Uno.XamlMerge.Task

An MSBuild task which enables the merging of WinUI XAML resource dictionaries into a single flat ResourceDictionary.

This code is derived from WinUI's BatchMergeXaml class, to be reused in any WinUI and Uno Platform Controls library.

Why merge ResourceDictionary instances?

This task is generally used to allow for separate resource dictionary authoring (which makes them easier to load and read) to avoid impacting resources lookup runtime performance. In WinUI, ResourceDictionary resolution is performed through a graph traversal of MergedDictionaries, which generally implies that a worse case resolution can require as many lookups as there are dictionaries.

To limit the impact of the traversal, this task takes all resource dictionaries found in a specific MSBuild item group, and merges them into a single file. This file is then generally either named Themes\Generic.xaml or referenced as a merged dictionary from another Themes\Generic.xaml file.

Using the task

  1. In your csproj file, include the following block:

    <PropertyGroup>
  <_Uno_XamlMerge_Task_Version>1.0.0</_Uno_XamlMerge_Task_Version>
</PropertyGroup>
<ItemGroup>
  <PackageReference Include="Uno.XamlMerge.Task" Version="$(_Uno_XamlMerge_Task_Version)" />
</ItemGroup>

  2. Specify the resource dictionaries to be merged:

    <ItemGroup>
  <XamlMergeInput Include="Styles\**\*.xaml" Exclude="Styles\Generic.xaml" />
</ItemGroup>
    Note

    The Exclude portion is only needed if some of your dictionaries still need to be referenced explicitly as some libraries require valid full path, such as Uno.Themes.

  3. Add the following block at the end your project library or app:

    <Import Project="$(NuGetPackageRoot)uno.xamlmerge.task\$(_Uno_XamlMerge_Task_Version)\build\Uno.XamlMerge.Task.targets"
        Condition="'$(TargetFramework)' == '' and '$(TargetFrameworks)'!='' and exists('$(NuGetPackageRoot)\uno.xamlmerge.task\$(_Uno_XamlMerge_Task_Version)')" />

  4. By default, the generated file is called Generated\mergedpages.xaml, but can be overridden as follows:

    <PropertyGroup>
  <XamlMergeOutputFile>Themes\Generic.xaml</XamlMergeOutputFile>
</PropertyGroup>

  5. The generated file can be referenced as follows:

    <ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TestLibrary">

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="ms-appx:///REPLACE_ME/Generated/mergedpages.xaml" />
    </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

Multiple generated files

There are scenarios where placing groups of XAML files in separate merged files are useful, particularly, when IL Linking must be used.

In this case, the project file must be defined as follows:

<ItemGroup>
  <!-- Output files -->
  <XamlMergeOutputFiles Include="Generated\MergedPart1.xaml"  />
  <XamlMergeOutputFiles Include="Generated\MergedPart2.xaml"  />

  <!-- Input files, with the `MergeFile` metadata -->
  <XamlMergeInput Include="Styles\Part1\**\*.xaml" Exclude="Styles\Generic.xaml" MergeFile="MergedPart1.xaml" />
  <XamlMergeInput Include="Styles\Part2\**\*.xaml" Exclude="Styles\Generic.xaml" MergeFile="MergedPart2.xaml" />
</ItemGroup>

In this case, the XamlMergeOutputFile cannot be used, as the XamlMergeOutputFiles is defined.

Further reading