I am building an app which can fetch a Warning object from an API which I do not control nor do they want to remove this behaivor to which I am trying to work around.

I would like to be able to parse either a single Warning object or a List<Warning> objects. (Because the API returns either 1 object if there is only 1 or > 1 a list of objects.)

Question

How can I parse an object as a list directly or parse list this with Moshi?

The data I get looks either like this:

{

  # List<Warning>

  "warnings": [{...}, {...}]

}

or like this:

{

  # Warning

  "warnings": {...}

}

I have read and found some sources on this topic, but no examples and I am not sure how to proceed.. so any pointers would be highly appreciated.

Attempt

I initially tried to shoehorn an autogenerated moshi adapter thinking I might be able to build ontop of it but I couldn't figure out what was going on so it didn't really lead me anywhere. But I included the generated code anyway incase it might be useful for the task still.

Edit: This is what I currently have, though im not very pleased by the fact it takes an instance of Moshi to work.

Solution

Generalised approach with a Factory

I tried to port adapter which Eric wrote to kotlin
since I since has realised that a more general approach is better much like
Eric points out in his reply. As soon a this is working, I will re-write parts of this post to make it more understandable, its a bit messy now I apologize for that.

EDIT: I ended up using the solution Eric suggested in another thread but ported to Kotlin.

Adapter with Factory

class SingleToArrayAdapter(

    val delegateAdapter: JsonAdapter<List<Any>>,

    val elementAdapter: JsonAdapter<Any>

) : JsonAdapter<Any>() {



    companion object {

        val INSTANCE = SingleToArrayAdapterFactory()

    }



    override fun fromJson(reader: JsonReader): Any? =

        if (reader.peek() != JsonReader.Token.BEGIN_ARRAY) {

            Collections.singletonList(elementAdapter.fromJson(reader))

        } else delegateAdapter.fromJson(reader)



    override fun toJson(writer: JsonWriter, value: Any?) =

        throw UnsupportedOperationException("SingleToArrayAdapter is only used to deserialize objects")



    class SingleToArrayAdapterFactory : JsonAdapter.Factory {

        override fun create(type: Type, annotations: Set<Annotation>, moshi: Moshi): JsonAdapter<Any>? {

            val delegateAnnotations = Types.nextAnnotations(annotations, SingleToArray::class.java)

            if (delegateAnnotations == null) return null

            if (Types.getRawType(type) != List::class.java) throw IllegalArgumentException("Only lists may be annotated with @SingleToArray. Found: $type")

            val elementType = Types.collectionElementType(type, List::class.java)

            val delegateAdapter: JsonAdapter<List<Any>> = moshi.adapter(type, delegateAnnotations)

            val elementAdapter: JsonAdapter<Any> = moshi.adapter(elementType)



            return SingleToArrayAdapter(delegateAdapter, elementAdapter)

        }

    }

}

Qualifier

Note: I had to add the Target(FIELD).

@Retention(RUNTIME)

@Target(FIELD)

@JsonQualifier

annotation class SingleToArray

Usage

Annotate a field you want to make sure is parsed as a list with @SingleToArray.

data class Alert(

    @SingleToArray

    @Json(name = "alert")

    val alert: List<Warning>

)

and dont forget to add the adapter to your moshi instance:

val moshi = Moshi.Builder()

            .add(SingleToArrayAdapter.INSTANCE)

            .build()

Related issues/topics:

• Parse JSON key that is either object or array of object


• Moshi Determine if JSON is array or single object


• https://github.com/square/moshi

the API returns either 1 object if there is only 1 or > 1 a list of objects.

Create an adapter that peeks to see if you're getting an array first.
Here is exactly what you want. It includes a qualifier, so you can apply it only to lists that may have this behavior for single items. @SingleToArray List<Warning>.

There is another example of dealing with multiple formats here for further reading.