[generator] Add support for peerConstructorPartialMethod (#1087)

Context: #1085
Context: dotnet/runtime#82121

Some Java objects are *big*, e.g. [`Bitmap`][0] instances, but as
far as MonoVM is concerned, the `Bitmap` instances are *tiny*: a
few pointers, and that's it.  MonoVM doesn't know that it could be
referencing several MB of data in the Java VM.

MonoVM is gaining support for [`GC.AddMemoryPressure()`][1] and
[`GC.RemoveMemoryPressure()`][2], which potentially allows for the
parent of all cross-VM GC integrations: using the `GC` methods to
inform MonoVM of how much non-managed memory has been allocated.
This could allow MonoVM to collect more frequently when total process
memory is low.

How should we call `GC.AddMemoryPressure()` and
`GC.RemoveMemoryPressure()`?

`GC.RemoveMemoryPressure()` is straightforward: a class can override
`Java.Lang.Object.Dispose(bool)`.

`GC.AddMemoryPressure()` is the problem: where and how can it be
called from a class binding?  This is trickier, as there was no way
to have custom code called by the bound type.  Instead, various
forms of "hacky workarounds" are often employed, e.g. copying
`generator`-emitted content into a `partial` class, then using
`metadata` to *prevent* `generator` from binding those members.
It's all around fugly.

Fortunately C# has a solution: [`partial` methods][3]!

Add support for a `peerConstructorPartialMethod` metadata entry,
applicable to `<class/>` elements, which contains the name of a
`partial` method to both declare and invoke from the "peer constructor":

	<attr
	    path="//class[@name='Bitmap']"
	    name="peerConstructorPartialMethod"
	>_OnBitmapCreated</attr>

This will alter our existing "peer constructor" generation code, a'la:

	partial class Bitmap : Java.Lang.Object {
	    internal Bitmap (IntPtr h, JniHandleOwnership t) : base (h, t)
	    {
	    }
	}

to instead become:

	partial class Bitmap : Java.Lang.Object {
	    partial void _OnBitmapCreated ();
	    internal Bitmap (IntPtr h, JniHandleOwnership t) : base (h, t)
	    {
	        _OnBitmapCreated ();
	    }
	}

This allows a hand-written `partial class Bitmap` to do:

        // Hand-written code
        partial class Bitmap {
            int _memoryPressure;

            partial void _OnBitmapCreated ()
            {
                _memoryPressure = ByteCount;
                GC.AddMemoryPressure (_memoryPressure);
            }

            protected override void Dispose (bool disposing)
            {
                if (_memoryPressure != 0) {
                    GC.RemoveMemoryPressure (_memoryPressure);
                    _memoryPressure = 0;
                }
            }
        }

TODO: "extend" this for `<method/>`s as well?

[0]: https://developer.android.com/reference/android/graphics/Bitmap
[1]: https://learn.microsoft.com/dotnet/api/system.gc.addmemorypressure?view=net-7.0
[2]: https://learn.microsoft.com/dotnet/api/system.gc.removememorypressure?view=net-7.0
[3]: https://learn.microsoft.com/dotnet/csharp/language-reference/keywords/partial-method

src/Xamarin.SourceWriter/Models/MethodWriter.cs

@@ -19,6 +19,7 @@ public class MethodWriter : ISourceWriter, ITakeParameters
 		public List<string> Body { get; set; } = new List<string> ();
 		public bool IsSealed { get; set; }
 		public bool IsStatic { get; set; }
+		public bool IsPartial { get; set; }
 		public bool IsPrivate { get => visibility == Visibility.Private; set => visibility = value ? Visibility.Private : Visibility.Default; }
 		public bool IsProtected { get => visibility == Visibility.Protected; set => visibility = value ? Visibility.Protected : Visibility.Default; }
 		public bool IsOverride { get; set; }
@@ -103,6 +104,9 @@ public virtual void WriteSignature (CodeWriter writer)
 			if (IsUnsafe)
 				writer.Write ("unsafe ");
 
+			if (IsPartial)
+				writer.Write ("partial ");
+
 			WriteReturnType (writer);
 
 			if (ExplicitInterfaceImplementation.HasValue ())

tests/generator-Tests/Unit-Tests/CodeGeneratorTests.cs

@@ -376,6 +376,34 @@ public void CompatVirtualMethod_Class ()
 			Assert.True (writer.ToString ().NormalizeLineEndings ().Contains ("catch (Java.Lang.NoSuchMethodError) { throw new Java.Lang.AbstractMethodError (__id); }".NormalizeLineEndings ()), $"was: `{writer}`");
 		}
 
