为什么此ListView不与ObservableCollection元素绑定？

问题描述 投票：0回答：3

我正在尝试将ListView与可观察的集合数据绑定，但是它仍然为空。我有一些断点，并且可以看到ListaProfissionais（Observable collection）已正确填充。在下面，您可以看到我的.cs和.xaml代码（重要的部分）。有人看到这里有什么问题吗？


using System.ComponentModel;

public class Profissionais : INotifyPropertyChanged
{
    private string id;
    public string Id
    {
        get { return this.id; }
        set
        {
            if (this.id != value)
            {
                this.id = value;
                this.NotifyPropertyChanged("Id");
            }
        }
    }

    private string categoria;
    public string Categoria
    {
        get { return this.categoria; }
        set
        {
            if (this.categoria != value)
            {
                this.categoria = value;
                this.NotifyPropertyChanged("Categoria");
            }
        }
    }

    private string titulo;
    public string Titulo
    {
        get { return this.titulo; }
        set
        {
            if (this.titulo != value)
            {
                this.titulo = value;
                this.NotifyPropertyChanged("Titulo");
            }
        }
    }

    private string fotoperfil;
    public string FotoPerfil
    {
        get { return this.fotoperfil; }
        set
        {
            if (this.fotoperfil != value)
            {
                this.fotoperfil = value;
                this.NotifyPropertyChanged("FotoPerfil");
            }
        }
    }

    private string endereco;
    public string Endereco
    {
        get { return this.endereco; }
        set
        {
            if (this.endereco != value)
            {
                this.endereco = value;
                this.NotifyPropertyChanged("Endereco");
            }
        }
    }

    private string distancia;
    public string Distancia
    {
        get { return this.distancia; }
        set
        {
            if (this.distancia != value)
            {
                this.distancia = value;
                this.NotifyPropertyChanged("Distancia");
            }
        }
    }

    private string sexo;
    public string Sexo
    {
        get { return this.sexo; }
        set
        {
            if (this.sexo != value)
            {
                this.sexo = value;
                this.NotifyPropertyChanged("Sexo");
            }
        }
    }


    public event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged(string propName)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
    }

}



public static ObservableCollection<Profissionais> ListaProfissionais = new ObservableCollection<Profissionais>();


try
{
   using (WebClient client = new WebClient())
   {
      Uri uri = new Uri("xxxx.xxxxx.php");
      byte[] retorno = await client.UploadValuesTaskAsync(uri, DataSend);
      string resultado = System.Text.Encoding.UTF8.GetString(retorno);
      ListaProfissionais = JsonConvert.DeserializeObject<ObservableCollection<Profissionais>>(resultado);

   }
}
catch
{
  throw;
}
finally
{
  ListViewProfissionais.ItemsSource = ListaProfissionais; 
  // crashes here but I can see that ItemsSource has the 4 elements as expected
}

我的XAML：


<ListView x:Name="ListViewProfissionais" x:FieldModifier="public static" HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand" VerticalScrollBarVisibility="Never" HorizontalScrollBarVisibility="Never" BackgroundColor="Transparent" SeparatorColor="Transparent">
      <ListView.ItemTemplate>
         <DataTemplate>
            <Frame HasShadow="True" CornerRadius="5" HeightRequest="215" Margin="8" Padding="4" BackgroundColor="White">
               <Grid>
                 <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*"></ColumnDefinition>
                 </Grid.ColumnDefinitions>

                 <Grid.RowDefinitions>
                    <RowDefinition Height="15"></RowDefinition>
                    <RowDefinition Height="25"></RowDefinition>
                    <RowDefinition Height="105"></RowDefinition>
                    <RowDefinition Height="25"></RowDefinition>
                    <RowDefinition Height="25"></RowDefinition>
                 </Grid.RowDefinitions>

                 <Label Text="{Binding Categoria}" TextColor="#8e8e8e" FontSize="10" Grid.Column="0" Grid.Row="0" VerticalOptions="StartAndExpand" HorizontalOptions="StartAndExpand" Margin="2,0,0,0"></Label>
                 <Label Text="{Binding Titulo}" FontAttributes="Bold" TextColor="#337760" FontSize="14" Grid.Column="0" Grid.Row="1" VerticalOptions="StartAndExpand" HorizontalOptions="StartAndExpand" Margin="2,0,0,0"></Label>

                <Frame Grid.Column="0" Grid.Row="2" BackgroundColor="Transparent" CornerRadius="4" Padding="0" Margin="0">
                <Image Source="{Binding FotoPerfil}" HorizontalOptions="FillAndExpand" Aspect="AspectFill" VerticalOptions="FillAndExpand" />
                 </Frame>

