Platform-specific XAML markup in Uno

Uno allows you to reuse views and business logic across platforms. Sometimes though you may want to write different code per platform, either because you need to access platform-specific native APIs and 3rd-party libraries, or because you want your app to look and behave differently depending on the platform.

This guide covers multiple approaches to managing per-platform markup in XAML. See this guide for managing per-platform C#.

Project structure

There are two ways to restrict code or XAML markup to be used only on a specific platform:

  • Use conditionals within a shared file
  • Place the code in a file which is only included in the desired platform head.

The structure of an Uno app created with the default Visual Studio template is explained in more detail here. The key point to understand is that files in a shared project referenced from a platform head are treated in exactly the same way as files included directly under the head, and are compiled together into a single assembly.

XAML conditional prefixes

The Uno platform uses pre-defined prefixes to include or exclude parts of XAML markup depending on the platform. These prefixes can be applied to XAML objects or to individual object properties.

Conditional prefixes you wish to use in XAML file must be defined at the top of the file, like other XAML prefixes. They can be then applied to any object or property within the body of the file.

For prefixes which will be excluded on Windows (e.g. android, ios), the actual namespace is arbitrary, since the Uno parser ignores it. The prefix should be put in the mc:Ignorable list. For prefixes which will be included on Windows (e.g. win, not_android) the namespace should be http://schemas.microsoft.com/winfx/2006/xaml/presentation and the prefix should not be put in the mc:Ignorable list.

Examples

Example 1

Using the following XAML:

<Page x:Class="HelloWorld.MainPage"
	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	  xmlns:android="http://uno.ui/android"
	  xmlns:ios="http://uno.ui/ios"
	  xmlns:wasm="http://uno.ui/wasm"
	  xmlns:win="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:not_android="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	  mc:Ignorable="d android ios wasm">

	<StackPanel Margin="20,70,0,0">
		<TextBlock Text="This text will be large on Windows, and pink on WASM"
				   win:FontSize="24"
				   wasm:Foreground="DeepPink"
				   TextWrapping="Wrap"/>
		<TextBlock android:Text="This version will be used on Android"
				   not_android:Text="This version will be used on every other platform" />
		<ios:TextBlock Text="This TextBlock will only be created on iOS" />
	</StackPanel>
</Page>

Results in:

Visual output

In this example note how the properties FontSize and Foreground are selectively used based on platform. The TextBlock property Text also has two different values based on whether or not the app is running in Android. Finally, an entire TextBlock is added if the app is running in iOS. This shows:

  1. How certain properties can be used based on the platform
  2. How the values of certain properties can be changed based on the platform
  3. How entire controls can be added or removed for certain platforms

Example 2

Platform-specific XAML also allows you to exclude a parent element and all its children. This is especially useful for cases where children are already part of a namespace but need to be excluded on certain platforms.

Consider the following XAML which is using the Windows Community Toolkit's Blur animation. While this runs for UWP, it is not currently supported in the Uno Platform and needs to be conditionally disabled. It isn't possible to add something like <win:interactivity:Interaction.Behaviors> to disable the behavior itself. Instead, the entire Grid is disabled on any platforms except Windows and child elements will be disabled along with it.

<Page x:Class="HelloWorld.MainPage"
	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	  xmlns:behaviors="using:Microsoft.Toolkit.Uwp.UI.Animations.Behaviors"
	  xmlns:toolkit="using:Microsoft.Toolkit.Uwp.UI.Controls"
	  xmlns:interactivity="using:Microsoft.Xaml.Interactivity"
	  xmlns:win="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	  mc:Ignorable="d android ios wasm">

	<Grid>
		<win:Grid Background="Gray">
			<interactivity:Interaction.Behaviors>
				<behaviors:Blur Value="7" Duration="0" Delay="0" AutomaticallyStart="True" />
			</interactivity:Interaction.Behaviors>
		</win:Grid>
		<Grid>
			<!-- Other page content -->
		</Grid>
	</Grid>
</Page>

Available prefixes

The pre-defined prefixes are listed below:

