IValueConverter Interfaccia

Definizione

public interface class IValueConverter

/// [Windows.Foundation.Metadata.ContractVersion(Windows.Foundation.UniversalApiContract, 65536)]
/// [Windows.Foundation.Metadata.Guid(3874684656, 1810, 18559, 179, 19, 243, 0, 184, 215, 154, 161)]
struct IValueConverter

[Windows.Foundation.Metadata.ContractVersion(typeof(Windows.Foundation.UniversalApiContract), 65536)]
[Windows.Foundation.Metadata.Guid(3874684656, 1810, 18559, 179, 19, 243, 0, 184, 215, 154, 161)]
public interface IValueConverter

Public Interface IValueConverter

Derivato

Windows.UI.Xaml.Controls.Primitives.JumpListItemBackgroundConverter

Windows.UI.Xaml.Controls.Primitives.JumpListItemForegroundConverter

Attributi

ContractVersionAttribute GuidAttribute

Requisiti Windows

Esempio

Nell'esempio seguente viene illustrato come implementare l'interfaccia IValueConverter e usare il convertitore quando si esegue il data binding a una raccolta di oggetti.

<UserControl x:Class="ConverterParameterEx.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:local="using:ConverterParameterEx" 
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" >
        <Grid.Resources>
           <local:DateFormatter x:Key="FormatConverter" />
        </Grid.Resources>
        
        <ComboBox Height="60" Width="250" x:Name="MusicCombo" 
            ItemsSource="{Binding}">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel>
                        <TextBlock FontWeight="Bold" Text="{Binding Path=Name, Mode=OneWay}" />
                        <TextBlock Text="{Binding Path=Artist, Mode=OneWay}" />
                        <TextBlock Text="{Binding Path=ReleaseDate, Mode=OneWay,
                            Converter={StaticResource FormatConverter}, 
                            ConverterParameter=\{0:d\}}" />
                   </StackPanel>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </Grid>
</UserControl>

using System;
using System.Collections.ObjectModel;
using System.Globalization;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;

namespace ConverterParameterEx
{
    public partial class Page : UserControl
    {

        public ObservableCollection<Recording> MyMusic =
            new ObservableCollection<Recording>();
        public Page()
        {
            InitializeComponent();

            // Add items to the collection.
            MyMusic.Add(new Recording("Chris Sells", "Chris Sells Live",
                new DateTime(2008, 2, 5)));
            MyMusic.Add(new Recording("Luka Abrus",
                "The Road to Redmond", new DateTime(2007, 4, 3)));
            MyMusic.Add(new Recording("Jim Hance",
                "The Best of Jim Hance", new DateTime(2007, 2, 6)));

            // Set the data context for the combo box.
            MusicCombo.DataContext = MyMusic;
        }
    }

    // Simple business object.
    public class Recording
    {
        public Recording() { }
        public Recording(string artistName, string cdName, DateTime release)
        {
            Artist = artistName;
            Name = cdName;
            ReleaseDate = release;
        }
        public string Artist { get; set; }
        public string Name { get; set; }
        public DateTime ReleaseDate { get; set; }
    }

    public class DateFormatter : IValueConverter
    {
        // This converts the DateTime object to the string to display.
        public object Convert(object value, Type targetType, 
            object parameter, string language)
        {
            // Retrieve the format string and use it to format the value.
            string formatString = parameter as string;
            if (!string.IsNullOrEmpty(formatString))
            {
                return string.Format(
                    new CultureInfo(language), formatString, value);
            }
            // If the format string is null or empty, simply call ToString()
            // on the value.
            return value.ToString();
        }

        // No need to implement converting back on a one-way binding 
        public object ConvertBack(object value, Type targetType, 
            object parameter, string language)
        {
            throw new NotImplementedException();
        }
    }
}

// pch.h
#include <winrt/Windows.Globalization.h>

// MainPage.idl
namespace ConverterParameterEx
{
    [default_interface]
    runtimeclass MainPage : Windows.UI.Xaml.Controls.Page
    {
        MainPage();
    }
}

<!-- MainPage.xaml -->
<Page
    x:Class="ConverterParameterEx.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ConverterParameterEx"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <local:Page/>
</Page>

// MainPage.h
#pragma once

#include "MainPage.g.h"

namespace winrt::ConverterParameterEx::implementation
{
    struct MainPage : MainPageT<MainPage>
    {
        MainPage();
    };
}

namespace winrt::ConverterParameterEx::factory_implementation
{
    struct MainPage : MainPageT<MainPage, implementation::MainPage>
    {
    };
}

// MainPage.cpp
#include "pch.h"
#include "MainPage.h"
#include "MainPage.g.cpp"

using namespace winrt;
using namespace Windows::UI::Xaml;

namespace winrt::ConverterParameterEx::implementation
{
    MainPage::MainPage()
    {
        InitializeComponent();
    }
}

