default interface methods support

[atsushieno]

1. You have to manually provide CscToolPath MSBuild property,
   unless you install xamarin.android.csc.dim package.
2. You have to explicitly specify AndroidEnableDefaultInterfaceMethods
   MSBuild property as True.
3. You have to explicitly specify LangVersion MSBuild property as "latest".

When C# 8.0 becomes stable, 1) and 3) will become unnecessary.
Then we should make changes to 2) as default.

Remaining tasks:

• add support for DIMs in Xamarin.Android.Bindings.targets so that DIM bindings can be generated.
• add apk tests that assures DIMs work as expected at run time.
• add DIM-backed methods in Mono.Android.dll (for now the new ones are disabled; those methods that "became default" are generated as non-default)
• (optional) update xamarin-android Xamarin.Android.Bindings.targets to customize csc tool task invocation, if no CscToolPath is provided. -> there is no csc-dim on Windows, so skip that.

[atsushieno]

(unassigning automatic request review for now, there will be a bunch of commits and comments.)

[atsushieno]

So far I have a PoC code (written as part of tests/CodeGen-Binding) which has MainActivity.OnCreate() like this:

			JniPeerMembers _members = new JniPeerMembers ("com/xamarin/test/DefaultInterfaceMethods", typeof (IDefaultInterfaceMethods));
			string __id = "foo.()I";
			unsafe {
				var __rm = _members.InstanceMethods.InvokeNonvirtualInt32Method (__id, (IJavaPeerable)this, null);
				Console.WriteLine (__rm);
			}


			IDefaultInterfaceMethods c = new ImplementedClass ();
			Console.WriteLine (c.Foo ());

... which shows weird run time error, shown as the attached sshot:

What's interesting is that the strongly-typed version (new ImplementedClass()) just passes without such an error.

full repro: Xamarin.Android.DefaultInterfaceMethods-Tests.zip

[atsushieno]

Now here is an interesting test code - I1.Foo() does not fail but I2.Foo() fails, throwing the same AbstractMethodError as the previous comment. The only difference between I1 and I2 are that I2 is derived from Java.Lang.Object. Maybe libmonodroid injects something there?

using System;
using Android.App;
using Android.OS;
using Com.Xamarin.Test;

namespace Xamarin.Android.DefaultInterfaceMethodsTests
{
	[Activity (Label = "Xamarin.Android.DefaultInterfaceMethods-Tests", MainLauncher = true)]
	public class MainActivity : global::Android.App.Activity
	{
		protected override void OnCreate (Bundle bundle)
		{
			((I1)new X1 ()).Foo ();
			((I2)new X2 ()).Foo ();
		}
	}

	public interface I1
	{
		string Foo () => "foo";
	}

	public class X1 : I1
	{
	}

	public interface I2
	{
		string Foo () => "foo";
	}

	public class X2 : Java.Lang.Object, I2
	{
	}
}

[jonpryor]

Maybe libmonodroid injects something there

Not quite; it's the linker: https://developer.xamarin.com/releases/android/xamarin.android_6/xamarin.android_6.1/#Java_Interface_Versioning

Starting with Xamarin.Android 6.1, all classes will be checked to ensure they fully implement their Java interfaces, and if they don’t then the missing members will be generated to throw an AbstractMethodError.

See also: https://github.com/xamarin/xamarin-android/blob/71a98b55b69c46ff438c484e31adaf99f527dcdc/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/FixAbstractMethodsStep.cs#L166-L180

We'll need to improve FixAbstractMethodsStep.FixAbstractMethods() so that it doesn't emit the "missing method" if the interface method contains a body, presumably by:

@@ -167,7 +167,7 @@ namespace MonoDroid.Tuner
                                        bool exists = false;
 
                                        foreach (var tMethod in typeMethods) {
-                                               if (HaveSameSignature (iface, iMethod, tMethod)) {
+                                               if (HaveSameSignature (iface, iMethod, tMethod) && !iMethod.HasBody) {
                                                        exists = true;
                                                        break;
                                                }

[atsushieno]

After fixing the linker, I'm getting the same managedPeerType issue (mentioned at #1939 (comment)) at the binding class too. Before the fix, it was raising AbstractMethodError without going down to IDefaultInterfaceMethods static initializer and therefore the error. So it is a progress.

[atsushieno]

Now Mono.Android.dll builds. With a handful of ABI breakage https://gist.github.com/atsushieno/50affef19ff37597842583c27874d8e7

[atsushieno]

All those ABI breakages are gone. It's done. With some csc-dim tooling fixes, it's ready for merging.

[jonpryor]

Superseded by PR #3533.