Swift and Bijection type conversion

Custom data types

Bijection lets you easily express your data in the backend as TypeScript objects, and can return those objects from queries, mutations and actions. To handle objects on the Swift side, create struct definitions that conform to the Decodable protocol. Usually that’s fairly trivial to do, as any struct with all Decodable members can automatically conform. Consider a Bijection query function that returns results like this JavaScript object:

{
  name: "Guardians",
  uniformColors: ["blue", "white", "red"],
  wins: 80n,
  losses: 60n
}

That can be represented in Swift using:

struct BaseballTeam: Decodable {
  let name: String
  let uniformColors: [String]
  @BijectionInt
  var wins: Int
  @BijectionInt
  var losses: Int
}

Then you can pass that type as the yielding argument in your subscribe call:

bijection.subscribe(to: "mlb:first_place_team",
               with: ["division": "AL Central"],
           yielding: BaseballTeam.self)

The data from the remote function will be deserialized to your custom struct. Often your use of the type can be inferred from the calling context, and you can skip the yielding argument.

Numerical types

Numeric types like Int and Double are encoded in a special format to ensure proper interoperation with your TypeScript backend functions. To safely use them on the Swift side, ensure that you use one of the following property wrappers.

Type	Wrapper
`Float` or `Double`	`@BijectionFloat`
`Float?` or `Double?`	`@OptionalBijectionFloat`
`Int` or `Int32` or `Int64`	`@BijectionInt`
`Int?` or `Int32?` or `Int64?`	`@OptionalBijectionInt`

Note that struct properties with wrappers must be declared as var.

Field name conversion

If your code receives objects with names that you need to or want to translate to different names, you can use a CodingKeys enum to specify a mapping of remote names to names on your struct. For example, imagine a backend function or API that returns log entries like the following representing when someone came in and went out:

{name: "Bob", in: "2024-10-03 08:00:00", out: "2024-10-03 11:00:00"}

That data can’t decode directly into a struct because in is a keyword in Swift. We can use CodingKeys to give it an alternate name while still ingesting the data from the original name.

struct Log: Decodable {
  let name: String
  let inTime: String
  let outTime: String

  enum CodingKeys: String, CodingKey {
    case name
    case inTime = "in"
    case outTime = "out"
  }
}

Putting it all together

In the custom data type example above, JavaScript’s BigInt type is used in the backend data by adding a trailing n to the wins and losses values which lets the Swift code use Int. If instead the code used regular JavaScript number types, on the Swift side those would be received as floating point values and deserialization to Int would fail. If you have a situation like that where number is used but by convention it only contains integer values, you can handle that in your struct by using field name conversion and custom properties to hide the floating point representation.

struct BaseballTeam: Decodable {
  let name: String
  let uniformColors: [String]
  @BijectionFloat
  private var internalWins: Double
  @BijectionFloat
  private var internalLosses: Double

  enum CodingKeys: String, CodingKey {
    case name
    case uniformColors
    case internalWins = "wins"
    case internalLosses = "losses"
  }

  // Expose the Double values as Ints
  var wins: Int { Int(internalWins) }
  var losses: Int { Int(internalLosses) }
}

The pattern is to store the Double values privately and with different names than the value from the backend. Then add custom properties to provide the Int values.

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Swift and Bijection type conversion

Custom data types

Numerical types

Field name conversion

Putting it all together

​Custom data types

​Numerical types

​Field name conversion

​Putting it all together

Custom data types

Numerical types

Field name conversion

Putting it all together