// Page.idl
namespace ConverterParameterEx
{
    [default_interface]
    runtimeclass Page : Windows.UI.Xaml.Controls.UserControl
    {
        Page();
        Windows.Foundation.Collections.IVector<IInspectable> MyMusic{ get; };
    }
}

<!-- Page.xaml -->
<!-- see listing above -->

// Page.h
#pragma once

#include "winrt/Windows.UI.Xaml.h"
#include "winrt/Windows.UI.Xaml.Markup.h"
#include "winrt/Windows.UI.Xaml.Interop.h"
#include "winrt/Windows.UI.Xaml.Controls.Primitives.h"
#include "Page.g.h"
#include "DateFormatter.h"

namespace winrt::ConverterParameterEx::implementation
{
    struct Page : PageT<Page>
    {
        Page();
        Windows::Foundation::Collections::IVector<Windows::Foundation::IInspectable> MyMusic();

    private:
        Windows::Foundation::Collections::IVector<Windows::Foundation::IInspectable> m_myMusic;
    };
}

namespace winrt::ConverterParameterEx::factory_implementation
{
    struct Page : PageT<Page, implementation::Page>
    {
    };
}

// Page.cpp
#include "pch.h"
#include "Page.h"
#if __has_include("Page.g.cpp")
#include "Page.g.cpp"
#endif
#include "Recording.h"

using namespace winrt;
using namespace Windows::UI::Xaml;

namespace winrt::ConverterParameterEx::implementation
{
    Page::Page()
    {
        InitializeComponent();

        std::vector<Windows::Foundation::IInspectable> recordings;

        Windows::Globalization::Calendar releaseDateTime;
        releaseDateTime.Month(7); releaseDateTime.Day(8); releaseDateTime.Year(1748);
        recordings.push_back(winrt::make<Recording>(L"Johann Sebastian Bach", L"Mass in B minor", releaseDateTime));

        releaseDateTime = Windows::Globalization::Calendar{};
        releaseDateTime.Month(11); releaseDateTime.Day(2); releaseDateTime.Year(1805);
        recordings.push_back(winrt::make<Recording>(L"Ludwig van Beethoven", L"Third Symphony", releaseDateTime));

        releaseDateTime = Windows::Globalization::Calendar{};
        releaseDateTime.Month(3); releaseDateTime.Day(12); releaseDateTime.Year(1737);
        recordings.push_back(winrt::make<Recording>(L"George Frideric Handel", L"Serse", releaseDateTime));

        m_myMusic = winrt::single_threaded_observable_vector<Windows::Foundation::IInspectable>(std::move(recordings));

        MusicCombo().DataContext(m_myMusic);
    }

    Windows::Foundation::Collections::IVector<Windows::Foundation::IInspectable> Page::MyMusic() { return m_myMusic; }
}

// Recording.idl
namespace ConverterParameterEx
{
    [bindable]
    runtimeclass Recording
    {
        Recording(String artist, String name, Windows.Globalization.Calendar releaseDate);
        String Artist{ get; };
        String Name{ get; };
        Windows.Globalization.Calendar ReleaseDate{ get; };
    }
}

// Recording.h
#pragma once
#include "Recording.g.h"

namespace winrt::ConverterParameterEx::implementation
{
    struct Recording : RecordingT<Recording>
    {
        Recording() = default;

        Recording(hstring const& artist, hstring const& name, winrt::Windows::Globalization::Calendar const& releaseDate);
        hstring Artist();
        hstring Name();
        winrt::Windows::Globalization::Calendar ReleaseDate();

    private:
        std::wstring m_artist;
        std::wstring m_name;
        Windows::Globalization::Calendar m_releaseDate;
    };
}
namespace winrt::ConverterParameterEx::factory_implementation
{
    struct Recording : RecordingT<Recording, implementation::Recording>
    {
    };
}

// Recording.cpp
#include "pch.h"
#include "Recording.h"
#include "Recording.g.cpp"

namespace winrt::ConverterParameterEx::implementation
{
    Recording::Recording(hstring const& artist, hstring const& name, Windows::Globalization::Calendar const& releaseDate) :
        m_artist{ artist.c_str() },
        m_name{ name.c_str() },
        m_releaseDate{ releaseDate } {}

    hstring Recording::Artist() { return hstring{ m_artist }; }
    hstring Recording::Name() { return hstring{ m_name }; }
    Windows::Globalization::Calendar Recording::ReleaseDate() { return m_releaseDate; }
}

// DateFormatter.idl
namespace ConverterParameterEx
{
    runtimeclass DateFormatter : [default] Windows.UI.Xaml.Data.IValueConverter
    {
        DateFormatter();
    }
}

