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Xamarin.Forms Multi-Bindings

  • Artikkeli

Multi-bindings provide the ability to attach a collection of Binding objects to a single binding target property. They are created with the MultiBinding class, which evaluates all of its Binding objects, and returns a single value through a IMultiValueConverter instance provided by your application. In addition, MultiBinding reevaluates all of its Binding objects when any of the bound data changes.

The MultiBinding class defines the following properties:

  • Bindings, of type IList<BindingBase>, which represents the collection of Binding objects within the MultiBinding instance.
  • Converter, of type IMultiValueConverter, which represents the converter to use to convert the source values to or from the target value.
  • ConverterParameter, of type object, which represents an optional parameter to pass to the Converter.

The Bindings property is the content property of the MultiBinding class, and therefore does not need to be explicitly set from XAML.

In addition, the MultiBinding class inherits the following properties from the BindingBase class:

  • FallbackValue, of type object, which represents the value to use when the multi-binding is unable to return a value.
  • Mode, of type BindingMode, which indicates the direction of the data flow of the multi-binding.
  • StringFormat, of type string, which specifies how to format the multi-binding result if it's displayed as a string.
  • TargetNullValue, of type object, which represents the value that is used in the target wen the value of the source is null.

A MultiBinding must use a IMultiValueConverter to produce a value for the binding target, based on the value of the bindings in the Bindings collection. For example, a Color might be computed from red, blue, and green values, which can be values from the same or different binding source objects. When a value moves from the target to the sources, the target property value is translated to a set of values that are fed back into the bindings.

Important

Individual bindings in the Bindings collection can have their own value converters.

The value of the Mode property determines the functionality of the MultiBinding, and is used as the binding mode for all the bindings in the collection unless an individual binding overrides the property. For example, if the Mode property on a MultiBinding object is set to TwoWay, then all the bindings in the collection are considered TwoWay unless you explicitly set a different Mode value on one of the bindings.

Define a IMultiValueConverter

The IMultiValueConverter interface enables custom logic to be applied to a MultiBinding. To associate a converter with a MultiBinding, create a class that implements the IMultiValueConverter interface, and then implement the Convert and ConvertBack methods:

public class AllTrueMultiConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values == null || !targetType.IsAssignableFrom(typeof(bool)))
        {
            return false;
            // Alternatively, return BindableProperty.UnsetValue to use the binding FallbackValue
        }

        foreach (var value in values)
        {
            if (!(value is bool b))
            {
                return false;
                // Alternatively, return BindableProperty.UnsetValue to use the binding FallbackValue
            }
            else if (!b)
            {
                return false;
            }
        }
        return true;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        if (!(value is bool b) || targetTypes.Any(t => !t.IsAssignableFrom(typeof(bool))))
        {
            // Return null to indicate conversion back is not possible
            return null;
        }

        if (b)
        {
            return targetTypes.Select(t => (object)true).ToArray();
        }
        else
        {
            // Can't convert back from false because of ambiguity
            return null;
        }
    }
}

The Convert method converts source values to a value for the binding target. Xamarin.Forms calls this method when it propagates values from source bindings to the binding target. This method accepts four arguments:

  • values, of type object[], is an array of values that the source bindings in the MultiBinding produces.
  • targetType, of type Type, is the type of the binding target property.
  • parameter, of type object, is the converter parameter to use.
  • culture, of type CultureInfo, is the culture to use in the converter.

The Convert method returns an object that represents a converted value. This method should return:

  • BindableProperty.UnsetValue to indicate that the converter did not produce a value, and that the binding will use the FallbackValue.
  • Binding.DoNothing to instruct Xamarin.Forms not to perform any action. For example, to instruct Xamarin.Forms not to transfer a value to the binding target, or not to use the FallbackValue.
  • null to indicate that the converter cannot perform the conversion, and that the binding will use the TargetNullValue.

Important

A MultiBinding that receives BindableProperty.UnsetValue from a Convert method must define its FallbackValue property. Similarly, a MultiBinding that receives null from a Convert method must define its TargetNullValue propety.

The ConvertBack method converts a binding target to the source binding values. This method accepts four arguments:

  • value, of type object, is the value that the binding target produces.
  • targetTypes, of type Type[], is the array of types to convert to. The array length indicates the number and types of values that are suggested for the method to return.
  • parameter, of type object, is the converter parameter to use.
  • culture, of type CultureInfo, is the culture to use in the converter.

