Fragments in Apollo Kotlin

Apollo Kotlin supports both forms of GraphQL fragments:

• Named fragments, which enable you to reuse a set of fields across multiple operations

• Inline fragments, which enable you to access fields of polymorphic types

Named fragments

Take a look at the following GraphQL definitions:

1# Declaration of named fragment
2fragment LaunchFragment on Launch {
3  id
4  site
5  mission {
6    name
7  }
8}
9
10query LaunchDetails($id:ID!) {
11  launch(id: $id) {
12    # Usage of named fragment
13    ...LaunchFragment
14  }
15}

Based on the fragment definition, Apollo Kotlin generates the following LaunchFragment class that you can reuse in different queries:

1data class LaunchFragment(
2  val id: String,
3  val site: String?,
4  val mission: Mission?
5)

Generated models for operations that use this fragment have a .launchFragment property for accessing the fragment's fields:

1println("Mission site: ${launch.launchFragment?.site}")

The launchFragment property is null if the returned object is not a Launch.

You can reuse the fragment in any number of operations:

1 # ...previous definitions...
2
3query LaunchList {
4  launches {
5    launches {
6      ...LaunchFragment
7    }
8  }
9}

You can even use a fragment in operations that are defined in a different .graphql file from the fragment itself. This is because Apollo Kotlin codegen merges all .graphql files in to a single file before generating models.

Inline fragments

Take a look at this query definition that uses two inline fragments:

1query HeroForEpisode($ep: Episode!) {
2  hero(episode: $ep) {
3    name
4    ... on Droid {
5      primaryFunction
6    }
7    ... on Human {
8      height
9    }
10  }
11}

For this operation, Apollo Kotlin generates a Hero class that contains OnDroid and OnHuman nested classes. Hero also includes onDroid and onHuman fields that enable you to access fields that are specific to Droid and Human:

1println("Droid primaryFunction: ${hero.onDroid?.primaryFunction}")
2println("Human height: ${hero.onHuman?.height}")

These on<ShapeName> fields (onDroid and onHuman) are null if the returned object is not of the corresponding type.