XAML Custom Namespace Schemas in Xamarin.Forms

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Types in a library can be referenced in XAML by declaring a XAML namespace for the library, with the namespace declaration specifying the Common Language Runtime (CLR) namespace name and an assembly name:

<ContentPage ...
             xmlns:controls="clr-namespace:MyCompany.Controls;assembly="MyCompany.Controls">
    ...
</ContentPage>

However, specifying a CLR namespace and assembly name in a xmlns definition can be awkward and error prone. In addition, multiple XAML namespace declarations may be required if the library contains types in multiple namespaces.

An alternative approach is to define a custom namespace schema, such as http://mycompany.com/schemas/controls, that maps to one or more CLR namespaces. This enables a single XAML namespace declaration to reference all the types in an assembly, even if they are in different namespaces. It also enables a single XAML namespace declaration to reference types in multiple assemblies.

For more information about XAML namespaces, see XAML Namespaces in Xamarin.Forms.

Defining a custom namespace schema

The sample application contains a library that exposes some simple controls, such as CircleButton:

using Xamarin.Forms;

namespace MyCompany.Controls
{
    public class CircleButton : Button
    {
        ...
    }
}

All the controls in the library reside in the MyCompany.Controls namespace. These controls can be exposed to a calling assembly through a custom namespace schema.

A custom namespace schema is defined with the XmlnsDefinitionAttribute class, which specifies the mapping between a XAML namespace and one or more CLR namespaces. The XmlnsDefinitionAttribute takes two arguments: the XAML namespace name, and the CLR namespace name. The XAML namespace name is stored in the XmlnsDefinitionAttribute.XmlNamespace property, and the CLR namespace name is stored in the XmlnsDefinitionAttribute.ClrNamespace property.

Note

The XmlnsDefinitionAttribute class also has a property named AssemblyName, which can be optionally set to the name of the assembly. This is only required when a CLR namespace referenced from a XmlnsDefinitionAttribute is in a external assembly.

The XmlnsDefinitionAttribute should be defined at the assembly level in the project that contains the CLR namespaces that will be mapped in the custom namespace schema. The following example shows the AssemblyInfo.cs file from the sample application:

using Xamarin.Forms;
using MyCompany.Controls;

[assembly: Preserve]
[assembly: XmlnsDefinition("http://mycompany.com/schemas/controls", "MyCompany.Controls")]

This code creates a custom namespace schema that maps the http://mycompany.com/schemas/controls URL to the MyCompany.Controls CLR namespace. In addition, the Preserve attribute is specified on the assembly, to ensure that the linker preserves all the types in the assembly.

Important

The Preserve attribute should be applied to classes in the assembly that are mapped through the custom namespace schema, or applied to the entire assembly.

The custom namespace schema can then be used for type resolution in XAML files.

Consuming a custom namespace schema

To consume types from the custom namespace schema, the XAML compiler requires that there's a code reference from the assembly that consumes the types, to the assembly that defines the types. This can be accomplished by adding a class containing an Init method to the assembly that defines the types that will be consumed through XAML:

namespace MyCompany.Controls
{
    public static class Controls
    {
        public static void Init()
        {
        }
    }
}

The Init method can then be called from the assembly that consumes types from the custom namespace schema:

using Xamarin.Forms;
using MyCompany.Controls;

namespace CustomNamespaceSchemaDemo
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            Controls.Init();
            InitializeComponent();
        }
    }
}

Warning

Failure to include such a code reference will result in the XAML compiler being unable to locate the assembly containing the custom namespace schema types.

To consume the CircleButton control, a XAML namespace is declared, with the namespace declaration specifying the custom namespace schema URL:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="http://mycompany.com/schemas/controls"
             x:Class="CustomNamespaceSchemaDemo.MainPage">
    <StackLayout Margin="20,35,20,20">
        ...
        <controls:CircleButton Text="+"
                               BackgroundColor="Fuchsia"
                               BorderColor="Black"
                               CircleDiameter="100" />
        <controls:CircleButton Text="-"
                               BackgroundColor="Teal"
                               BorderColor="Silver"
                               CircleDiameter="70" />
        ...
    </StackLayout>
</ContentPage>

CircleButton instances can then be added to the ContentPage by declaring them with the controls namespace prefix.

To find the custom namespace schema types, Xamarin.Forms will search referenced assemblies for XmlnsDefinitionAttribute instances. If the xmlns attribute for an element in a XAML file matches the XmlNamespace property value in a XmlnsDefinitionAttribute, Xamarin.Forms will attempt to use the XmlnsDefinitionAttribute.ClrNamespace property value for resolution of the type. If type resolution fails, Xamarin.Forms will continue to attempt type resolution based on any additional matching XmlnsDefinitionAttribute instances.

The result is that two CircleButton instances are displayed:

Circle buttons