The ConvertBack method returns an array of values, of type object[], that have been converted from the target value back to the source values. This method should return:

  • BindableProperty.UnsetValue at position i to indicate that the converter is unable to provide a value for the source binding at index i, and that no value is to be set on it.
  • Binding.DoNothing at position i to indicate that no value is to be set on the source binding at index i.
  • null to indicate that the converter cannot perform the conversion or that it does not support conversion in this direction.

Consume a IMultiValueConverter

A IMultiValueConverter is consumed by instantiating it in a resource dictionary, and then referencing it using the StaticResource markup extension to set the MultiBinding.Converter property:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DataBindingDemos"
             x:Class="DataBindingDemos.MultiBindingConverterPage"
             Title="MultiBinding Converter demo">

    <ContentPage.Resources>
        <local:AllTrueMultiConverter x:Key="AllTrueConverter" />
        <local:InverterConverter x:Key="InverterConverter" />
    </ContentPage.Resources>

    <CheckBox>
        <CheckBox.IsChecked>
            <MultiBinding Converter="{StaticResource AllTrueConverter}">
                <Binding Path="Employee.IsOver16" />
                <Binding Path="Employee.HasPassedTest" />
                <Binding Path="Employee.IsSuspended"
                         Converter="{StaticResource InverterConverter}" />
            </MultiBinding>
        </CheckBox.IsChecked>
    </CheckBox>
</ContentPage>

In this example, the MultiBinding object uses the AllTrueMultiConverter instance to set the CheckBox.IsChecked property to true, provided that the three Binding objects evaluate to true. Otherwise, the CheckBox.IsChecked property is set to false.

By default, the CheckBox.IsChecked property uses a TwoWay binding. Therefore, the ConvertBack method of the AllTrueMultiConverter instance is executed when the CheckBox is unchecked by the user, which sets the source binding values to the value of the CheckBox.IsChecked property.

The equivalent C# code is shown below:

public class MultiBindingConverterCodePage : ContentPage
{
    public MultiBindingConverterCodePage()
    {
        BindingContext = new GroupViewModel();

        CheckBox checkBox = new CheckBox();
        checkBox.SetBinding(CheckBox.IsCheckedProperty, new MultiBinding
        {
            Bindings = new Collection<BindingBase>
            {
                new Binding("Employee1.IsOver16"),
                new Binding("Employee1.HasPassedTest"),
                new Binding("Employee1.IsSuspended", converter: new InverterConverter())
            },
            Converter = new AllTrueMultiConverter()
        });

        Title = "MultiBinding converter demo";
        Content = checkBox;
    }
}

Format strings

A MultiBinding can format any multi-binding result that's displayed as a string, with the StringFormat property. This property can be set to a standard .NET formatting string, with placeholders, that specifies how to format the multi-binding result:

<Label>
    <Label.Text>
        <MultiBinding StringFormat="{}{0} {1} {2}">
            <Binding Path="Employee1.Forename" />
            <Binding Path="Employee1.MiddleName" />
            <Binding Path="Employee1.Surname" />
        </MultiBinding>
    </Label.Text>
</Label>

In this example, the StringFormat property combines the three bound values into a single string that's displayed by the Label.

The equivalent C# code is shown below:

Label label = new Label();
label.SetBinding(Label.TextProperty, new MultiBinding
{
    Bindings = new Collection<BindingBase>
    {
        new Binding("Employee1.Forename"),
        new Binding("Employee1.MiddleName"),
        new Binding("Employee1.Surname")
    },
    StringFormat = "{0} {1} {2}"
});

Important

The number of parameters in a composite string format can't exceed the number of child Binding objects in the MultiBinding.

When setting the Converter and StringFormat properties, the converter is applied to the data value first, and then the StringFormat is applied.

For more information about string formatting in Xamarin.Forms, see Xamarin.Forms String Formatting.

Provide fallback values

Data bindings can be made more robust by defining fallback values to use if the binding process fails. This can be accomplished by optionally defining the FallbackValue and TargetNullValue properties on a MultiBinding object.

A MultiBinding will use its FallbackValue when the Convert method of an IMultiValueConverter instance returns BindableProperty.UnsetValue, which indicates that the converter did not produce a value. A MultiBinding will use its TargetNullValue when the Convert method of an IMultiValueConverter instance returns null, which indicates that the converter cannot perform the conversion.

