对象在绑定中未在ListView中显示。

Question

我有一个通过ListView属性绑定项的ItemsSource。我不明白为什么不显示DataTemplate数据。他们仍然正确地从互联网下载，但没有显示。我尝试过从c#和xaml插入ItemsSource属性，但是结果没有改变。

xaml

<yummy:PancakeView x:Name="ViewFrasi" IsVisible="False" Grid.Row="1" CornerRadius="30,30,0,0" BackgroundColor="White" VerticalOptions="FillAndExpand">
        <yummy:PancakeView.Border>
            <yummy:Border Color="Blue" Thickness="4"/>
        </yummy:PancakeView.Border>
        <Grid Margin="15">
            <ListView x:Name="Frasi"
                      ItemsSource="{Binding FrasiJsonOnline1}">
                <ListView.Header>
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="40"/>
                        </Grid.RowDefinitions>
                        <BoxView CornerRadius="100" Grid.Row="0" HorizontalOptions="End" WidthRequest="40">
                            <BoxView.Background>
                                <LinearGradientBrush  StartPoint="1,0" EndPoint="1,1">
                                    <GradientStop Color="Blue" Offset="0" />
                                    <GradientStop Color="LightBlue" Offset="1.0" />
                                </LinearGradientBrush>
                            </BoxView.Background>
                        </BoxView>
                        <Image Source="checked.png" Grid.Row="0" HorizontalOptions="End" HeightRequest="20" Margin="0,0,10,0"/>
                        <Button x:Name="BtSave" IsVisible="False" Clicked="BtSave_Clicked" BackgroundColor="Transparent" Grid.Row="0" HorizontalOptions="End" HeightRequest="25"/>
                        <ImageButton Source="close.png" Grid.Row="0" HorizontalOptions="Start" HeightRequest="20" Margin="5,0,0,0" Clicked="Close_Clicked" BackgroundColor="Transparent"/>
                    </Grid>
                </ListView.Header>
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <Grid>
                            <ScrollView>
                                <Grid>
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="50"/>
                                    </Grid.RowDefinitions>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="1*"/>
                                        <ColumnDefinition Width="5*"/>
                                    </Grid.ColumnDefinitions>
                                    <ImageButton Grid.Row="0" Grid.Column="0" Source="IconCopy.png" Clicked="CopyClipboard_Clicked"/>
                                    <Label Grid.Row="0" Grid.Column="1" Text="Test" TextColor="Black" FontSize="15"/>
                                </Grid>
                            </ScrollView>
                        </Grid>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>

        </Grid>
    </yummy:PancakeView>

c#

private async void CategoryView_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        var selected = e.CurrentSelection;         
        ClassCategory model = e.CurrentSelection.FirstOrDefault() as ClassCategory;

        WebClient clientw = new WebClient();
        clientw.Credentials = new NetworkCredential("xxxxxxx", "xxxxxx");
        string Frasi1 = "ftp://epiz_27426656@ftpupload.net/htdocs/" + model.Titolo + ".json";
        string contents1 = await clientw.DownloadStringTaskAsync(Frasi1);
        ObservableCollection<FraseClass> FrasiJsonOnline1 = JsonConvert.DeserializeObject<ObservableCollection<FraseClass>>(contents1);

        ViewFrasi.IsVisible = true;
        ViewFrasi.TranslationY = 600;
        ViewFrasi.TranslateTo(0, 0, 500, Easing.SinInOut);

    }

Answer 1

您正在使用局部变量，使其成为方法外部的公共属性，并在方法中更改其值，如下所示：

public ObservableCollection<FraseClass> FrasiJsonOnline1 { get; set; }

private async void CategoryView_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        var selected = e.CurrentSelection;         
        ClassCategory model = e.CurrentSelection.FirstOrDefault() as ClassCategory;

        WebClient clientw = new WebClient();
        clientw.Credentials = new NetworkCredential("xxxxxxx", "xxxxxx");
        string Frasi1 = "ftp://epiz_27426656@ftpupload.net/htdocs/" + model.Titolo + ".json";
        string contents1 = await clientw.DownloadStringTaskAsync(Frasi1);
        FrasiJsonOnline1 = JsonConvert.DeserializeObject<ObservableCollection<FraseClass>>(contents1);

        ViewFrasi.IsVisible = true;
        ViewFrasi.TranslationY = 600;
        ViewFrasi.TranslateTo(0, 0, 500, Easing.SinInOut);

    }

Answer 2

当您将ListView的ItemsSource绑定到FrasiJsonOnline1时。

这意味着您正在绑定到BindingContext.FrasiJsonOnline1。

 ItemsSource="{Binding BindingContext.FrasiJsonOnline1}".

所以FrasiJsonOnline1应该是BindingContext的一个属性。例如，如果您的BindingContext是myViewModel，那么FrasiJsonOnline1应该是myViewModel的属性，那么绑定就会成功：

public partial class MainPage : ContentPage
{
    public MainPage()
    {
        InitializeComponent();

        BindingContext = new myViewModel();
    }
}

public class myViewModel
{
    public ObservableCollection<FraseClass> FrasiJsonOnline1 { get; set; }

    public myViewModel()
    {
        FrasiJsonOnline1 = new ObservableCollection<FraseClass>();
        //...your codes
    }
}

如果您按照您的方式定义FrasiJsonOnline1：

public class myViewModel
{
    public myViewModel()
    {

    }

    private async void CategoryView_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        ObservableCollection<FraseClass> FrasiJsonOnline1 = JsonConvert.DeserializeObject<ObservableCollection<FraseClass>>(contents1);
    }
}

FrasiJsonOnline1是在方法中定义的，它是一个局部变量。BindingContext(myViewModel)没有一个名为FrasiJsonOnline1的属性，并且绑定无法工作。