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:

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# Declaration of named fragment
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fragment LaunchFragment on Launch {
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  id
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  site
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  mission {
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    name
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  }
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}
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query LaunchDetails($id:ID!) {
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  launch(id: $id) {
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    # Usage of named fragment
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    ...LaunchFragment
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  }
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}

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

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data class LaunchFragment(
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  val id: String,
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  val site: String?,
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  val mission: Mission?
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)

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

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

You can reuse the fragment in any number of operations:

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# ...previous definitions...
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query LaunchList {
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  launches {
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    launches {
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      ...LaunchFragment
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    }
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  }
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}

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:

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query HeroForEpisode($ep: Episode!) {
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  hero(episode: $ep) {
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    name
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    ... on Droid {
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      primaryFunction
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    }
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    ... on Human {
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      height
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    }
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  }
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}

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:

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