Fixing the Moshi enumeration downside utilizing generics | Iconic Tech

How reified sort parameters will prevent from “copy paste” when parsing enums.

Moshi is without doubt one of the hottest JSON parsing libraries on Android that permits us to transform a JSON object to an information class and vice versa. It really works nicely with Retrofit, some of the well-liked networking libraries, and since server responses are largely rendered in JSON format, Moshi will save us from writing boilerplate code when parsing them on the shopper.

However for DRY-loving Android builders, Moshi has one deadly flaw; enumerations. Out of the field, Moshi will both require customized parsers for every enum in its API or require you to symbolize enums as strings, which no backend database will ever do.

Can we keep away from repeating boilerplate code whereas nonetheless having the ability to parse enumerations with out representing them as strings? after all we are able to (it is software program!)however the reply could shock you.

As a result of all you want is an annotation. We now have the next JSON illustration of a Individual which may be mechanically transformed to an information class, just by including the @JsonClass(generateAdapter = true) annotation. This annotation will generate an adapter at compile time that can do the parsing for you.

For extra advanced knowledge sorts, Moshi offers us the pliability to write down our personal customized adapters, basically supporting all types of modeling we need to accomplish.

Let’s add a further property in our Individual knowledge class, which is Function. That will be an enum class with two doable values ​​for now, Admin both Moderator.

Now what sort of worth do you anticipate within the JSON illustration to map it right into a Function? Is it a quantity or a string?

Moshi helps enumeration parsing utilizing the EnumJsonAdapter, however it comes with a draw back. The worth in JSON should be a string that matches the title of the enumeration fixed on the shopper. This clearly makes the code error susceptible and tougher to refactor, since merely renaming an enum fixed will break parsing.

What in case you use numbers to symbolize the values ​​as a substitute?

Since representing values ​​as strings is error susceptible, what if we use plain integers that the shopper will parse and assign to the respective enum fixed?

This makes our API much less verbose and our shopper code simpler to keep up, since we now not permit ourselves to interrupt parsing just by renaming an enum fixed.

Nevertheless, now we are able to now not use the EnumJsonAdapter offered by Moshi, because the values ​​aren’t represented by strings. We’ll want to write down a customized adapter that maps the integer to an enum fixed.

First, let’s add a worth property in our Function enum class that can match the one now we have outlined in our JSON illustration:

As we are able to see within the above snippet, a Admin is now related to a worth of 1 whereas a Moderator is related to a worth of 2. the fromValueOrNull The operate will take the worth and assign it to an enum fixed.

Now let’s write a customized adapter that can do the fromJson Y toJson mapping:

We are actually capable of outline the enumeration courses as plain numbers in our server responses, as a substitute of utilizing strings. However we did it at the price of writing a customized adapter. What if now we have dozens of enums with an related worth? Do we have to write all that boilerplate code each time? Properly, that is the place generics come into play.

First, we agree that we have to affiliate our enums with a quantity. Meaning we’ll have to outline an interface that ought to be carried out by our enums:

To keep away from now defining a fromValueOrNull operate in every enum, we’ll benefit from the interface we simply created and create a GenericEnumFactory with a fromValueOrNull operate that can take an integer and the sort T of the enum and can do the mapping to return us the fixed of the enum. The sort parameter T after all you’ll have to implement the IEnumValue Interface:

We have simply eliminated among the boilerplate code from our enums, however we nonetheless have to outline a brand new JsonAdapter per enum. May we in some way outline only one adapter and reuse it throughout all of our enums?

The magic of reified sort parameters

If we need to create a generic adapter, we’ll want entry to the sort T of the enum in order that we are able to entry the enum constants and their worth that he IEnumValue supplies the interface. However we all know that the sort is cleared at run time and is barely out there at compile time. That is precisely the place reified sort parameters are available: they’re going to permit us to entry a kind handed as a parameter as if it had been a traditional class.

Nevertheless, on the time of penning this publish, reified sort parameters aren’t supported on the class stage in Kotlin, so any try to create a generic adapter like the next would fail:

Nevertheless, there may be one other trick we are able to do. We all know that reified sort parameters are appropriate with inline features, so as a substitute of defining a generic class, what occurs if we outline a generic operate that can return the generic worth? JsonAdapter<T>? And that will probably be a wonderfully legitimate answer!

There now we have a generic operate that can mechanically generate for us the enum adapter to do the fromJson Y toJson mapping, and that helps integer illustration of our enums in server responses.

Observe: firstOrNull as its title signifies it may possibly return null if the entire worth we offer can’t be mapped to an enumeration fixed. Because of this the enums within the knowledge courses that mannequin our server’s responses should be nullable. Nevertheless, now we have the choice of defining a default worth, for instance by returning enumValues<T>().first() Sure firstOrNull returns null.

Utilizing this operate is kind of easy, we simply want so as to add the adapters in our Moshi.Builder definition, just like how we might do it with every other customized Moshi adapter:

This method permits us to symbolize enumerations as integers in server responses with out us having to write down and preserve a separate adapter for every of our enumeration courses. On the identical time, the code turns into simpler to keep up and refactor, and extra resilient to bugs.

Lucas Cavalcante Y Stelios Frantzeskakis are builders of Perry Avenue Software programpublishers of LGBTQ+ courting apps NAPE Y Jack’dwith greater than 30 million members worldwide.