Value Marshaling with Expressions

[jonpryor]

Fix marshaling

• I don't like JniMarshalInfo; it's too convoluted, makes the simple cases hard.

• Do in conjunction w/ Java.Interop.Dynamic

• Need to permit use of custom attributes on return types, parameter types, "inline" into marshal methods.

• Need to use System.Linq.Expressions/etc. to "merge"/do the work of monodroid/tools/generator, so that Java.Interop.Export can support value types, etc.

• Use case: Android.Graphics.Color, which is marshaled from JNI as an int but to managed as an Android.Graphics.Color struct, and vice-versa.

• Should probably split JniMarshalInfo into separate "to managed" and "from managed" types?

  • No; needless added complexity, and when it comes to System.Linq.Expressions many of the type information will be shared between them.

• "New" marshaling support should try to leverage Expression<TDelegate> to make it easier to write marshalers:

  public class JnIValueMarshaler<TJni, TManaged> : JniValueMarshaler {
      public sealed override Expression<Func<JniValueMarshalerContext, TJni, TManaged>> JniTypeToManagedType (JniValueMarshalerContext context,  Expression jniSourceValue,     Expression manageDestinationdValue);
      public abstract Expression<Func<JniValueMarshaler, TJni, TManaged> JniTypeToManagedType ();
  }
  class Int32Marshaler : JniValueMarshaler<int, int> {
      public override Expression<Func<JniValueMarshalerContext, TJni, TManaged>> JniTypeToManagedType (JniValueMarshalerContext context,  Expression jniSourceValue,     Expression manageDestinationdValue)
      {
          return (c, jni) => jni;
      }
  }
  

• With such a system in place, we'll need to be able to "rebind" the Expression<Func<...>> parameters with the generated parameters

  http://stackoverflow.com/questions/8610506/binding-parameter-in-expression-trees

  class MyVisitor : ExpressionVisitor {
      ParameterExpression[] old, cur;
      public MyVisitor(ParameterExpression[] old, ParameterExpression[] cur)
      {
          this.old = old;
          this.cur = cur;
      }
      protected override Expression VisitParameter(ParameterExpression node)
      {
          for (int i = 0; i < old.Length; ++i) {
              if (old [i].Name == node.Name && old [i].Type == node.Type)
                  return cur [i];
              }
          return node;
      }
  }
  

  With MyVisitor, we can then convert the result of Int32Marshaler.JniTypeToManagedType():

  var p = Expression.Parameter (typeof (int), "p");
  var c = Expression.Parameter (typeof (JniValueMarshalerContext), "c");
  var e = new Int32Marshaler ().JniValueMarshalerContext ();
  var v = new MyVisitor (e.Parameters, new[]{c, p});
  var e = Expression.Lambda<Func<JniValueMarshalerContext, int, int>>(v.Visit (e.Body), new[]{c, p});
  

Probably related: JniMarshalInfoAttribute, a custom attribute to control marshaling behavior?

We'll also need similar custom attributes to control parameter and return type marshaling, via the above Expression architecture.