JSONCodable
09 Dec 2015Soon after Swift was released a war started, a war that will be remembered as the JSON parsing libraries war.
The war is not over yet, if you’ve been following The iOS Times you might have noticed that every couple pf weeks a new parsing library is published.
If you ask me though, the war is over. And there is a winner*.
JSONCodable, by Matthew Cheok, provides decode and encode of JSON dictionaries to Swift objects, using Swift 2’s throw for proper error handling, and it uses of sharped focused protocols.
Let’s bring up the example app we used in the Alamofire tutorial and see how to manipulate JSON data using JSONCodable.
How to decode a JSON to a Swift object
In the toy app made for the Alamofire example we connected to the JSON placeholder APIs and get a list of posts.
We could write a model object representing the post using a struct for example.
struct Post {
let userId: Int
let id: Int
let title: String
let body: String
}
To be able to parse a JSON dictionary into a Post all we have to do is conform to the JSONDecodable protocol, which defines the init?(JSONDictionary: JSONObject) method, and describe the keys mappings.
extension Post: JSONDecodable {
init?(JSONDictionary: JSONObject) {
let decoder = JSONDecoder(object: JSONDictionary)
do {
id = try decoder.decode("id")
userId = try decoder.decode("userId")
title = try decoder.decode("title")
body = try decoder.decode("body")
} catch {
return nil
}
}
}
That’s it. Now we can replace the silly manual parsing in the app with a clearer and safer:
Alamofire.request(.GET, "http://jsonplaceholder.typicode.com/posts")
.responseJSON { response in
if let data = response.result.value as? [[String: AnyObject]] {
for postJSON in data {
if let post = Post(JSONDictionary: postJSON) {
self.posts.append(post)
}
}
} else {
// TODO: Handle errors
}
How to decode a JSON with nested objects
The library’s parsing power doesn’t end here. We can decode nested objects as well.
Consider this payload for a “user” resource:
{
"id": 1,
"name": "Leanne Graham",
"username": "Bret",
"address": {
"street": "Kulas Light",
"suite": "Apt. 556",
"city": "Gwenborough",
"zipcode": "92998-3874"
}
}
We can model the user resource like this:
struct User {
let id: Int
let name: String
let username: String
let address: Address
}
struct Address {
let street: String
let suite: String
let city: String
let zipCode: String
}
JSONCodable won’t break a sweat when parsing a user JSON dictionary, as long as we defined the mappings for both User and Address:
extension Address: JSONDecodable {
init?(JSONDictionary: JSONObject) {
let decoder = JSONDecoder(object: JSONDictionary)
do {
street = try decoder.decode("street")
suite = try decoder.decode("suite")
city = try decoder.decode("city")
zipCode = try decoder.decode("zipcode")
}
catch {
return nil
}
}
}
extension User: JSONDecodable {
init?(JSONDictionary: JSONObject) {
let decoder = JSONDecoder(object: JSONDictionary)
do {
id = try decoder.decode("id")
name = try decoder.decode("name")
username = try decoder.decode("username")
address = try decoder.decode("address")
} catch {
return nil
}
}
}
Ok you might say, but what about types that are not your usual String or Int, what if I had an NSURL?. Well, you can decode a string into an NSURL like this:
website = try decoder.decode("website", transformer: JSONTransformers.StringToNSURL)
JSONTransformer<EncodedType, DecodedType> is an object you can initialize passing two closures, one to go from EncodedType to DecodedType, the other to do the opposite.
In the case of the String to NSURL the transformer is created this way:
public struct JSONTransformers {
public static let StringToNSURL = JSONTransformer<String, NSURL>(
decoding: {NSURL(string: $0)},
encoding: {$0.absoluteString})
I’m sure you can already see how easy it is to create your custom transformers.
So far we’ve talked about how to decode a dictionary into an object, but how can we go the other way around?
How to encode a model into a JSON dictionary
The same way you can conform to the JSONDecodable protocol to go from JSON dictionary to Swift object, by conforming to JSONEncodable you can do the opposite.
extension Post: JSONEncodable {
func toJSON() throws -> AnyObject {
return try JSONEncoder.create { encoder in
try encoder.encode(id, key: "id")
try encoder.encode(userId, key: "userId")
try encoder.encode(title, key: "title")
try encoder.encode(body, key: "body")
}
}
}
Pro Tip
Writing strings values for keys multiple times is always dangerous as it is easy to make typos. We can have a stronger and easier to change system by leveraging Swift’s enum’s raw value feature:
extension Post {
enum JSONKeys: String {
case id = "id"
case userId = "userId"
case title = "title"
case body = "body"
}
}
Then get the key values like this:
extension Post: JSONEncodable {
func toJSON() throws -> AnyObject {
return try JSONEncoder.create { encoder in
try encoder.encode(id, key: Post.JSONKeys.id.rawValue)
try encoder.encode(userId, key: Post.JSONKeys.userId.rawValue)
try encoder.encode(title, key: Post.JSONKeys.title.rawValue)
try encoder.encode(body, key: Post.JSONKeys.body.rawValue)
}
}
}
There is a bit more typing involved, but it results in dryer code that is easier to maintain.
Next Steps
The iOS Times Advent Calendar’s post certainly don’t aim to be comprehensive tutorials on how to use a library, but rather provide a little taste of it, to ignite your curiosity.
I suggest you keep JSONCodable in mind when looking for a JSON parsing solution for your next project. Head over to the project’s repo on GitHub, or feel free to play around with the code from this tutorial.
That’s it for today. See you tomorrow with a library to describe layouts in code the right way. Subscribe to the email list to avoid missing out.
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To be honest with you my very favourite JSON parsing library is Argo, but that's because I like to feel smart and be using monadic operators.