+
+ Decodable
+
+Simple and strict, yet powerful object mapping made possible by Swift 2’s error handling. Greatly inspired by Argo, but without a bizillion functional operators.
+
+
+
+
+
+
+struct Repository {
+ let name: String
+ let description: String
+ let stargazersCount: Int
+ let language: String?
+ let sometimesMissingKey: String?
+
+ let owner: User // Struct conforming to Decodable
+ let defaultBranch: Branch // Struct NOT conforming to Decodable
+
+ var fullName: String { return "\(owner.login)/\(name)" }
+}
+
+extension Repository: Decodable {
+ static func decode(j: Any) throws -> Repository {
+ return try Repository(
+ name: j => "nested" => "name",
+ description: j => "description",
+ stargazersCount: j => "stargazers_count",
+ language: j => "language",
+ sometimesMissingKey: j =>? "sometimesMissingKey",
+ owner: j => "owner",
+ defaultBranch: Branch(name: j => "default_branch")
+ )
+ }
+}
+
+do {
+ let json = try NSJSONSerialization.JSONObjectWithData(data, options: [])
+ let repo = try [Repository].decode(json)
+} catch {
+ print(error)
+}
+
+How does it work?
+A protocol
+public protocol Decodable {
+ static func decode(json: Any) throws -> Self
+}
+
+A parse-function
+public func parse<T>(json: Any, path: [String], decode: (Any throws -> T)) throws -> T
+
+And shameless operator-overloading
+
+The too-many generated overloads, all calling the parse
-function, can be found in Overloads.swift. Return types include T?
, [T?]
, [T?]?
, Any
and [String: T]?
. When conditional protocol conformance is supported in Swift this won’t be necessary, and automatic decoding of infinitely nested generic types (like [[[[[[[[[A???]]: B]]]?]]?]]
) would work.
+
+An overload may look like this:
+public func => <T: Decodable>(json: Any, keyPath: KeyPath) throws -> T
+
+KeyPaths
+
+Keypaths can be created from string and array literals as well as with explicit initializers. They can also be joined using the operators =>
and =>?
. =>?
is another operator that indicates that nil
should be returned if the key to the right is missing.
+
+
+- When composing
=>
and =>?
operators in the same keypath, the strictness of =>
is still honoured.
+- Optional key paths (
=>?
) require an optional return type
+
+let a: KeyPath = "a"
+let b: KeyPath = ["a", "b"]
+let c: KeyPath = "a" => "b" => "c"
+let string: String? = json =>? "key1" => "key2" => "key3"`
+ ^^^^ allowed to be missing
+
+Errors
+
+Errors will be caught and rethrown in the decoding process to backpropagate metadata, like the JSON object that failed decoding, the key path to it, and the root JSON object.
+
+From DecodingError.swift:
+public enum DecodingError: ErrorProtocol, Equatable {
+ /// Thrown when optional casting from `Any` fails.
+ ///
+ /// This can happen both when trying to access a key on a object
+ /// that isn't a `NSDictionary`, and failing to cast a `Castable`
+ /// primitive.
+ case typeMismatch(expected: Any.Type, actual: Any.Type, Metadata)
+
+ /// Thrown when a given, required, key was not found in a dictionary.
+ case missingKey(String, Metadata)
+
+ /// Thrown from the `RawRepresentable` extension when
+ /// `init(rawValue:)` returned `nil`.
+ case rawRepresentableInitializationError(rawValue: Any, Metadata)
+
+ /// When an error is thrown that isn't `DecodingError`, it
+ /// will be wrapped in `DecodingError.other` in order to also provide
+ /// metadata about where the error was thrown.
+ case other(ErrorProtocol, Metadata)
+}
+
+let dict: NSDictionary = ["object": ["repo": ["owner": ["id" : 1, "login": "anviking"]]]]
+
+do {
+ let username: String = try dict => "object" => "repo" => "owner" => "name"
+} catch let error {
+ print(error)
+}
+//
+// MissingKeyError at object.repo.owner: name in {
+// id = 1;
+// login = anviking;
+// }
+
+Handling Errors
+
+Expressions like j => "key"
will throw directly, and catch
-statements can be used to create the most complex error handling behaviours. This also means that try?
can be used to return nil if anything goes wrong instead of throwing.
+
+For convenience there is an operator, =>?
, that only returns nil on missing keys, for APIs that indicate null
in that manner, and to aid working with different response formats.
+
+
+
+Overload |
+Null Behaviour |
+Missing Key Behavior |
+Type Mismatch Behaviour |
+Errors in subjects |
+
+
+
+=> -> T |
+throws |
+throws |
+throws |
+uncaught (throws) |
+
+
+=> -> T? |
+nil |
+throws |
+throws |
+uncaught (throws) |
+
+
+=>? -> T? |
+nil |
+nil |
+throws |
+uncaught (throws) |
+
+
+try? => -> T |
+nil |
+nil |
+nil |
+caught (nil) |
+
+
+Customization
+
+Int
, Double
,String
, Bool
, Date
(ISO8601), NSArray
, and NSDictionary
types that conform to DynamicDecodable
with the following declaration:
+public protocol DynamicDecodable {
+ associatedtype DecodedType
+ static var decoder: (Any) throws -> DecodedType {get set}
+}
+
+
+This allows Decodable to implement default decoding closures while allowing you to override them as needed.
+// Lets extend Bool.decoder so that it accepts certain strings:
+Bool.decoder = { json in
+ switch json {
+ case let str as String where str == "true":
+ return true
+ case let str as String where str == "false":
+ return false
+ default:
+ return try cast(json)
+ }
+}
+
+
+Note that when extending new types to conform to Decodable
there is really no point in conforming to DynamicDecodable
since you already control the implementation. Also note that the decoder
properties are intended as set once
. If you need different behaviour on different occasions, please create custom decode functions.
+
+The default Date.decoder
uses a ISO8601 date formatter. If you don’t want to create your own decode closure there’s a helper:
+Date.decoder = Date.decoder(using: formatter)
+
+
+
+Don’t be afraid of not conforming to Decodable
.
+let array = try NSArray.decode(json => "list").map {
+ try Contribution(json: $0, repository: repo)
+}
+
+Tips
+
+
+- You can use
Decodable
with classes. Just make sure to either call a required
initializer on self (e.g self.init
) and return Self
, or make your class final
. ( This might be a problem though)
+- The
Decodable
-protocol and the =>
-operator should in no way make you committed to use them everywhere.
+
+Compatibility
+
+
+
+Swift version |
+Compatible tag or branch |
+
+
+
+Swift 5.0 |
+0.7.0 |
+
+
+Swift 4.0 |
+0.6.0 |
+
+
+Swift 3.0 |
+v0.5 |
+
+
+Swift 2.3 |
+v0.4.4 |
+
+
+Swift 2.2 |
+v0.4.3 |
+
+
+Note on Swift 4.0 usage
+
+Due to collisions with the standard library you will have to import ambiguous symbols specifically, in addition to Decodable
as a whole.
+
+This means you likely want the following
+import Decodable
+import protocol Decodable.Decodable
+
+
+and you can import other symbols, e.g KeyPath
, DecodingError
, in a similar fashion (using import struct
and import enum
)
+
+
+