c# - 使用 BindableObject 实现 IValueConverter 并且绑定(bind)的 Property 始终为默认值

Question

我正在尝试找到一种方法来处理 CommandParameter 不能与 IValueConverter 绑定(bind)的事实。通过阅读，我发现了在实现 IValueConverter 时从 BindableObject 继承的建议。

尝试访问该属性时，它始终为 0，可能采用默认值。

作为引用，我通过使用 Xamarin.Forms Shell 模板作为示例来简化问题:

当使项目使用移动应用程序(Xamarin.Forms)并在创建项目时选择 Shell。从那里我做了少量修改。请注意，我只是设置一个断点来查看分配给 BindableProperty 的内容，它始终为 0。

此示例向已提供的 Item.cs 类添加两个属性。

public class Item {
    public string Id { get; set; }

    public string Text { get; set; }

    public string Description { get; set; }

    // Added these two properties. The idea is to Bind Number1 to the Text property of a label, and pass Number2 to the Property
    public int Number1 { get; set; }

    public int Number2 { get; set; }
}

在 MockDataStore.cs 的构造函数中，我为 Number1 和 Number2 属性添加了值。

var mockItems = new List<Item>{
    new Item { Id = Guid.NewGuid().ToString(), Text = "First item", Description="This is an item description.", Number1 = 5, Number2 = 20},
    new Item { Id = Guid.NewGuid().ToString(), Text = "Second item", Description="This is an item description.", Number1 = 2, Number2 = 8},
    new Item { Id = Guid.NewGuid().ToString(), Text = "Third item", Description="This is an item description.", Number1 = 3, Number2 = 6},
    new Item { Id = Guid.NewGuid().ToString(), Text = "Fourth item", Description="This is an item description.", Number1 = 4, Number2 = 7},
    new Item { Id = Guid.NewGuid().ToString(), Text = "Fifth item", Description="This is an item description.", Number1 = 1, Number2 = 10},
    new Item { Id = Guid.NewGuid().ToString(), Text = "Sixth item", Description="This is an item description.", Number1 = 8, Number2 = 2}
};

带有 BindableObject 和 IValueConverter 的转换器

public class Number1Number2Converter : BindableObject, IValueConverter {
    public static readonly BindableProperty ValueProperty =
        BindableProperty.Create(nameof(Value), typeof(int), typeof(Number1Number2Converter), null);

    public int Value {
        get {
            return (int)GetValue(ValueProperty);
        }
        set {
            SetValue(ValueProperty, value);
        }
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        // object value does have a value assigned, which is good however the bound Property of Value does not. This should have Number2 being passed in.
        var testing123 = Value;

        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
        return null;
    }
}

在 var testing123 = Value; 行上，我放置了一个断点。我还没有尝试使用 Value，我只是想看到它不是 0。我在 MockDataStore 中分配给 Number2 的值没有一个是 0。

这是修改后的 ItemsPage.xaml。请注意绑定(bind)了 Number1 的标签，当然还有转换器。

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:converters="clr-namespace:App2.Converters;assembly=App2"
             mc:Ignorable="d"
             x:Class="App2.Views.ItemsPage"
             Title="{Binding Title}"
             x:Name="BrowseItemsPage">
    <ContentPage.ToolbarItems>
        <ToolbarItem Text="Add" Clicked="AddItem_Clicked" />
    </ContentPage.ToolbarItems>
    <StackLayout>
        <ListView x:Name="ItemsListView"
                ItemsSource="{Binding Items}"
                VerticalOptions="FillAndExpand"
                HasUnevenRows="true"
                RefreshCommand="{Binding LoadItemsCommand}"
                IsPullToRefreshEnabled="true"
                IsRefreshing="{Binding IsBusy, Mode=OneWay}"
                CachingStrategy="RecycleElement"
                ItemSelected="OnItemSelected">
            <d:ListView.ItemsSource>
                <x:Array Type="{x:Type x:String}">
                    <x:String>First Item</x:String>
                    <x:String>Second Item</x:String>
                    <x:String>Third Item</x:String>
                    <x:String>Forth Item</x:String>
                    <x:String>Fifth Item</x:String>
                    <x:String>Sixth Item</x:String>
                </x:Array>
            </d:ListView.ItemsSource>
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <StackLayout Padding="10">
                            <Label Text="{Binding Text}" 
                                d:Text="{Binding .}"
                                LineBreakMode="NoWrap" 
                                Style="{DynamicResource ListItemTextStyle}" 
                                FontSize="16" />
                            <Label Text="{Binding Description}" 
                                d:Text="Item description"
                                LineBreakMode="NoWrap"
                                Style="{DynamicResource ListItemDetailTextStyle}"
                                FontSize="13" />
                            <Label>
                                <Label.Text>
                                    <Binding Path="Number1">
                                        <Binding.Converter>
                                            <converters:Number1Number2Converter Value="{Binding Path=Number2}" />
                                        </Binding.Converter>
                                    </Binding>
                                </Label.Text>
                            </Label>
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout>
</ContentPage>

Value 在此示例中始终为 0。参数 object value 被赋予了 Number1 的 int 值，但是 Value 的可绑定(bind)属性应该被赋予 Number2。这个概念是否存在根本性缺陷，或者我是否在正确的轨道上但未能实现某些东西？

Answer 1

您可以尝试将整个 Item 对象发送到您的转换器:

<Label Text="{Binding Path=., Converter={StaticResource Number1Number2Converter}}" />

然后，在您的转换器中您应该可以访问这两个属性:

public class Number1Number2Converter : IValueConverter {    
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        if (value is Item item)
        {
             var number1 = item.Number1;
             var number2 = item.Number2,
        }
        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
        return null;
    }
}