Prefix Included platforms Excluded platforms Namespace Put in mc:Ignorable?
win Windows Android, iOS, web, macOS, Skia http://schemas.microsoft.com/winfx/2006/xaml/presentation no
xamarin Android, iOS, web, macOS, Skia Windows http://uno.ui/xamarin yes
not_win Android, iOS, web, macOS, Skia Windows http://uno.ui/not_win yes
android Android Windows, iOS, web, macOS, Skia http://uno.ui/android yes
ios iOS Windows, Android, web, macOS, Skia http://uno.ui/ios yes
wasm web Windows, Android, iOS, macOS, Skia http://uno.ui/wasm yes
macos macOS Windows, Android, iOS, web, Skia http://uno.ui/macos yes
skia Skia Windows, Android, iOS, web, macOs http://schemas.microsoft.com/winfx/2006/xaml/presentation no
not_android Windows, iOS, web, macOS, Skia Android http://schemas.microsoft.com/winfx/2006/xaml/presentation no
not_ios Windows, Android, web, macOS, Skia iOS http://schemas.microsoft.com/winfx/2006/xaml/presentation no
not_wasm Windows, Android, iOS, macOS, Skia web http://schemas.microsoft.com/winfx/2006/xaml/presentation no
not_macos Windows, Android, iOS, web, Skia macOS http://schemas.microsoft.com/winfx/2006/xaml/presentation no
not_skia Windows, Android, iOS, web, macOS Skia http://schemas.microsoft.com/winfx/2006/xaml/presentation no

More visually, platform support for the pre-defined prefixes is shown in the below table:

Prefix Win Droid iOS Web macOS Skia
win
android
ios
wasm
macos
skia
xamarin
not_win
not_android
not_ios
not_wasm
not_macos
not_skia

Where:

  • 'Win' represents Windows, and
  • 'Droid' represents Android

XAML prefixes in cross-targeted libraries

For Uno 3.0 and above, XAML prefixes behave differently in class libraries than when used directly in application code. Specifically, it isn't possible to distinguish Skia and Wasm in a library, since both platforms use the .NET Standard 2.0 target. The wasm and skia prefixes will always evaluate to false inside of a library.

The prefix netstdref is available and will include the objects or properties in both Skia and Wasm build. A prefix not_nestdref can also be used to exclude them. Since Skia and Wasm are similar, it is often not necessary to make the distinction.

In cases where it is needed (fonts are one example) then the XAML files must be placed directly in the platform specific project or a shared project.

Prefix Namespace Put in mc:Ignorable?
netstdref http://uno.ui/netstdref yes
not_netstdref http://uno.ui/not_netstdref yes

XAML Conditional Methods

You can use standard WinUI conditional XAML prefixes with Uno Platform, as documented here.

Currently the following conditional methods are supported:

  • IsApiContractPresent(ContractName, VersionNumber)
  • IsApiContractNotPresent(ContractName, VersionNumber)
  • IsTypePresent(ControlType)
  • IsTypeNotPresent(ControlType)

IsApiContractPresent

The following ContractName values are currently supported:

  • "Windows.Foundation.UniversalApiContract": resolves to true if VersionNumber is 10 or below, and false otherwise.
  • "Uno.WinUI": resolves to true if the Uno.WinUI NuGet package (ie the WinUI 3 API mapping) is in use, false if the Uno.UI NuGet package is in use.

All other contract names will resolve to false.

IsTypePresent

IsTypePresent() will resolve to true if the type is found and it isn't marked with the [NotImplemented] attribute. This is useful for conditionally enabling XAML in a more declarative way, instead of referring to specific platforms.

Limitations in cross-targeted libraries

IsTypePresent() is resolved at compile time. In a cross-targeted library, this means that for WebAssembly and Skia, it doesn't use the correct implementation status for those platforms, but instead uses the implementation status reported by the 'reference' Uno.UI .NET Standard 2.0 assembly. Therefore it may report an incorrect status for WebAssembly and/or Skia inside of a cross-targeted library.

Example

The following example uses IsTypePresent to use WebView to display content on platforms where it's available, and a custom control to display the content on platforms where WebView isn't supported:

<Page x:Class="HelloWorld.MainPage"
	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	  xmlns:local="using:HelloWorld"
	  xmlns:webviewpresent="http://schemas.microsoft.com/winfx/2006/xaml/presentation?IsTypePresent(Windows.UI.Xaml.Controls.WebView)"
	  xmlns:webviewnotpresent="http://schemas.microsoft.com/winfx/2006/xaml/presentation?IsTypeNotPresent(Windows.UI.Xaml.Controls.WebView)"
	  xmlns:local_webviewnotpresent="using:HelloWorld?IsTypeNotPresent(Windows.UI.Xaml.Controls.WebView)"
	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	  mc:Ignorable="d">

	<Grid>
		<webviewpresent:WebView Source="{Binding DisplayContent}" />
		<local_webviewnotpresent:WebViewSubstitute ContentSource="{Binding DisplayContent}" />
		<webviewnotpresent:TextBlock VerticalAlignment="Bottom" Text="Showing substitute for WebView">
	</Grid>
</Page>