Moshi: Choose adapter based on field value

[vganin]

Hi!

I'm stuck with a seemingly simple problem. I want to choose adapter based on some field value (let's call it type) to construct types from class hierarchy. Obvious approach is to write some delegating adapter like this:

public class BaseTypeAdapter {

    @NonNull
    private final JsonAdapter<Derived1> derivedTypeAdapter1;

    @NonNull
    private final JsonAdapter<Derived2> derivedTypeAdapter2;

    @NonNull
    private final JsonAdapter<Map> mapAdapter;

    public BaseTypeAdapter(@NonNull Moshi subMoshi) {
        this.derivedTypeAdapter1 = subMoshi.adapter(Derived1.class);
        this.derivedTypeAdapter2 = subMoshi.adapter(Derived2.class);
        this.mapAdapter = subMoshi.adapter(Map.class);
    }

    @ToJson
    public String toJson(Base base) {
        ...
    }

    @FromJson
    public Base fromJson(Map<String, Object> map) {
        try {
            // FIXME: Redundantly transforming json from one form to another.
            // FIXME: Moshi's API doesn't allow to accept json as string directly.
            JSONObject jsonObject = new JSONObject(map);
            String json = mapAdapter.toJson(map);

            String type = jsonObject.getString("type");
            if (type != null) {
                switch (type) {
                    case "derived1":
                        return derivedTypeAdapter1.fromJson(json);
                    case "derived2":
                        return derivedTypeAdapter2.fromJson(json);
                }
            }
        } catch (JSONException | IOException e) {
            throw new RuntimeException(e);
        }

        return null;
    }
}

In comments you can see that I'm stuck with API limitations that force me to write such bizarre code. Is there any better way to do the same thing?

[NightlyNexus]

you could read the type off the map instead of converting to a string.
or

You could use the the JsonReader API directly. And, you can put in the delegating adapters as your method arguments to reuse the same Moshi instance.

@FromJson Base fromJson(JsonReader reader, JsonAdapter<Derived1> derivedTypeAdapter1,
    JsonAdapter<Derived2> derivedTypeAdapter2) throws IOException {
  Map<Object, String> value = (Map<Object, String>) reader.readJsonValue();
  String type = value.get("type");
  if (type == null) {
    return null;
  }
  switch (type) {
    case "derived1":
      return derivedTypeAdapter1.fromJsonValue(value);
    case "derived2":
      return derivedTypeAdapter2.fromJsonValue(value);
    default:
      return null;
  }
}

a couple examples: https://github.com/square/moshi/pull/264 and https://github.com/square/moshi/pull/256

I've been trying to write a blog post to demonstrate this better. hopefully soon.

[NightlyNexus]

you could probably copy-paste the mentioned RuntimeJsonAdapterFactory and use it for this, too, without writing all this code yourself.

[vganin]

Ah, I'm deeply sorry. I was using outdated Moshi version (1.3.0) and hadn't those methods in JsonReader yet I tried the approach with custom JsonAdapter. Thank you for answer and examples. Closing.