                 <Label Text="{Binding Endereco}" TextColor="#8e8e8e" FontSize="10" Grid.Column="0" Grid.Row="3" VerticalOptions="EndAndExpand" HorizontalOptions="StartAndExpand"></Label>
                  <Label Text="&#xf3c5;" Grid.Column="0" Grid.Row="4" FontSize="10" HorizontalOptions="StartAndExpand" TextColor="#ff9000" Padding="0,6,0,0">
                   <Label.FontFamily>
                     <OnPlatform x:TypeArguments="x:String" Android="Font-Awesome-Free-Solid.otf#FontAwesome5Free-Solid" iOS="FontAwesome5Free-Solid" />
                   </Label.FontFamily>
                  </Label>
                  <Label Text="{Binding Distancia}" TextColor="#ff9000" FontSize="10" Grid.Column="0" Grid.Row="4" VerticalOptions="Center" HorizontalOptions="StartAndExpand" Margin="10,0,0,0"></Label>
               </Grid>
            </Frame>
         </DataTemplate>
       </ListView.ItemTemplate>
  </ListView>

它崩溃并显示消息“ System.InvalidCastException：指定的转换无效。”。见下文：

System.InvalidCastException: Specified cast is not valid.
  at at (wrapper castclass) System.Object.__castclass_with_cache(object,intptr,intptr)
  at Xamarin.Forms.Internals.TemplatedItemsList`2[TView,TItem].ActivateContent (System.Int32 index, System.Object item) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\TemplatedItemsList.cs:534
  at Xamarin.Forms.Internals.TemplatedItemsList`2[TView,TItem].CreateContent (System.Int32 index, System.Object item, System.Boolean insert) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\TemplatedItemsList.cs:543
  at Xamarin.Forms.Internals.TemplatedItemsList`2[TView,TItem].GetOrCreateContent (System.Int32 index, System.Object item) [0x00023] in D:\a\1\s\Xamarin.Forms.Core\TemplatedItemsList.cs:602
  at Xamarin.Forms.Internals.TemplatedItemsList`2[TView,TItem].get_Item (System.Int32 index) [0x00000] in D:\a\1\s\Xamarin.Forms.Core\TemplatedItemsList.cs:337
  at Xamarin.Forms.Platform.Android.ListViewAdapter.GetCellsFromPosition (System.Int32 position, System.Int32 take) [0x0003b] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ListViewAdapter.cs:539
  at Xamarin.Forms.Platform.Android.ListViewAdapter.GetCellForPosition (System.Int32 position) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ListViewAdapter.cs:454
  at Xamarin.Forms.Platform.Android.ListViewAdapter.GetView (System.Int32 position, Android.Views.View convertView, Android.Views.ViewGroup parent) [0x0006d] in D:\a\1\s\Xamarin.Forms.Platform.Android\Renderers\ListViewAdapter.cs:225
  at Android.Widget.BaseAdapter.n_GetView_ILandroid_view_View_Landroid_view_ViewGroup_ (System.IntPtr jnienv, System.IntPtr native__this, System.Int32 position, System.IntPtr native_convertView, System.IntPtr native_parent) [0x0001a] in <11f101b564894ca7af6c482ddc51c698>:0
  at at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.74(intptr,intptr,int,intptr,intptr)
c# visual-studio listview xamarin observablecollection
3个回答
0
投票

我认为您必须通知属性已更改，例如为您的类实现INotifyPropertyChanged接口。此外，我看不到是否设置了DataContext。不错的简单示例https://www.wpf-tutorial.com/data-binding/responding-to-changes/

0
投票

假设您正在尝试从后面的代码中绑定属性ListaProfissionais：

方法1（不好的做法）：

您必须将ListaProfissionais添加到类的BindingContext属性中。您可以按以下方式进行处理：

... assuming you already called your API ...
ListaProfissionais = JsonConvert.DeserializeObject<ObservableCollection<Profissionais>>(resultado);
BindingContext = ListaProfissionais;

在您的XAML而不是ItemsSource="{Binding ListaProfissionais}"上使用ItemSource="{Binding BindingContext}"

方法2（一种更好的方法）：

在XAML页面中为该类设置名称（Name =“ listPage”）。在您的ListView中，您可以执行以下操作：

ItemsSource="{Binding ListaProfissionais, Source='{x:Reference listPage}'}"

注意：我将在以后进行测试。

  1. 将INotifyPropertyChanged接口添加到cs文件并实现其方法。

将ListaProfissionais更改为这样的属性：

private ObservableCollection<Profissionais> _listaProfissionais;

public ObservableCollection<Profissionais> ListaProfissionais
{
    get => _listaProfissionais;
    set 
    {
        _listaProfissionais = value;
        // This should be the method implemented by INotifyPropertyChanged.
        OnPropertyChanged(nameof(ListaProfissionais);
    }
}

注意：如果您不通知绑定引擎该属性已更改，则该列表不会通知ListView。这样，我们可以确保ListView的绑定上下文正在侦听ListProfessionais中的属性更改。

希望有帮助。

0
投票

已找到问题的根源。它在xaml中，我不知道为什么，但是标签Grid不能直接在DataTemplate内部，它必须在ViewCell标签内部，我只添加了一个ViewCell，现在它可以很好地工作。

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