Parameters about Moshi custom adapter

Question

I am using Moshi with Kotlin. According to the README, an custom TypeAdapter may like this:

class CardAdapter {
  @ToJson String toJson(Card card) {
    return card.rank + card.suit.name().substring(0, 1);
  }

  @FromJson Card fromJson(String card) {
    return //...
  }
}

It receive an Object or String, transform to another. But I made an Adapter like this:

class ConfigAdapter{

    @ToJson
    fun toJson(writer: JsonWriter, value: WfBaseConfig?) {
        val moshi = Moshi.Builder().build()
        when (value) {
            is BoolConfig -> moshi.adapter(BoolConfig::class.java).toJson(writer, value)
            // Other types ...
            else -> throw
            IllegalArgumentException("Unknown wf config type: ${value::class.java.simpleName}")
        }
    }

    @FromJson
    fun fromJson(reader: JsonReader): BaseConfig? {
        var type = ""
        val copy = reader.peekJson()
        copy.readObject {
            when (copy.selectName(JsonReader.Options.of("type"))) {
                0 -> type = copy.nextString()
                else -> copy.skipNameAndValue()
            }
        }
        val moshi = Moshi.Builder().build()
        return when (type) {
            "boole" -> moshi.adapter(BoolConfig::class.java).fromJson(reader)
            // Other operations...
            else -> {
                e("Unknown config type: $type")
                null
            }
        }
    }

    private fun JsonReader.skipNameAndValue() {
        skipName()
        skipValue()
    }
}

Original JSON format:

[
 {
    "id": "show_weather",
    "type": "boole",
    "title_en": "Show Weather",
    "def": false
 },
 {
    "id": "text_size",
    "type": "list",
    "title_en": "Text Size",
    "item_type": "text",
    "def": 1,
    "data": [{
            "id": 0,
            "title_en": "Large",
        },
        {
            "id": 1,
            "title_en": "Middle",
        },
        {
            "id": 2,
            "title_en": "Small",
        }
    ]
 }
]

Explanation: The format of each object is determined by type, if it was list, format of data will be determined by item_type.

Use it:

val listType = Types.newParameterizedType(List::class.java, BaseConfig::class.java)
val moshi = Moshi.Builder().add(ConfigAdapter()).build()
return moshi.adapter<List<BaseConfig>>(listType).fromJson(conf)

It works well too. The logic of parsing is not important. I just can't work out what type or parameters can we use in toJson() or fromJson(). I failed to find any documnets about that.

By the way, these parameters were generated by JsonAdapter<T>, but I remove the 'extent' at last because it cause Unable to resolve Lcom/squareup/moshi/JsonAdapter; annotation class 7823 warning logcat outputing. I am also very confused about when should I extend JsonAdapter class or just ues @ToJson/@FromJson annotations.

Any explanation will be appreciated.