Support gson RuntimeTypeAdapterFactory generation

[GediminasZukas]

gson-extras has such class: https://github.com/google/gson/blob/master/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java which is helpful for polymorphic types serialization without reflection.

For example, it can be used to handle Kotlin sealed class serialization:

sealed class Shape {
        class Rectangle() : Shape()
        class Circle() : Shape()
}

val adapter: RuntimeTypeAdapterFactory<Shape> = RuntimeTypeAdapterFactory.of(Shape::class.java)
        .registerSubtype(Rectangle::class.java)
        .registerSubtype(Circle::class.java)

And then in your gson instance builder:

registerTypeAdapterFactory(adapter)

However, seems it is a boilerplate and not safe solution to write such factory for every sealed class in the project. Would be nice to handle it with annotations like kotlinx-serialization does with polymorphic serialization problems: https://medium.com/@andreclassen1337/goodbye-runtimetypeadapterfactory-polymorphic-serialization-using-kotlinx-serialization-46a8cec36fdc

@Polymorphic
sealed class Shape {
        @SerialName("Rectangle")
        class Rectangle() : Shape()
        @SerialName("Circle")
        class Circle() : Shape()
}

[aafanasev]

@GediminasZukas that's a great idea! I'll have a look and keep you updated

[GediminasZukas]

I've made a tiny library which tries to solve this problem: https://github.com/GediminasZukas/PolyAssistant Feel free to use any idea from it if you find it helpful.