Moshi + Kotlin + SealedClass

Is there a way of deserializing json using

sealed class Layer

data class ShapeLayer(var type: LayerType) : Layer
data class TextLayer(var type: LayerType) : Layer
data class ImageLayer(var type: LayerType) : Layer

LayerType is just some enum which can be used to distinguish which type should this object have.

I thought I could add Adapter this way:

class LayerAdapter{
@FromJson
fun fromJson(layerJson: LayerJson): Layer {
return when (layerJson.layerType) {
LayerType.SHAPE -> PreCompLayer()
LayerType.SOLID -> SolidLayer()
LayerType.Text -> TextLayer()
}
}
}
Where LayerJson would be object which has every possible field of all LayerTypes.

Now the problem is:
> Cannot serialize abstract class com.example.models.layers.Layer

I could try to use interface, but I don't think it would be correct to use empty interface in this.

Yes, you can create a custom type adapter to parse json according to the `layerType` like this:

class LayerAdapter {
@FromJson
fun fromJson(layerJson: LayerJson): Layer = when (layerJson.layerType) {
LayerType.SHAPE -> ShapeLayer(layerJson.layerType, layerJson.shape ?: "")
LayerType.TEXT -> TextLayer(layerJson.layerType, layerJson.text ?: "")
LayerType.IMAGE -> ImageLayer(layerJson.layerType, layerJson.image ?: "")
}

@ToJson
fun toJson(layer: Layer): LayerJson = when (layer) {
is ShapeLayer -> LayerJson(layer.type, shape = layer.shape)
is TextLayer -> LayerJson(layer.type, text = layer.text)
is ImageLayer -> LayerJson(layer.type, image = layer.image)
else -> throw RuntimeException("Not support data type")
}
}

Here I have make some changes to your data class for clarity (an extra property to each of the `Layer` type, e.g. `shape` for `ShapeLayer`):

sealed class Layer

data class ShapeLayer(val type: LayerType, val shape: String) : Layer()
data class TextLayer(val type: LayerType, val text: String) : Layer()
data class ImageLayer(val type: LayerType, val image: String) : Layer()

//LayerJson contains every possible property of all layers
data class LayerJson(val layerType: LayerType, val shape: String? = null, val text: String? = null, val image: String? = null) : Layer()

enum class LayerType {
SHAPE, TEXT, IMAGE
}

Testing code:

val moshi = Moshi.Builder()
.add(LayerAdapter())
.build()
val type = Types.newParameterizedType(List::class.java, Layer::class.java)
val adapter = moshi.adapter<List<Layer>>(type)

//Convert from json string to List<Layer>
val layers: List<Layer>? = adapter.fromJson("""
[
{"layerType":"SHAPE", "shape":"I am rectangle"},
{"layerType":"TEXT", "text":"I am text"},
{"layerType":"IMAGE", "image":"I am image"}
]
""".trimIndent())
layers?.forEach(::println)

//Convert a list back to json string
val jsonString: String = adapter.toJson(layers)
println(jsonString)

Output:

ShapeLayer(type=SHAPE, shape=I am rectangle)
TextLayer(type=TEXT, text=I am text)
ImageLayer(type=IMAGE, image=I am image)
[{"layerType":"SHAPE","shape":"I am rectangle"},{"layerType":"TEXT","text":"I am text"},{"image":"I am image","layerType":"IMAGE"}]

---
Edit:
You can add the adapter as usual when you are trying to parse other object which contain `Layer`. Suppose you have an object like this:

data class LayerContainer(val layers: List<Layer>)

Testing code:

val moshi = Moshi.Builder()
.add(LayerAdapter())
.build()

val adapter = moshi.adapter(LayerContainer::class.java)
val layerContainer: LayerContainer? = adapter.fromJson("""
{
"layers": [
{"layerType":"SHAPE", "shape":"I am rectangle"},
{"layerType":"TEXT", "text":"I am text"},
{"layerType":"IMAGE", "image":"I am image"}
]
}
""".trimIndent())
layerContainer?.layers?.forEach(::println)

val jsonString: String = adapter.toJson(layerContainer)
println(jsonString)

It turned out that my code was actually correct from beginning!

Problem was with field declaration inside data Class:

data class LayerContainer(var/val layers: List<Layer>)

It works with val, and doesn't work with var!
Kotlin somehow creates different code down below.
This is my final code for this part of model:

@JvmSuppressWildcards var layers: List<Layer>