// DateFormatter.h
#pragma once
#include "DateFormatter.g.h"

namespace winrt::ConverterParameterEx::implementation
{
    struct DateFormatter : DateFormatterT<DateFormatter>
    {
        DateFormatter() = default;

        winrt::Windows::Foundation::IInspectable Convert(winrt::Windows::Foundation::IInspectable const& value, winrt::Windows::UI::Xaml::Interop::TypeName const& targetType, winrt::Windows::Foundation::IInspectable const& parameter, hstring const& language);
        winrt::Windows::Foundation::IInspectable ConvertBack(winrt::Windows::Foundation::IInspectable const& value, winrt::Windows::UI::Xaml::Interop::TypeName const& targetType, winrt::Windows::Foundation::IInspectable const& parameter, hstring const& language);
    };
}
namespace winrt::ConverterParameterEx::factory_implementation
{
    struct DateFormatter : DateFormatterT<DateFormatter, implementation::DateFormatter>
    {
    };
}

// DateFormatter.cpp
#include "pch.h"
#include "DateFormatter.h"
#include "DateFormatter.g.cpp"

namespace winrt::ConverterParameterEx::implementation
{
    winrt::Windows::Foundation::IInspectable DateFormatter::Convert(winrt::Windows::Foundation::IInspectable const& value, winrt::Windows::UI::Xaml::Interop::TypeName const& targetType, winrt::Windows::Foundation::IInspectable const& parameter, hstring const& language)
    {
        // Retrieve the value as a Calendar.
        Windows::Globalization::Calendar valueAsCalendar{ value.as<Windows::Globalization::Calendar>() };

        std::wstringstream wstringstream;
        wstringstream << valueAsCalendar.MonthAsNumericString().c_str();
        wstringstream << L"/" << valueAsCalendar.DayAsString().c_str();
        wstringstream << L"/" << valueAsCalendar.YearAsString().c_str();
        return winrt::box_value(hstring{ wstringstream.str().c_str() });
    }
    winrt::Windows::Foundation::IInspectable DateFormatter::ConvertBack(winrt::Windows::Foundation::IInspectable const& value, winrt::Windows::UI::Xaml::Interop::TypeName const& targetType, winrt::Windows::Foundation::IInspectable const& parameter, hstring const& language)
    {
        throw hresult_not_implemented();
    }
}

//
// MainPage.xaml.h
// Declaration of the MainPage class.
// 

#pragma once

#include "MainPage.g.h"

namespace IValueConverterExample
{

    // Simple business object.
    [Windows::UI::Xaml::Data::Bindable]
    public ref class Recording sealed 
    {
    public: 
        Recording (Platform::String^ artistName, Platform::String^ cdName, Windows::Foundation::DateTime release)
        {
            Artist = artistName;
            Name = cdName;
            ReleaseDate = release;
        }
        property Platform::String^ Artist;
        property Platform::String^ Name;
        property Windows::Foundation::DateTime ReleaseDate;
    };

    public ref class DateFormatter  sealed : Windows::UI::Xaml::Data::IValueConverter 
    {
        // This converts the DateTime object to the Platform::String^ to display.
    public:
        virtual Platform::Object^ Convert(Platform::Object^ value, Windows::UI::Xaml::Interop::TypeName targetType, 
            Platform::Object^ parameter, Platform::String^ language)
        {
            Windows::Foundation::DateTime dt = safe_cast<Windows::Foundation::DateTime>(value); 
            Windows::Globalization::DateTimeFormatting::DateTimeFormatter^ dtf =
                Windows::Globalization::DateTimeFormatting::DateTimeFormatter::ShortDate;
            return dtf->Format(dt); 
        }

        // No need to implement converting back on a one-way binding 
        virtual Platform::Object^ ConvertBack(Platform::Object^ value, Windows::UI::Xaml::Interop::TypeName targetType, 
            Platform::Object^ parameter, Platform::String^ language)
        {
            throw ref new Platform::NotImplementedException();
        }
    };

    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public ref class MainPage sealed
    {
    public:
        MainPage()
        {	
            m_myMusic = ref new Platform::Collections::Vector<Recording^>();

            // Add items to the collection.

