)
        {
            return null;
        }

        public IEnumerable<AlertLevel> DataBaseConnection(int mapCode)
        {
            string ConnectionString = "server=192.168.0.1;uid=userName;port=3387;pwd=password;database=dbName;";
            MySqlConnection Conn = new MySqlConnection(ConnectionString);
            var listAlert = new List<AlertLevel>();

            try
            {
                Conn.Open();

                //replace(2) with mapCode
                string query = "CALL Get_Alert_levels_Station(" + mapCode + ");";
                MySqlCommand myCommand = new MySqlCommand(query, Conn);
                MySqlDataReader myReader;

                myReader = myCommand.ExecuteReader();

                try
                {
                    while (myReader.Read())
                    {
                        var currentData = new AlertLevel()
                        {
                            
                            dateForecast = myReader.GetDateTime(0),
                            levelForecast = myReader.GetInt32(1)
                            
                        };

                        listAlert.Add(currentData);
                    } 
                }
                finally
                {
                    myReader.Close();
                    Conn.Close();
                }
            }

            catch (Exception ex)
            {
                Console.WriteLine("Database Connection", "Not Connected ..." + Environment.NewLine + ex.ToString(), "OK");
            }

            return listAlert;
        }

        CustomPin GetCustomPin(Marker annotation)
        {
            string id = annotation.Id.Substring(1);

            int mapCode = int.Parse(id);

            var result = DataBaseConnection(mapCode);

            MessagingCenter.Send<object, IEnumerable<AlertLevel>>(this, "PinSelected", result);

            var position = new Position(annotation.Position.Latitude, annotation.Position.Longitude);

            foreach (var pin in customPins)
            {
                if (pin.Position == position)
                {
                    return pin;
                }
            }
            return null;
        }
    }
}

我能举个例子来解决这个错误吗？

Answer 1

使用BindingContext属性的ListView。

如果您的发件人是您的 ListView 那么：

        ListView yourListView = (sender as ListView);
        yourListView.BindingContext = yourCollection;

如果没有，则给您的 ListView 一个名称（在 xaml 中使用 x:Name 属性）然后访问 BindingContext 属性你的代码在后面。

如果您的方法位于您的 ViewModel 中，只需将您的 ListView 绑定到它即可：

<ListView x:Name="lstLevel" ItemsSource="{Binding yourCollectionName}" ItemTapped="RowItemTapped">

Answer 2

只需分配 ListView

的 ItemSource

MessagingCenter.Subscribe<object, IEnumerable<AlertLevel>>(this, "PinSelected", async (sender, arg) =>
    {
       lstLevel.ItemsSource = arg;
    });

如何在 ListView Xamarin 上显示 System.Collection

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