Parent of @Serializable, how to obtain it?

[chumpa]

Suppose I have two classes:

@Serializable
class A(
    @XmlElement(true)
    val b: B,
)

@Serializable
class B(
    @XmlElement(true)
    val contentB: String,
) {
    var a:A? = null
    init {
// How can I obtain reference to parent A here?
        println("contentB=$contentB, A=$a")
    }
}

val a = xml.decodeFromString<A>("<A><B><contentB>content</contentB></B></A>")

I need some way to get instance of A in init block of B when decoding legacy XMLs. Is there any way?

[pdvrieze]

Unfortunately this is not possible directly. The way that the framework/library works is that it will first create all members (B), collect them and then call the constructor of the container (A). In other words, you can't as it doesn't exist.

This is a place where you will probably want to use some delegate for serialization rather than use a production class directly (Note that this delegate type could be file private or internal). So you basically have a very dumb data type that you (de)serialize (for A and/or B) and then you use that type to create the final structure (with reference to container/parent).

You can make this transparent by creating a custom serializer for A that deserializes A by deserializing the delegate (and maybe a delegate for B) and then calls the constructor for A with this delegate as parameter. The constructor for A can then call the constructor B with itself as one of the parameters.

[ComBatVision]

@pdvrieze Is is possible to post-process de-serialized class somehow and assign transient parent attribute when we already have parent object de-serialized?
I have a similar problem as topic author, but I do not need to have parent in init block. I need to address parent of class later in business logic.

[pdvrieze]

Yes it is possible to postprocess. The way I would do this is to have a ASerialDelegate private member class, and then have a custom deserializer for A that deserializes through the delegate, and then postprocesses that to create an A instance.