Custom Scalars

In addition to its built-in scalar types (Int, String, etc.), GraphQL supports defining custom scalars. For example, your schema might provide a custom scalar for Date, UUID, or GeoLocation.

Custom Scalars are initially defined as part of a schema. To interact with a schema using custom scalars, your client must define a Swift type to use for each custom scalar.

Apollo iOS automatically defines Swift types for all of the built-in scalar types:

By default, each custom scalar is treated as a Swift String, but you can customize the type of all of your custom scalars with Apollo iOS!

Generating custom scalar types

If any part of your GraphQL application references a custom scalar defined by the schema, a file for it will be generated in your generated schema output. This file can be used to define the Swift type for the custom scalar.

For example, in a schema that defines a custom scalar:

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scalar UUID

Apollo iOS generates a UUID custom scalar type in your generated schema output. This generated file defines UUID as a String by default.

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public extension MySchema {
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  typealias UUID = String
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}

The MySchema.UUID type will be referenced in any other generated objects that reference the UUID scalar.

Because custom scalar files are only generated once, they can then be edited, and your changes will never be overwritten by subsequent code generation execution.

You can edit this file to define a different type for the UUID scalar type.

Defining a custom scalar type

You can define the type for a custom scalar by creating a new type, or by pointing the typealias to another existing type.

The type for your custom scalar must conform to the CustomScalarType protocol. This requires you to implement the JSON serialization functionality for your custom scalar type.

To implement the CustomScalarType protocol:

1. Implement the _jsonValue property.

This converts the scalar value into a JSONValue that can be serialized into a JSON dictionary. The value of the _jsonValue property will be stored in the cache as the JSON value for the custom scalar.

Usually, this should be identical to the value received from the network response to represent the custom scalar.

2. Implement the init(_jsonValue:) initializer.

This initializer is used to construct the custom scalar value from a JSONValue. When constructing the scalar from a network response, the value of the _jsonValue parameter will be the value from the network response. When constructing the scalar from a cached value, this will be the value provided in the _jsonValue property.

If the value provided is unrecognized, you should throw an error from this function. Apollo iOS provides JSONDecodingError in the ApolloAPI library, but you may throw any custom error you wish.

3. Implement the Hashable and Equatable protocols if needed.

If your custom scalar type does not already conform to Hashable and Equatable, you will need to implement hash(into:) and the == operator to conform to each of these protocols respectively.

Example: UUID

For example, you could point the typealias for UUID to the Foundation.UUID type:

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import Foundation
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public extension MySchema {
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  typealias UUID = Foundation.UUID
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}
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extension Foundation.UUID: CustomScalarType {
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  public init (_jsonValue value: JSONValue) throws {
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    guard let uuidString = value as? String,
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    let uuid = UUID(uuidString: uuidString) else {
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      throw JSONDecodingError.couldNotConvert(value: value, to: Foundation.UUID.self)
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    }
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    self = uuid
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  }
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  public var _jsonValue: JSONValue {
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    uuidString
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  }
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}

Example: GeoPoint

Alternatively, you could create your own custom scalar type. In this case, replace the typealias with the new type.

For example, a custom scalar GeoPoint that has a JSON representation of "100.0,10.0" could be implemented like this:

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import Foundation
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extension MySchema {
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    public struct GeoPoint: CustomScalarType, Hashable {
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        let x: Float
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        let y: Float
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        public init (_jsonValue value: JSONValue) throws {
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            let coordinates = try (value as? String)?
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            .components(separatedBy: ",")
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            .map { try Float(_jsonValue: $0) }
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            guard let coordinates, coordinates.count == 2 else {
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                throw JSONDecodingError.couldNotConvert(value: value, to: GeoPoint.self)
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            }
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            self.x = coordinates[0]
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            self.y = coordinates[1]
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        }
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        public var _jsonValue: JSONValue {
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            "\(String(format: "%.1f", x)),\(String(format: "%.1f", y))"
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        }
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    }
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}

The GeoPoint struct conforms to both CustomScalarType and Hashable. You must explicitly declare the conformance to Hashable, which inherits Equatable conformance. Because Swift can synthesize the Hashable and Equatable conformancs here, you do not need to implement them.

The init(_jsonValue:) initializer casts the JSONValue as a String and separates it into two coordinates. It converts those coordinates using Float(_jsonValue:) which is provided by Apollo. Each of the built-in scalar types has JSON serialization support that you can use within your custom scalar implementations.

To ensure consistency of the serialized JSON, the _jsonValue function ensures that the coordinates are formatted with a single decimal point using String(format:,_:).

JSON and other custom scalars with multiple return types

Some custom scalars are set up to potentially return multiple types at runtime. This is not ideal since you lose type safety, but if you're using an API you don't have control over, there's often not a great alternative to this.

When this happens, because you don't know the type that's coming in, you can't set up a single typealias for that scalar. Instead, you need to define some other way of instantiating your custom scalar object.

This happens most often with JSON, which can return either an array or a dictionary. Here's an example of how you can use an enum to allow dynamic-but-limited types to parse (with CustomJSON as a placeholder type name`):

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extension MySchema {
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    public enum CustomJSON: CustomScalarType, Hashable {
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        case dictionary([String: AnyHashable])
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        case array([AnyHashable])
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        public init(_jsonValue value: JSONValue) throws {
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            if let dict = value as? [String: AnyHashable] {
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                self = .dictionary(dict)
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            } else if let array = value as? [AnyHashable] {
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                self = .array(array)
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            } else {
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                throw JSONDecodingError.couldNotConvert(value: value, to: CustomJSON.self)
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            }
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        }
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        public var _jsonValue: JSONValue {
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            switch self {
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            case let .dictionary(json as AnyHashable),
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                 let .array(json as AnyHashable):
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                return json
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            }
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        }
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        public static func == (lhs: CustomJSON, rhs: CustomJSON) -> Bool {
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            lhs._jsonValue == rhs._jsonValue
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        }
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        public func hash(into hasher: inout Hasher) {
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            hasher.combine(_jsonValue)
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        }
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    }
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}