For more information about binding fallbacks, see Xamarin.Forms Binding Fallbacks.

Nest MultiBinding objects

MultiBinding objects can be nested so that multiple MultiBinding objects are evaluated to return a value through an IMultiValueConverter instance:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DataBindingDemos"
             x:Class="DataBindingDemos.NestedMultiBindingPage"
             Title="Nested MultiBinding demo">

    <ContentPage.Resources>
        <local:AllTrueMultiConverter x:Key="AllTrueConverter" />
        <local:AnyTrueMultiConverter x:Key="AnyTrueConverter" />
        <local:InverterConverter x:Key="InverterConverter" />
    </ContentPage.Resources>

    <CheckBox>
        <CheckBox.IsChecked>
            <MultiBinding Converter="{StaticResource AnyTrueConverter}">
                <MultiBinding Converter="{StaticResource AllTrueConverter}">
                    <Binding Path="Employee.IsOver16" />
                    <Binding Path="Employee.HasPassedTest" />
                    <Binding Path="Employee.IsSuspended" Converter="{StaticResource InverterConverter}" />                        
                </MultiBinding>
                <Binding Path="Employee.IsMonarch" />
            </MultiBinding>
        </CheckBox.IsChecked>
    </CheckBox>
</ContentPage>

In this example, the MultiBinding object uses its AnyTrueMultiConverter instance to set the CheckBox.IsChecked property to true, provided that all of the Binding objects in the inner MultiBinding object evaluate to true, or provided that the Binding object in the outer MultiBinding object evaluates to true. Otherwise, the CheckBox.IsChecked property is set to false.

Use a RelativeSource binding in a MultiBinding

MultiBinding objects support relative bindings, which provide the ability to set the binding source relative to the position of the binding target:

<ContentPage ...
             xmlns:local="clr-namespace:DataBindingDemos"
             xmlns:xct="clr-namespace:Xamarin.CommunityToolkit.UI.Views;assembly=Xamarin.CommunityToolkit">
    <ContentPage.Resources>
        <local:AllTrueMultiConverter x:Key="AllTrueConverter" />

        <ControlTemplate x:Key="CardViewExpanderControlTemplate">
            <xct:Expander BindingContext="{Binding Source={RelativeSource TemplatedParent}}"
                          IsExpanded="{Binding IsExpanded, Source={RelativeSource TemplatedParent}}"
                          BackgroundColor="{Binding CardColor}">
                <xct:Expander.IsVisible>
                    <MultiBinding Converter="{StaticResource AllTrueConverter}">
                        <Binding Path="IsExpanded" />
                        <Binding Path="IsEnabled" />
                    </MultiBinding>
                </xct:Expander.IsVisible>
                <xct:Expander.Header>
                    <Grid>
                        <!-- XAML that defines Expander header goes here -->
                    </Grid>
                </xct:Expander.Header>
                <Grid>
                    <!-- XAML that defines Expander content goes here -->
                </Grid>
            </xct:Expander>
        </ControlTemplate>
    </ContentPage.Resources>

    <StackLayout>
        <controls:CardViewExpander BorderColor="DarkGray"
                                   CardTitle="John Doe"
                                   CardDescription="Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla elit dolor, convallis non interdum."
                                   IconBackgroundColor="SlateGray"
                                   IconImageSource="user.png"
                                   ControlTemplate="{StaticResource CardViewExpanderControlTemplate}"
                                   IsEnabled="True"
                                   IsExpanded="True" />
    </StackLayout>
</ContentPage>

Note

The Expander control is now part of the Xamarin Community Toolkit.

In this example, the TemplatedParent relative binding mode is used to bind from within a control template to the runtime object instance to which the template is applied. The Expander, which is the root element of the ControlTemplate, has its BindingContext set to the runtime object instance to which the template is applied. Therefore, the Expander and its children resolve their binding expressions, and Binding objects, against the properties of the CardViewExpander object. The MultiBinding uses the AllTrueMultiConverter instance to set the Expander.IsVisible property to true provided that the two Binding objects evaluate to true. Otherwise, the Expander.IsVisible property is set to false.

For more information about relative bindings, see Xamarin.Forms Relative Bindings. For more information about control templates, see Xamarin.Forms Control Templates.