需要从 Xamarin 中的选取器中选择至少一项

在我的应用程序中，有一个选择器和按钮。我想要两件事。

1. 需要从选取器中选择至少一项，否则将无法单击按钮。例如就像HTML 中的必需属性

2. 如果值为提供者1，然后导航到此页面，否则如果提供商2，然后导航至此页面。

视图中选择器和按钮的代码

<Picker x:Name="picker_provider" 
        Title="Select a Provider">
        <Picker.ItemsSource>
            <x:Array Type="{x:Type x:String}">
                <x:String>Provider 1</x:String>
                <x:String>Provider 2</x:String>
            </x:Array>
        </Picker.ItemsSource>
</Picker>

<Button
    Text="Next"
    Command="{Binding NextPageCommand}"/>

ViewModel.cs 中的按钮绑定代码

public async Task NextPage(RegisterViewModel nextpage) => await NavigationService.NavigateToAsync(nextpage, null, NavigationType.Modal);

#region Bindable Command
public ICommand NextPageCommand => new Command<RegisterViewModel>(async (nextpage) =>
    {
        await NextPage(nextpage);
    });
#endregion

您可以在按钮中使用多数据触发器来启用或禁用它，具体取决于选择器选择属性的状态。

因为您要绑定到字符串，所以您需要添加一个转换器来测试它们是 null 还是空，例如：

namespace Osma.Mobile.App.Converters
{
    public class StringNullOrEmptyValueBoolConverter: IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value is string str && !string.IsNullOrEmpty(str))
            {
                // String is not null or empty
                return false;
            }
            
            // String is null or empty
            return true;
        }
    
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

将所需的属性添加到您的ViewModel:

public class ProviderViewModel : ABaseViewModel
{
    public ProviderViewModel(
        IUserDialogs userDialogs,
        INavigationService navigationService
        ) : base(
            nameof(ProviderViewModel),
            userDialogs,
            navigationService)
    {
    }

    public override async Task InitializeAsync(object navigationData)
    {
        await base.InitializeAsync(navigationData);
    }

   private string _selectedProvider;
   public string SelectedProvider {
       get { return _selectedProvider; }
       set 
       { 
            this.RaiseAndSetIfChanged(ref _selectedProvider, value) }
       }
  }

   private string _selectedCountry;
   public string SelectedCountry {
       get { return _selectedCountry; }
       set 
       { 
           this.RaiseAndSetIfChanged(ref _selectedCountry, value) }
       }
  }

    public async Task RegistrationPage(RegisterViewModel registration) => await NavigationService.NavigateToAsync(registration, null, NavigationType.Modal);

    public ICommand RegistrationPageCommand => new Command<RegisterViewModel>(async (registration) =>
    {
        await RegistrationPage(registration);
    }, (x) => false);
}

将转换器命名空间和资源添加到页面并实现多重触发器和绑定：

<?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:converters="clr-namespace:Osma.Mobile.App.Converters"
             x:Class="Osma.Mobile.App.Views.ProviderPage"
             NavigationPage.HasNavigationBar="False"
             BackgroundColor="#004B86">

   <ContentPage.Resources>
    <ResourceDictionary>
        <converters:StringNullOrEmptyValueBoolConverter x:Key="stringNullOrEmptyValueBoolConverter" />
    </ResourceDictionary>
   </ContentPage.Resources>


    <ContentPage.Content>
        <StackLayout
            Spacing="30">

            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="70" />
                    <RowDefinition Height="70" />
                </Grid.RowDefinitions>
                
                <Grid Grid.Row="0">
                    <Picker 
                        x:Name="picker_country" 
                        Title="Select a Country" 
                        TextColor="Silver" 
                        TitleColor="Silver" 
                        HorizontalOptions="Start" 
                        WidthRequest = "200" 
                        VerticalOptions="Start" 
                        Margin="130,0,0,0"
                        SelectedItem="{Binding SelectedCountry}">
                        <Picker.ItemsSource>
                            <x:Array Type="{x:Type x:String}">
                                <x:String>Sweden</x:String>
                            </x:Array>
                        </Picker.ItemsSource>
                    </Picker>
                </Grid>

                <Grid Grid.Row="1">
                    <Picker 
                        x:Name="picker_provider" 
                        Title="Select a Provider" 
                        TextColor="Silver" 
                        TitleColor="Silver" 
                        HorizontalOptions="Start" 
                        WidthRequest = "200" 
                        VerticalOptions="Start" 
                        Margin="130,0,0,0"
                        SelectedItem="{Binding SelectedProvider}">
                        <Picker.ItemsSource>
                            <x:Array Type="{x:Type x:String}">
                                <x:String>Provider 1</x:String>
                            </x:Array>
                        </Picker.ItemsSource>
                    </Picker>
                </Grid>
            </Grid>

        <Button 
            BackgroundColor="#2194EF"
            TextColor="White" 
            Text="Next" 
            HeightRequest="60"
            MinimumHeightRequest="60"
            IsEnabled="False"
            Command="{Binding RegistrationPageCommand}">
            <Button.Triggers>
                <MultiTrigger TargetType="Button">
                    <MultiTrigger.Conditions>
                        <BindingCondition Binding="{Binding SelectedCountry,
                                                       Converter={StaticResource stringNullOrEmptyValueBoolConverter }}" Value="false" />
                        <BindingCondition Binding="{Binding SelectedProvider,
                                                       Converter={StaticResource stringNullOrEmptyValueBoolConverter }}" Value="false" />
                    </MultiTrigger.Conditions>
        
                    <Setter Property="IsEnabled" Value="True" />
                </MultiTrigger>
            </Button.Triggers>
        </Button>

        </StackLayout>
    </ContentPage.Content>
</ContentPage>

正如您在代码中看到的，多重触发条件是两个属性都不为 null 或为空，在这种情况下，按钮将被启用。