[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

Author: jonpryor

src/Xamarin.SourceWriter/Models/MethodWriter.cs

@@ -19,6 +19,7 @@ public class MethodWriter : ISourceWriter, ITakeParameters
 publicList<string>Body{get;set;}=newList<string>();
 publicboolIsSealed{get;set;}
 publicboolIsStatic{get;set;}
+publicboolIsPartial{get;set;}
 publicboolIsPrivate{get=>visibility==Visibility.Private;set=>visibility=value?Visibility.Private:Visibility.Default;}
 publicboolIsProtected{get=>visibility==Visibility.Protected;set=>visibility=value?Visibility.Protected:Visibility.Default;}
 publicboolIsOverride{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]
+publicvoidPeerConstructorPartialMethod_Class()
+{
+varxml=@"<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>";
+
+vargens=ParseApiDefinition(xml);
+varklass=gens.Single(g =>g.Name=="MyClass");
+
+generator.Context.ContextTypes.Push(klass);
+generator.WriteType(klass,string.Empty,newGenerationInfo("","","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]
 publicvoidWriteDuplicateInterfaceEventArgs()
 {

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)
 
 publicboolNeedsNew{get;set;}
 
+publicstringPeerConstructorPartialMethod{get;set;}
+
 protectedoverrideboolOnValidate(CodeGenerationOptionsopt,GenericParameterDefinitionListtype_params,CodeGeneratorContextcontext)
 {
 if(validated)

tools/generator/SourceWriters/BoundClass.cs

@@ -126,8 +126,12 @@ void AddBindingInfrastructure (ClassGen klass, CodeGenerationOptions opt)
 voidAddConstructors(ClassGenklass,CodeGenerationOptionsopt,CodeGeneratorContextcontext)
 {
 // Add required constructor for all JLO inheriting classes
-if(klass.FullName!="Java.Lang.Object"&&klass.InheritsObject)
-Constructors.Add(newJavaLangObjectConstructor(klass,opt));
+if(klass.FullName!="Java.Lang.Object"&&klass.InheritsObject){
+if(!string.IsNullOrWhiteSpace(klass.PeerConstructorPartialMethod)){
+Methods.Add(newConstructorPartialMethod(klass.PeerConstructorPartialMethod));
+}
+Constructors.Add(newJavaLangObjectConstructor(klass,opt,klass.PeerConstructorPartialMethod));
+}
 
 foreach(varctorinklass.Ctors){
 // Don't bind final or protected constructors

tools/generator/SourceWriters/ConstructorPartialMethod.cs

@@ -0,0 +1,22 @@
+usingSystem;
+usingSystem.Collections.Generic;
+usingSystem.Linq;
+usingSystem.Text;
+usingSystem.Threading.Tasks;
+usingMonoDroid.Generation;
+usingXamarin.Android.Binder;
+usingXamarin.SourceWriter;
+
+namespacegenerator.SourceWriters
+{
+publicclassConstructorPartialMethod:MethodWriter
+{
+publicConstructorPartialMethod(stringpartialMethodName)
+{
+Name=partialMethodName;
+IsPartial=true;
+IsDeclaration=true;
+ReturnType=newTypeReferenceWriter("void");
+}
+}
+}

tools/generator/SourceWriters/JavaLangObjectConstructor.cs

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