c# - Xamarin 形成具有可绑定(bind)属性的 ValueConverter

Question

我有一个 Xamarin 项目，我正在尝试绑定(bind)我的 ValueConverter。我在我的 UWP 项目中完成了此操作，但在 Xamarin 中我遇到了编译错误:

No property, bindable property, or event found for 'UserData', or mismatching type between value and property.

这样做的目的是当你在ListView中选择一个item时，entry读取UserValue属性，当你编辑这个entry中的文本时，item的属性的内容被更新，这个变化是在 ListView 中可见。

这是我的代码:

MainPage.XAML:

<StackLayout>
    <Entry x:Name="Entry_NewValue" TextChanged="Entry_NewValue_TextChanged"/>

    <ListView x:Name="_ListView" ItemSelected="_ListView_ItemSelected">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <Grid>
                        <Grid.Resources>
                            <local:ViewCell_Converter x:Key="ViewCell_Converter" UserData="{Binding}"/>
                        </Grid.Resources>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="2*"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>

                        <Label Grid.Column="0" Text="{Binding UserName}"/>
                        <Label Grid.Column="1" Text="{Binding UserValue, Converter={StaticResource ViewCell_Converter}}"/>
                    </Grid>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>

MainPage.XAML.cs:

public MainPage()
{
    InitializeComponent();

    _ListView.ItemsSource = UserDatas();
}        

private ObservableCollection<UserData> UserDatas()
{
    return new ObservableCollection<UserData>()
    {
        new UserData() { UserName = "Name1", UserValue = 10, ValidatedByUser = true, },
        new UserData() { UserName = "Name2", UserValue = 11, ValidatedByUser = false, },
        new UserData() { UserName = "Name3", UserValue = 12, ValidatedByUser = true, },
        new UserData() { UserName = "Name4", UserValue = 13, ValidatedByUser = false, },
        new UserData() { UserName = "Name5", UserValue = 14, ValidatedByUser = true, },
    };
}

private void _ListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)
{
    if (e.SelectedItem is UserData userData)
    {
        Entry_NewValue.Text = userData.UserValue.ToString();
    }
}

private void Entry_NewValue_TextChanged(object sender, TextChangedEventArgs e)
{
    if (_ListView.SelectedItem is UserData userData)
    {
        if (double.TryParse(Entry_NewValue.Text, out double _double))
        {
            if(userData.UserValue != _double)
            {
                userData.UserValue = _double;
            }                
        }
    }
}

方法.cs:

public class UserData : Prism.Mvvm.BindableBase
{
    private bool _validatedByUser;
    public bool ValidatedByUser
    {
        get
        {
            return _validatedByUser;
        }
        set
        {
            SetProperty(ref _validatedByUser, value);
        }
    }

    private string _userName;
    public string UserName
    {
        get
        {
            return _userName;
        }
        set
        {
            SetProperty(ref _userName, value);
        }
    }

    private double _userValue;
    public double UserValue
    {
        get
        {
            return _userValue;
        }
        set
        {
            SetProperty(ref _userValue, value);
        }
    }
    }

    public class ViewCell_Converter : BindableObject, IValueConverter
    {            
    public UserData UserData
    {
        get { return (UserData)GetValue(bindableProperty); }
        set { SetValue(bindableProperty, value); }
    }

    public static readonly BindableProperty bindableProperty = BindableProperty.Create(
                                                               nameof(UserData),
                                                               typeof(UserData),
                                                               typeof(ViewCell_Converter),
                                                               null);

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (UserData != null)
        {
            if (value is double _value)
            {
                if (UserData.ValidatedByUser)
                {
                    return string.Format("{0:0.00}", _value);
                }
                else
                {
                    return string.Format("{0:0.0000}", _value);
                }
            }
        }

        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Answer 1

这是解决方案(快速示例):

    public class ViewCell_Converter : IValueConverter
{

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is double doubleValue)
        {
            if (value is double _value)
            {
                return string.Format("{0:0.00}", doubleValue);
            }
        }

        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

您不需要更多的值转换器。

<StackLayout>
    <Entry x:Name="Entry_NewValue" TextChanged="Entry_NewValue_TextChanged"/>

    <ListView x:Name="_ListView" ItemSelected="_ListView_ItemSelected">
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <Grid>
                        <Grid.Resources>
                            <local:ViewCell_Converter x:Key="ViewCell_Converter"/>
                        </Grid.Resources>

                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="2*"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>

                        <Label Grid.Column="0" Text="{Binding UserName}"/>
                        <Label Grid.Column="1" Text="{Binding UserValue, Converter={StaticResource ViewCell_Converter}}"/>
                    </Grid>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>