Hi! We’re using Moshi in our project and we’re qu...

# squarelibraries

[Lasse Magnussen]

Hi!

We’re using Moshi in our project and we’re quite heavily vested and we have a number of fairly advanced custom adapters.
It’s working quite well.

However, there’s a new problem that I’d like to solve -- if at all possible, in Moshi.

Given json like such:

{
  "title": "Some title",
  "description": "Description of something",
  "amount": 1000,
  "currency": "NOK",
  "metadata": {
    "more": "stuff"
  }
}

I’d like to unflatten amount and currency into a its own object when deserialising:

data class Item(val title: String, val description: String, val money: Money, val metadata: MetaData)

data class Money(val amount: Int, val currency: String)

Is this doable in a generic-ish way in Moshi? I’d like for that to extend to any Retrofit response object we deem worthy.

If no, I guess the alternative would be the less neat:

data class Item(val title: String, val description: String, val amount: Int, val currency String, val metadata: MetaData) {
    val money: Money = Money(amount, currency)
}

which I’d like to avoid.

[Benoit Quenaudon]

Isn’t it just another custom adapter?

[Benoit Quenaudon]

toJson extract money, fromJson doesn’t the reverse

[Lasse Magnussen]

This scheme have to work across all our different response objects. So the custom adapter needs to be custom in such a way that you don’t have to wire it up for each and every one of these responses.

[Benoit Quenaudon]

The adapter is hooked up to Moshi so that shouldn’t be a problem?

[Benoit Quenaudon]

If you do something like

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

and have a unique Moshi instance across your whole app, that’s all you need

[Lasse Magnussen]

What does the CardAdapter look like? It needs to handle generic-ish objects and it needs to somehow extract the two fields it needs and put into the respective val in the data class.

And the data class could be 1 of many different ones.
data class SomeOtherResponse(val index: Int, val money: Money) f.ex.

It needs to be generic, it needs to delegate the rest of the fields to other adapters.

[Benoit Quenaudon]

This comes from the readme page of Moshi

[Benoit Quenaudon]

If many different types will have those two fields you wanna merge, that's gonna be tough. You need a way to know when to merge or not

[Lasse Magnussen]

Probably a custom annotation yes.

[Lasse Magnussen]

I was thinking that if I had an annotation @Wrapped or something that could signal that this field should get its data from its “parent” (really it’ll be siblings), but I’m struggling to find a neat way to do that.

[Benoit Quenaudon]

I would look into composing adapters
https://github.com/square/moshi#composing-adapters

Use something like @AlwaysSerializeNulls on all parents, and then hook my logic somewhere

[Benoit Quenaudon]

@Zac Sweers I summon you pokemon

[Zac Sweers]

you’d need a custom adapter for Item and need to manually manage those keys yourself to create the intermediate Money type

[Zac Sweers]

easiest way to get it up and running is the readJsonValue() API, which will just give you back a Map<String, Any?> of everything in the current blob

[jessewilson]

On writing, you can use beginFlatten() :
https://github.com/square/moshi/blob/6f9223af8c1a69e5707e2ad8d5088956784ae8d5/moshi/src/main/java/com/squareup/moshi/JsonWriter.kt#L501

[Lasse Magnussen]

I couldn’t get beginFlatten to work, but I managed to come up with something.

Feel free to use it however you like.
Public domain.

UnwrappedJsonAdapterFactory.cpp