Consider using a Feature Switch for CFNetworkHandler and NSUrlSessionHandler

[eerhardt]

In .NET 5 we introduced the concept of a "Feature Switch". This allows .NET apps to change behavior by setting switches at build/publish time. It also allows for the trimmer to remove unused code based on the switch setting.

You can see the current list of Feature Switches the .NET Runtime has for ideas on how these work.

Today, we have code that reads which HttpMessageHandler (CFNetworkHandler or NSUrlSessionHandler) to use at runtime based on a setting in a Resource NSDictionary:

xamarin-macios/src/ObjCRuntime/RuntimeOptions.cs

Lines 156 to 189 in 3cce274

	internal static RuntimeOptions Read ()
	{
	// for iOS NSBundle.ResourcePath returns the path to the root of the app bundle
	// for macOS apps NSBundle.ResourcePath returns foo.app/Contents/Resources
	// for macOS frameworks NSBundle.ResourcePath returns foo.app/Versions/Current/Resources
	Class bundle_finder = new Class (typeof (NSObject.NSObject_Disposer));
	var resource_dir = NSBundle.FromClass (bundle_finder).ResourcePath;
	var plist_path = GetFileName (resource_dir);
	if (!File.Exists (plist_path))
	return null;
	using (var plist = NSDictionary.FromFile (plist_path)) {
	var options = new RuntimeOptions ();
	options.http_message_handler = (NSString) plist ["HttpMessageHandler"];
	return options;
	}
	}
	internal static HttpMessageHandler GetHttpMessageHandler ()
	{
	var options = RuntimeOptions.Read ();
	// all types will be present as this is executed only when the linker is not enabled
	var handler_name = options?.http_message_handler;
	#if NET
	switch (handler_name) {
	case CFNetworkHandlerValue:
	return new CFNetworkHandler ();
	case SocketsHandlerValue:
	return new SocketsHttpHandler ();
	default:
	if (handler_name != null && handler_name != NSUrlSessionHandlerValue)
	Runtime.NSLog ($"{handler_name} is not a valid HttpMessageHandler, defaulting to System.Net.Http.NSUrlSessionHandlerValue");
	return new NSUrlSessionHandler ();

This has the following disadvantages:

1. No matter which one is picked, they are both always compiled into the application. Using a feature switch would allow us to trim the unused implementation.
2. It is inconsistent from a user's perspective on how to configure the HttpMessageHandler in their application. If they want to use the SocketsHttpHandler that is built into .NET, they need to set the UseNativeHttpHandler MSBuild property to false in their .csproj. But if they want to switch between CFNetworkHandler or NSUrlSessionHandler, they need to set this setting in the resource file. Using a feature switch here would make it a consistent experience with the built-in .NET SocketsHttpHandler.

cc @rolfbjarne

[Agnik7]

Hi, I would like to work on this issue. Requesting the maintainers to kindly assign the issue to me

[rolfbjarne]

@Agnik7 done!

Feel free to ask any questions, either here, or in our Discord channel (https://discord.com/channels/732297728826277939/732297808148824115)

[Agnik7]

Thanks a lot @rolfbjarne !! Sure, I'll ask whenever I need help

[Agnik7]

Hi @rolfbjarne , I am new to open-source and contributing. It will be really appreciated if you could kindly guide me as to which file(s) needs to be updated, and give a basic idea about how to update them. Eagerly waiting for your guidance at the earliest. Thank you!!!

[rolfbjarne]

The idea would be to:

1. Add two internal static bool properties in ObjCRuntime.Runtime, named "UseCFNetworkHandler" and "UseNSUrlSessionHandler" around here:

   xamarin-macios/src/ObjCRuntime/Runtime.cs

   Line 251 in 638e6f5

2. Compute the values for these two properties:

   <_IsCFNetworkHandlerFeature Condition="'$(UseNativeHttpHandler)' == 'true' And '$(MtouchHttpClientHandler)' == 'CFNetworkHandler'">true</_IsCFNetworkHandlerFeature>
   <_IsCFNetworkHandlerFeature Condition="'$(_IsCFNetworkHandlerFeature)' == ''">false</_IsCFNetworkHandlerFeature>
   <_IsNSUrlSessionHandlerFeature Condition="'$(UseNativeHttpHandler)' == 'true' And '$(MtouchHttpClientHandler)' == 'NSUrlSessionHandler'">true</_IsNSUrlSessionHandlerFeature>
   <_IsNSUrlSessionHandlerFeature Condition="'$(_IsNSUrlSessionHandlerFeature)' == ''">false</_IsNSUrlSessionHandlerFeature>

   at the end here (note this is from the net8.0 branch):

   xamarin-macios/dotnet/targets/Xamarin.Shared.Sdk.targets

   Lines 495 to 507 in 638e6f5

   	<PropertyGroup>
   	<!-- Set linker feature flags for device/simulator builds -->
   	<_IsSimulatorFeature Condition="('$(_PlatformName)' == 'iOS' Or '$(_PlatformName)' == 'tvOS') And '$(_SdkIsSimulator)' == 'true'">true</_IsSimulatorFeature>
   	<_IsSimulatorFeature Condition="'$(_IsSimulatorFeature)' == ''">false</_IsSimulatorFeature>
   	<!-- Set managed static registrar value -->
   	<_IsManagedStaticRegistrarFeature Condition="'$(Registrar)' == 'managed-static'">true</_IsManagedStaticRegistrarFeature>
   	<_IsManagedStaticRegistrarFeature Condition="'$(Registrar)' != 'managed-static'">false</_IsManagedStaticRegistrarFeature>
   	<!-- Set NativeAOT value -->
   	<_IsNativeAOTFeature Condition="'$(_XamarinRuntime)' == 'NativeAOT'">true</_IsNativeAOTFeature>
   	<_IsNativeAOTFeature Condition="'$(_XamarinRuntime)' != 'NativeAOT'">false</_IsNativeAOTFeature>
   	</PropertyGroup>

3. Add two new RuntimeHostConfigurationOption entries here (note this is from the net8.0 branch):

   xamarin-macios/dotnet/targets/Xamarin.Shared.Sdk.targets

   Line 512 in 638e6f5

   <RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.IsNativeAOT" Value="$(_IsNativeAOTFeature)" Trim="true" />

   Something like this:

   <RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.UseCFNetworkHandler" Value="$(_IsCFNetworkHandlerFeature)" Trim="true" />
   <RuntimeHostConfigurationOption Include="ObjCRuntime.Runtime.UseNSUrlSessionHandler" Value="$(_IsNSUrlSessionHandlerFeature)" Trim="true" />

4. Rework the code in this file to use the two new Runtime properties instead of reading from a file:

   https://github.com/xamarin/xamarin-macios/blob/net8.0/src/ObjCRuntime/RuntimeOptions.cs

Note: these changes must be done for `#if NET" only.

[Agnik7]

Ok, so I need to make the changes in the net8.0 branch right?

[rolfbjarne]

Yes.

[Agnik7]

@rolfbjarne I have created a PR #19003 , where I have tried to implemented your valuable guidance. Kindly review it, and do tell me if there are any changes to be made or not. Thanks a lot!!