For Dart and Android, support for this feature is pretty straightforward. It can be solved by just updating the codec with enum indexes:

Dart:

enum Fruit {
  apple,
  banana,
}

class ACodec extends StandardMessageCodec {
  const ACodec();
  
  @override
  void writeValue(WriteBuffer buffer, Object? value) {
    if (value is Fruit) {
      buffer.putUint8(128);
      writeValue(buffer, value.index);
    } else {
      super.writeValue(buffer, value);
    }
  }

  @override
  Object? readValueOfType(int type, ReadBuffer buffer) {
    switch (type) {
      case 128:
        return Fruit.values[readValue(buffer)! as int];
      default:
        return super.readValueOfType(type, buffer);
    }
  }
}

Java:

public enum Fruit {
  apple(0),
  banana(1);

  private int index;
  Fruit(final int index) {
    this.index = index;
  }
}

private static class ACodec extends StandardMessageCodec {
  public static final ACodec INSTANCE = new ACodec();
  private ACodec() {}
  @Override
  protected Object readValueOfType(byte type, ByteBuffer buffer) {
    switch (type) {
      case (byte)128:
        return Fruit.values()[(int) readValue(buffer)];
      default:        
        return super.readValueOfType(type, buffer);
      
    }
  }
  @Override
  protected void writeValue(ByteArrayOutputStream stream, Object value)     {
    if (value instanceof Fruit) {
      stream.write(128);
      writeValue(stream, ((Fruit) value).index);
    } else {
      super.writeValue(stream, value);
    }
  }
}

But, the codec for Objective-C doesn't handle Objective-C type enums well. e.g:

typedef NS_ENUM(NSUInteger, Fruit) {
  FruitApple = 0,
  FruitBanana = 1,
};

FlutterStandardReader.readValueOfType and FlutterStandardWriter.writeValue require an object, not a primitive, to be returned or passed respectively. So, the solution won't be as straightforward as other languages. A couple of solutions I came up with are:

[Breaking Change] The Objective-C generation could create Enum like classes with static variables. e.g.

@interface Fruit : NSObject
@property(readonly) NSUInteger index;
- (instancetype)init NS_UNAVAILABLE;
+ (Fruit *)apple;
+ (Fruit *)banana;
@end

Methods for HostApis/FlutterApis could take and/or return enum wrappers. e.g.

typedef NS_ENUM(NSUInteger, Fruit) {
  FruitApple = 0,
  FruitBanana = 1,
};

@interface FruitEnum : NSObject
@property(readonly) Fruit value;
- (instancetype)initWithValue:(Fruit)value;
@end

@protocol AClass
- (void)aMethod:(NSArray<FruitEnum *> *)fruits error:(FlutterError *_Nullable *_Nonnull)error;
@end

pigeon: implement primitive enum return values and arguments #87307

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