Is there a way to use Kotlinx serialisation in an Android savedInstanceState Bundle?

Question

It looks like the compiler doesn't want to use Kotlinx serialized classes in putSerializable and getSerializable . It says Type mismatch: inferred type is MyViewModel.SavedState but Serializable? was expected Type mismatch: inferred type is MyViewModel.SavedState but Serializable? was expected .

In my Activity:

override fun onCreate(savedInstanceState: Bundle?) {
    AndroidInjection.inject(this)
    super.onCreate(savedInstanceState)

    setContentView(R.layout.my_activity_layout)

    viewModel.init(savedInstanceState?.getSerializable(SAVE_STATE) as? SavedState) // compiler complains here
}

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putSerializable(SAVE_STATE, viewModel.buildSaveState()) // and here
}

In my ViewModel:

fun buildSaveState(): SavedState =
        SavedState(value1, value2, value3, value4)

@Serializable
data class SavedState(val foo: Boolean?,
                      val foo1: Enum1?,
                      val foo2: Enum2?,
                      val foo3: MyType?)

MyType:

@Serializable
sealed class MyType {
    data class MyType1(val foo4: Enum3) : MyType()
    data class MyType2(val foo5: Enum4) : MyType()

    enum class Enum3 {
        ...
    }

    enum class Enum4 {
        ...
    }
}

Answer 1

I'm pretty sure Kotlinx.Serialization is not OOB compatible with Bundle's putSerializable. However, you could just stringify your SavedState , send it via putString and on the receiving end deserialize string back to your class.

Answer 2

You could use the kotlin-parcelize plugin ( https://developer.android.com/kotlin/parcelize )

First add the plugin to your app/build.gradle:

plugins {
    ..
    id 'kotlin-parcelize'
}

Then add the @Parcelize annotation and the Parcelable interface to a class:

import kotlinx.parcelize.Parcelize

@Parcelize
class User(val firstName: String, val lastName: String, val age: Int): Parcelable

And then you can add an instance to a bundle:

val user = User("John", "Doe", 33)
bundle.putParcelable("mykey", user)

However, the kotlin-parcelize plugin does not seem to work with sealed classes, so it may not be the right solution for your usecase.