+		[Test]
+		public void PeerConstructorPartialMethod_Class ()
+		{
+			var xml = @"<api>
+			  <package name='java.lang' jni-name='java/lang'>
+			    <class abstract='false' deprecated='not deprecated' final='false' name='Object' static='false' visibility='public' jni-signature='Ljava/lang/Object;' />
+			  </package>
+			  <package name='com.xamarin.android' jni-name='com/xamarin/android'>
+			    <class abstract='false' deprecated='not deprecated'
+			        extends='java.lang.Object' extends-generic-aware='java.lang.Object' jni-extends='Ljava/lang/Object;'
+			        peerConstructorPartialMethod='_OnMyClassCreated'
+			        jni-signature='Lcom/xamarin/android/MyClass;'
+			        name='MyClass'
+			        final='false' static='false' visibility='public'>
+			    </class>
+			  </package>
+			</api>";
+
+			var gens = ParseApiDefinition (xml);
+			var klass = gens.Single (g => g.Name == "MyClass");
+
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
+
+			Assert.True (writer.ToString ().NormalizeLineEndings ().Contains ("partial void _OnMyClassCreated ();".NormalizeLineEndings ()), $"was: `{writer}`");
+			Assert.True (writer.ToString ().NormalizeLineEndings ().Contains ("{ _OnMyClassCreated (); }".NormalizeLineEndings ()), $"was: `{writer}`");
+		}
 		[Test]
 		public void WriteDuplicateInterfaceEventArgs ()
 		{

tools/generator/Java.Interop.Tools.Generator.Importers/XmlApiImporter.cs

@@ -110,6 +110,7 @@ public static ClassGen CreateClass (XElement pkg, XElement elem, CodeGenerationO
 				FromXml = true,
 				IsAbstract = elem.XGetAttribute ("abstract") == "true",
 				IsFinal = elem.XGetAttribute ("final") == "true",
+				PeerConstructorPartialMethod = elem.XGetAttribute ("peerConstructorPartialMethod"),
 				// Only use an explicitly set XML attribute
 				Unnest = elem.XGetAttribute ("unnest") == "true" ? true :
 					 elem.XGetAttribute ("unnest") == "false" ? false :

tools/generator/Java.Interop.Tools.Generator.ObjectModel/ClassGen.cs

@@ -302,6 +302,8 @@ public bool IsExplicitlyImplementedMethod (string sig)
 
 		public bool NeedsNew { get; set; }
 
+		public string PeerConstructorPartialMethod { get; set; }
+
 		protected override bool OnValidate (CodeGenerationOptions opt, GenericParameterDefinitionList type_params, CodeGeneratorContext context)
 		{
 			if (validated)

tools/generator/SourceWriters/BoundClass.cs

@@ -126,8 +126,12 @@ void AddBindingInfrastructure (ClassGen klass, CodeGenerationOptions opt)
 		void AddConstructors (ClassGen klass, CodeGenerationOptions opt, CodeGeneratorContext context)
 		{
 			// Add required constructor for all JLO inheriting classes
-			if (klass.FullName != "Java.Lang.Object" && klass.InheritsObject)
-				Constructors.Add (new JavaLangObjectConstructor (klass, opt));
+			if (klass.FullName != "Java.Lang.Object" && klass.InheritsObject) {
+				if (!string.IsNullOrWhiteSpace (klass.PeerConstructorPartialMethod)) {
+					Methods.Add (new ConstructorPartialMethod (klass.PeerConstructorPartialMethod));
+				}
+				Constructors.Add (new JavaLangObjectConstructor (klass, opt, klass.PeerConstructorPartialMethod));
+			}
 
 			foreach (var ctor in klass.Ctors) {
 				// Don't bind final or protected constructors

tools/generator/SourceWriters/ConstructorPartialMethod.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using MonoDroid.Generation;
+using Xamarin.Android.Binder;
+using Xamarin.SourceWriter;
+
+namespace generator.SourceWriters
+{
+	public class ConstructorPartialMethod : MethodWriter
+	{
+		public ConstructorPartialMethod (string partialMethodName)
+		{
+			Name = partialMethodName;
+			IsPartial = true;
+			IsDeclaration = true;
+			ReturnType = new TypeReferenceWriter ("void");
+		}
+	}
+}

tools/generator/SourceWriters/JavaLangObjectConstructor.cs

@@ -11,7 +11,7 @@ namespace generator.SourceWriters
 {
 	public class JavaLangObjectConstructor : ConstructorWriter
 	{
-		public JavaLangObjectConstructor (ClassGen klass, CodeGenerationOptions opt)
+		public JavaLangObjectConstructor (ClassGen klass, CodeGenerationOptions opt, string callPartialMethod)
 		{
 			Name = klass.Name;
 
@@ -31,6 +31,9 @@ public JavaLangObjectConstructor (ClassGen klass, CodeGenerationOptions opt)
 
 				BaseCall = "base (javaReference, transfer)";
 			}
+			if (!string.IsNullOrWhiteSpace (callPartialMethod)) {
+				Body.Add ($"{callPartialMethod} ();");
+			}
 		}
 	}
 }