Hi :wave: I have many model classes extending a ba...

# serialization

[natario1]

Hi 👋 I have many model classes extending a base interface like:

class BaseModel {
    val keys: Set<String> 
    fun <T> get(key: String): T?
    fun <T> set(key: String, value: T?)
}

Basically, it works as a Map<String, Any?> and we can assume that the actual values are something that can be serialized, like numbers or strings. Then I have concrete models e.g. class UserModel : BaseModel, class PostModel : BaseModel and so on.

How can I make these classes serializable? I tried to write a common KSerializer and can handle serialization like:

@Serializer(UserModel::class)
object UserSerializer : BaseSerializer<UserModel>({ UserModel() })

@Serializer(PostModel::class)
object PostSerializer : BaseSerializer<PostModel>({ PostModel() })

class BaseSerializer<M : BaseModel>: KSerializer<M> {

    override fun serialize(encoder: Encoder, model: T) {
        for (key in model.keys) {
            encoder.encodeString(key)
            val value = model.get(key)
            when (value) {
                is Int -> encoder.encodeInt(value)
                is Long -> encoder.encodeLong(value)
                is String -> encoder.encodeString(value)
                else -> error()
            }
        }
    }
}

But then I have no idea of how to implement deserialize (looks like order matters there...?) and descriptor: SerialDescriptor (not even sure what this is). I might get it working but I don't think I'm doing things in the correct way. Can anyone help?

The BaseModel also has a working Parcelable implementation, if that matters.

[ephemient]

something like this?

Untitled

[natario1]

This is very helpful @ephemient, thanks! Learned a lot by going through your code.

But what if I don't know the model fields? For example, User can have name and age, but occasionally I can also call user["_extraRuntimeProperty"] = foo and I want these properties to be serialized as well. Basically I don't know the model schema at compile time... How should I write the descriptor in this case?

[ephemient]

you can't

[ephemient]

😄

[ephemient]

in that case you may want to just treat the whole thing as a Map

[natario1]

In fact it really is a map (a bit more complex because these models can be nested, but we can ignore it for now). But I'm still not sure about how to implement serialization for it 😅

[ephemient]

https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#composite-serializer-via-surrogate

Untitled

[ephemient]

assuming you're not too picky about the exact format