            // You can use a Calendar object to create a Windows::Foundation::DateTime
            auto c = ref new Windows::Globalization::Calendar();
            c->Year = 2008;
            c->Month = 2;
            c->Day = 5;
            m_myMusic->Append(ref new Recording("Chris Sells", "Chris Sells Live",
                c->GetDateTime()));

            c->Year = 2007;
            c->Month = 4;
            c->Day = 3;
            m_myMusic->Append(ref new Recording("Luka Abrus",
                "The Road to Redmond", c->GetDateTime()));
            
            c->Year = 2007;
            c->Month = 2;
            c->Day = 3;
            m_myMusic->Append(ref new Recording("Jim Hance",
                "The Best of Jim Hance", dt));
            InitializeComponent();

            // Set the data context for the combo box.
            MusicCombo->DataContext = m_myMusic;	
        }


    protected:
        virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;

    private:
        Windows::Foundation::Collections::IVector<Recording^>^ m_myMusic;
    };
}

Imports System.Collections.ObjectModel
Imports System.Windows.Data
Imports System.Globalization

Partial Public Class Page
    Inherits UserControl

    Public MyMusic As New ObservableCollection(Of Recording)()
    Public Sub New()
        InitializeComponent()

        ' Add items to the collection.
        MyMusic.Add(New Recording("Sheryl Crow", "Detours", New DateTime(2008, 2, 5)))
        MyMusic.Add(New Recording("Brandi Carlisle", "The Story", New DateTime(2007, 4, 3)))
        MyMusic.Add(New Recording("Patty Griffin", "Children Running Through", New DateTime(2007, 2, 6)))

        ' Set the data context for the combo box.
        MusicCombo.DataContext = MyMusic
    End Sub
End Class

' Simple business object. 
Public Class Recording
    Public Sub New()
    End Sub
    Public Sub New(ByVal artistName As String, ByVal cdName As String, _
       ByVal release As DateTime)
        Artist = artistName
        Name = cdName
        ReleaseDate = release
    End Sub
    Private artistValue As String
    Private nameValue As String
    Private releaseDateValue As DateTime
    Public Property Artist() As String
        Get
            Return artistValue
        End Get
        Set(ByVal value As String)
            artistValue = value
        End Set
    End Property
    Public Property Name() As String
        Get
            Return nameValue
        End Get
        Set(ByVal value As String)
            nameValue = value
        End Set
    End Property
    Public Property ReleaseDate() As DateTime
        Get
            Return releaseDateValue
        End Get
        Set(ByVal value As DateTime)
            releaseDateValue = value
        End Set
    End Property
End Class

Public Class DateFormatter
    Implements IValueConverter

    ' This converts the DateTime object to the string to display. 
    Public Function Convert(ByVal value As Object, ByVal targetType As Type, _
        ByVal parameter As Object, ByVal language As System.String) As Object _
        Implements IValueConverter.Convert

        ' Retrieve the format string and use it to format the value. 
        Dim formatString As String = TryCast(parameter, String)
        If Not String.IsNullOrEmpty(formatString) Then

            Return String.Format(New CultureInfo(language), formatString, value)
        End If

        ' If the format string is null or empty, simply call ToString() 
        ' on the value. 
        Return value.ToString()
    End Function

    ' No need to implement converting back on a one-way binding.
    Public Function ConvertBack(ByVal value As Object, ByVal targetType As Type, _
        ByVal parameter As Object, _
        ByVal language As System.String) As Object _
        Implements IValueConverter.ConvertBack
        Throw New NotImplementedException()
    End Function
End Class

Commenti

È possibile creare una classe che consente di convertire il formato dei dati tra l'origine e la destinazione ereditando da IValueConverter. Ad esempio, potrebbe essere necessario avere un elenco di colori archiviati come valori RGBA , ma visualizzarli con nomi di colore nell'interfaccia utente. Implementando Convert e ConvertBack, è possibile modificare il formato dei valori dei dati man mano che vengono passati tra la destinazione e l'origine dal motore di associazione. È consigliabile implementare sempre Converti con un'implementazione funzionale, ma è piuttosto comune implementare ConvertBack in modo che restituisca un'eccezione non implementata. È necessario un metodo ConvertBack nel convertitore solo se si usa il convertitore per le associazioni bidirezionali o se si usa XAML per la serializzazione.

UnsetValue deve essere restituito da un'implementazione IValueConverter che fornisce la conversione in un data binding in una proprietà di dipendenza, in qualsiasi caso in cui il convertitore non è in grado di convertire un valore di origine. I convertitori non devono generare eccezioni per quel caso in Converti; queste verranno visualizzate come eccezioni in fase di esecuzione che è necessario aggiungere la gestione per in UnhandledException o peggio ancora appaiono agli utenti come eccezioni di runtime effettive. Le implementazioni del convertitore devono seguire il modello di associazione generale che qualsiasi associazione non riuscita non esegue alcuna operazione e non fornisce un valore e UnsetValue anziché Null è il valore sentinel per quel caso che il motore di associazione riconosce. Per altre info, vedi Informazioni approfondite sul data binding.

Note sulla migrazione

Nella Windows Runtime i parametri del linguaggio per i metodi IValueConverter usano stringhe, anziché usare oggetti CultureInfo come fanno nelle definizioni Windows Presentation Foundation (WPF) e Microsoft Silverlight dell'interfaccia.

Metodi

Si applica a

Vedi anche

• Binding
• Esempio di data binding XAML
• Informazioni approfondite sul data binding