如何将当前列表项传递给 x:Bind 函数？

Question

我有一个简单的 ListView 绑定到对象列表。在 ListView.ItemTemplate 中，我想格式化每个对象并使用 x:Bind 函数绑定将其绑定到 TextBlock，如下所示：<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(???)}" />。但是，我不确定如何将对当前项的引用传递给函数。

如果我需要将项目的属性或什至多个属性（例如 <TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(Name, Model)}" />）传递给函数，这可以正常工作，但我不知道如何传递整个对象。

我已经尝试了所有这些都无济于事：

<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(this)}" />
<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(self)}" />
<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget({x:Bind})}" />
<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget({Binding})}" />
<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(Item)}" />
<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(DataContext)}" />
<TextBlock Name="Root" Text="{x:Bind local:MainViewModel.FormatWidget(Root.DataContext)}" />

作为解决方法，我可以在 returns 本身的数据项上创建并绑定到属性，如下所示：

        public Widget MeMyselfAndI { get => this; }

<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(MeMyselfAndI)}" />

然而，这是一个 hack，在我的真实项目中我无法修改数据项 class 的源以添加属性.

如何将对当前项的引用传递给函数？

这是一个重现：

MainPage.xaml

<Page
    x:Class="ListItemFunctionBinding.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="using:ListItemFunctionBinding"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
    mc:Ignorable="d">
    <Grid>
        <ListView ItemsSource="{x:Bind ViewModel.Widgets}" SelectedItem="{x:Bind ViewModel.SelectedItem}">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="local:Widget">
                    <TextBlock Text="{x:Bind local:MainViewModel.FormatWidget(MeMyselfAndI)}" />
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </Grid>
</Page>

MainPage.xaml.cs

using Windows.UI.Xaml.Controls;

namespace ListItemFunctionBinding
{
    public sealed partial class MainPage : Page
    {
        public MainViewModel ViewModel { get; } = new MainViewModel();

        public MainPage()
        {
            InitializeComponent();
        }
    }
}

MainViewModel.cs

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;

namespace ListItemFunctionBinding
{
    public class MainViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private Widget _selectedItem;

        public Widget SelectedItem
        {
            get => _selectedItem;
            set
            {
                if (_selectedItem != value)
                {
                    _selectedItem = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(SelectedItem)));
                }
            }
        }

        public ObservableCollection<Widget> Widgets { get; } = new ObservableCollection<Widget>()
        {
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Regular Widget",
                Model = "WX2020-01",
                Description = "Your typical everyday widget."
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Super Widget",
                Model = "WX2020-02",
                Description = "An extra special upgraded widget."
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Broken Widget",
                Model = "WX2020-03",
                Description = "A widget that has been used and abused."
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Fake Widget",
                Model = "WX2020-04",
                Description = "It's not really a widget at all!"
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Surprise Widget",
                Model = "WX2020-05",
                Description = "What kind of widget will it be?"
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Invisible Widget",
                Model = "WX2020-06",
                Description = "Our most inexpensive widget."
            },
            new Widget
            {
                Id = Guid.NewGuid(),
                Name = "Backwards Widget",
                Model = "WX2020-07",
                Description = "Really more of a tegdiw, come to think of it."
            }
        };

        public static string FormatWidget(Widget widget)
        {
            if (widget == null)
                return "No widget selected";
            else
                return $"{widget.Name} [{widget.Model}] {widget.Description}";
        }
    }
}

Widget.cs

using System;
using System.ComponentModel;

namespace ListItemFunctionBinding
{
    public class Widget : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        private Guid _id;
        private string _name;
        private string _model;
        private string _description;

        public Widget MeMyselfAndI { get => this; }

        public Guid Id
        {
            get => _id;
            set
            {
                if (_id != value)
                {
                    _id = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Id)));
                }
            }
        }

        public string Name
        {
            get => _name;
            set
            {
                if (_name != value)
                {
                    _name = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Name)));
                }
            }
        }

        public string Model
        {
            get => _model;
            set
            {
                if (_model != value)
                {
                    _model = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Model)));
                }
            }
        }

        public string Description
        {
            get => _description;
            set
            {
                if (_description != value)
                {
                    _description = value;
                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Description)));
                }
            }
        }
    }
}

Answer 1

在这种情况下，您可以使用 Converter 方法传递您当前的项目（例如 Widget），然后在 Converter 中调用 MainViewModel.FormatWidget 方法。

.xaml:

<Page.Resources>
    <local:MyConverter x:Key="MyConverter"/>
</Page.Resources>

<Grid>
    <ListView ItemsSource="{x:Bind ViewModel.Widgets}" SelectedItem="{x:Bind ViewModel.SelectedItem}">
        <ListView.ItemTemplate>
            <DataTemplate x:DataType="local:Widget">
                <TextBlock Text="{x:Bind Converter={StaticResource MyConverter}}" />
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</Grid>

.cs:

public class MyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        var widget = value as Widget;
        return MainViewModel.FormatWidget(widget);
    }

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

Answer 2

我找到了解决方案 here:

Pathless casting

The native bind parser doesn't provide a keyword to represent this as a function parameter, but it does support pathless casting (for example, {x:Bind (x:String)}), which can be used as a function parameter. Therefore, {x:Bind MethodName((namespace:TypeOfThis))} is a valid way to perform what is conceptually equivalent to {x:Bind MethodName(this)}.

所以在我原来的例子中，我可以使用<TextBlock Text="{x:Bind local:MainViewModel.FormatWidget((local:Widget))}" />来绑定当前列表项。

这非常有效！耶！

如何将当前列表项传递给 x:Bind 函数？

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uwp-xaml