如何在 ListView Xamarin 上显示 System.Collection

Posted 2023-04-19

【中文标题】如何在 ListView Xamarin 上显示 System.Collection

【英文标题】：How to display System.Collection on ListView Xamarin

【发布时间】：2021-08-26 03:30:40

【问题描述】：

我有一个系统集合，想在ListView 上显示来自IEnumerable<AlertLevel> 的两列dateForecast 和levelForecast。

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;
        );
    

当我点击地图中的标记时，我会收到以下错误消息：

System.Reflection.TargetInvocationException has been thrown: 

Exception has been thrown by the target of an invocation. 

并且错误在CustomMapRenderer.cs类的android项目中发送给我这个方法：

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;
        
    

我可以举一些例子来解决这个错误吗？

【参考方案1】：

使用ListView 的BindingContext 属性。

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，则为您的 ListView 命名（在 xaml 中使用 x:Name 属性），然后在您的代码中访问 BindingContext 属性。

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

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

【讨论】：

您的方式有问题，我会更新问题。请检查一下..

在这一行设置断点 "ListView lstLevel = (sender as ListView); 并确保演员在工作并为您提供 ListView ，转到下一行并确保在设置BindingContext 您的 ItemsSource 包含预期的集合（如果不尝试直接设置 ItemsSource 属性而不是 BindingContext ），如果 ItemsSource 具有预期值，那么您的地图控件自定义渲染器有问题。

也尝试使用可观察的集合：new ObservableCollection(arg)

【参考方案2】：

只需分配您的ListView 的ItemSource

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

【讨论】：

Jason 用你的方法当我点击标记时，listview 正在显示，但是是空的。我没有任何错误，但我看不到 listview 上的数据；(

我只使用 lstLevel.ItemsSource = arg;并且这项工作没有任何错误，但是为什么我的列表视图是空的？

您确认arg 包含您的数据了吗？

是的，我在问题中的 arg 上上传了截图。

修复Text="Binding DateLevel" - 根据您的屏幕截图，您的模型上没有名为“DateLevel”的属性。更改它以匹配模型中的实际属性名称。