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Programatically writing query selection#

Selection lets you select fields that you want to fetch from the query on a particular type.

SwiftGraphQL has generated phantom types for your operations, objects, interfaces and unions. You can find them by typing Unions./Interfaces./Objects./Operations. followed by a name from your GraphQL schema. You plug those into the Scope parameter.

The other parameter Type is what your constructor should return.

We generate type alias in selection which let you use XCode intellisnse and may infer your return type. They work like Selection.ObjectName.

Nullable, list, and non-nullable fields#

Selection packs a collection of utility functions that let you select nullable and list fields using your existing selecitons. Each selection comes with three calculated properties that let you do that:

  • list - to query lists
  • nullable - to query nullable fields
  • nonNullOrFail - to query nullable fields that should be there
// Create a non-nullable selection. let human = Selection.Human { Human( id: try $, name: try $ ) } // Use it with nullable and list fields. let query = Selection.Query { let list = try $0.humans(human.list) let nullable = try $0.human(id: "100", human.nullable) }

You can achieve the same effect using Selection static functions .list, .nullable, and .nonNullOrFail.

// Use it with nullable and list fields. let query = Selection.Query { let list = try $0.humans(Selection.list(human)) }
Making selection on the entire type#

You might want to write a selection on the entire type from the selection composer itself. This usually happens if you have a distinct identifier reused in many types.

Consider the following scenario where we have an id field in Human type. There are many cases where we only query id field from the Human that's why we create a human id selection.

let humanId = Selection<HumanID, Objects.Human> { HumanID.fromString(try $ }

Now, we want to reuse that same selection when query a detailed human type. To do that, we can use selection helper method that lets you make a selection on the whole TypeLock from inside the selection.

struct Human { let id: HumanID let name: String } let human = Selection.Human { Human( id: try $0.selection(humanId), name: try $ ) }

An alternative approach would be to manually rewrite the selection inside Human again.

let human = Selection.Human { Human( id: HumanID.fromString(try $, name: try $ ) }

Having distinct types for ids of different object types is particularly useful in large projects as it gives you verification that you are not using a wrong identifier for a particular type of field. At first, this might seem useless and cumbersome, but it makes your code more robust once you get used to it.

Mapping Selection#

You might want to map the result of your selection to a new type and get a selection for that new type. You can do that by calling a map function on selection and provide a mapping.

struct Human { let id: String let name: String } // Create a selection. let human = Selection.Human { Human( id: try $, name: try $, ) } // Map the original selection on Human to return String. let humanName: Selection<String, Objects.Human> = { $ }

⚠️ Don't make any nested calls to the API. Use the first half of the initializer to fetch all the data and return the calculated result. Just don't make nested requests.

// WRONG! let human = Selection.Human { select in let message: String if try select.likesStrawberries() { message = try } else { message = try select.homePlanet() } return message } // Correct. let human = Selection.Human { select in /* Data */ let likesStrawberries = try select.likesStrawberries() let name = try let homePlanet = try select.homePlanet() /* Return */ let message: String if likesStrawberries { message = name } else { message = homePlanet } return message }


When fetching a union you should provide selections for each of the union sub-types. Additionally, all of those selections should resolve to the same type.

let characterUnion = Selection.CharacterUnion { try $0.on( human: .init { try $0.funFact() /* String */ }, droid: .init { try $0.primaryFunction() /* String */ } ) }

You'd usually want to create a Swift enumerator and have different selecitons return different cases.


Interfaces are very similar to unions. The only difference is that you may query for a common field from the intersection.

let characterInteface = Selection.Character { /* Common */ let name = try $ /* Fragments */ let about = try $0.on( droid: Selection<String, Objects.Droid> { droid in try droid.primaryFunction() /* String */ }, human: Selection<String, Objects.Human> { human in try human.homePlanet() /* String */ } ) return "\(name). \(about)" }

You'd usually want to create a Swift enumerator and have different selecitons return different cases.


  • SwiftGraphQL

GraphQL's null value in an input type may be entirely omitted to represent the absence of a value or supplied as null to provide null value. This comes in especially handy in mutations.

Because of that, every input object that has an optional property accepts an optional argument that may either be .present(value), .absent() or .null(). We use functions to support recursive type annotations that GraphQL allows.

NOTE: Every nullable argument is by default absent so you don't have to write boilerplate.