feat: Add a property to launch the debugger for XAML generation

doc/articles/uno-development/troubleshooting-source-generation.md

@@ -1,5 +1,36 @@
 # Troubleshooting Source Generation
 
+Source Generation in Uno is done in one of two ways:
+- Using [C# 9.0 source generators](https://devblogs.microsoft.com/dotnet/introducing-c-source-generators/)
+- Using [Uno.SourceGeneration](https://github.com/unoplatform/uno.sourcegeneration) generators
+
+C# 9.0 generators are enabled automatically when using .NET 5 and up projects, otherwise Uno.SourceGeneration generators are used.
+
+It is possible to enable C# 9.0 generators for non .NET 5+ projects by adding this:
+```xml
+<PropertyGroup>
+  <LangVersion>9.0</LangVersion>
+  <UnoUIUseRoslynSourceGenerators>true</UnoUIUseRoslynSourceGenerators>
+</PropertyGroup>
+```
+Conversely, Uno.SourceGeneration generators can be used by setting this:
+```xml
+<PropertyGroup>
+  <UnoUIUseRoslynSourceGenerators>false</UnoUIUseRoslynSourceGenerators>
+</PropertyGroup>
+```
+
+## Debugging the XAML Source Generator
+It is possible to step into the XAML generator by adding the following property:
+```xml
+<PropertyGroup>
+  <UnoUISourceGeneratorDebuggerBreak>True</UnoUISourceGeneratorDebuggerBreak>
+</PropertyGroup>
+```
+Setting this property will popup a Debugger window allowing the selection of a visual studio instance, giving the ability to set breakpoints and trace the generator.
+
+## Troubleshooting Uno.SourceGeneration based generation
+
 When building, if you're having build error messages that looks like one of those:
 
 - `the targets [Microsoft.Build.Execution.TargetResult] failed to execute.`

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlCodeGeneration.cs

@@ -12,6 +12,7 @@
 using Uno.Logging;
 using Uno.UI.SourceGenerators.Telemetry;
 using Uno.UI.Xaml;
+using System.Drawing;
 
 #if NETFRAMEWORK
 using Microsoft.Build.Execution;
@@ -87,13 +88,14 @@ internal partial class XamlCodeGeneration
 		public XamlCodeGeneration(GeneratorExecutionContext context)
 		{
 			// To easily debug XAML code generation:
-			// 1. Uncomment the line below
-			// 2. Build Uno.UI.SourceGenerators and override local files (following instructions here: doc/articles/uno-development/debugging-uno-ui.md#debugging-unoui)
-			// 3. Build project containing your XAML. When prompted to attach a Visual Studio instance:
-			//		- if it's in an external solution, attach the VS instance running Uno.UI
-			//		- if you're debugging XAML generation inside the Uno solution, opt to create a new VS instance
-			//
-			// Debugger.Launch();
+			// Add <UnoUISourceGeneratorDebuggerBreak>True</UnoUISourceGeneratorDebuggerBreak> to your project
+
+			if (!Helpers.DesignTimeHelper.IsDesignTime(context)
+				&& (context.GetMSBuildPropertyValue("UnoUISourceGeneratorDebuggerBreak")?.Equals("True", StringComparison.OrdinalIgnoreCase) ?? false))
+			{
+				Debugger.Launch();
+			}
+
 			_generatorContext = context;
 			InitTelemetry(context);