swift - deserializing json

Deserializing JSON

Swift provides an easy way to deserialize JSON to Swift objects via JSONDecoder.

import Foundation

var json = """
{
"climberId": 42,
"name": "Jess",
"type": "boulder",
"grade": 6,
}
""".data(using: .utf8)!

// create a struct called "Climber" that conforms to Codable
// the struct should have properties with matching names and data types to the JSON

struct Climber: Codable {
    let climberId: Int
    let name: String
    let type: String
    let grade: Int
}

// create a JSON decoder
let decoder = JSONDecoder()

// decode the JSON into your model object and print the result
do {
    let climber = try decoder.decode(Climber.self, from: json)
    print(climber) // prints Climber(climberId: 42, name: "Jess", type: "boulder", grade: 6)
} catch {
    print(error)
}

Mapping JSON Keys to Swift

What if the JSON keys aren’t easily mappable to swift? That’s ok! Use CodingKey protocol to map them as you wish.

import Foundation

var json = """
{
    "climber id": 42,
    "name": "Jess",
    "type_of_climb": "boulder",
    "grade": 6,
}
""".data(using: .utf8)!

// create a struct called "Climber" that conforms to Codable
// the struct should have properties with matching names and data types to the JSON

struct Climber: Codable {
    let climberId: Int
    let name: String
    let type: String
    let grade: Int

    // add CodingKeys enum here
    enum CodingKeys: String, CodingKey {
        case climberId = "climber id"
        case name = "name"
        case type = "type_of_climb"
        case grade = "grade" 
    }
}

// create a JSON decoder
let decoder = JSONDecoder()

// decode the JSON into your model object and print the result
do {
    let climber = try decoder.decode(Climber.self, from: json)
    print(climber) // prints Climber(climberId: 42, name: "Jess", type: "boulder", grade: 6)
} catch {
    print(error)
}

Mapping Arrays

import Foundation

var json = """
[
    {
        "climber id": 42,
        "name": "Jess",
        "type_of_climb": "boulder",
        "grade": 6,
        "ascent": ["01/05/2020", "02/10/2020", "02/12/2020"]
    },
    {
        "climber id": 42,
        "name": "Jess",
        "type_of_climb": "boulder",
        "grade": 8,
        "ascent": []
    }
]
""".data(using: .utf8)!

struct Climber: Codable {
    let climberId: Int
    let name: String
    let type: String
    let grade: Int
    let ascent: [String]

    // add CodingKeys enum here
    enum CodingKeys: String, CodingKey {
        case climberId = "climber id"
        case name = "name"
        case type = "type_of_climb"
        case grade
        case ascent
    }
}

// create a JSON decoder
let decoder = JSONDecoder()

// decode the JSON into your model object and print the result
do {
    let climber = try decoder.decode([Climber].self, from: json)
    print(climber) // prints [workspace.Climber(climberId: 42, name: "Jess", type: "boulder", grade: 6, ascent: ["01/05/2020", "02/10/2020", "02/12/2020"]), workspace.Climber(climberId: 42, name: "Jess", type: "boulder", grade: 8, ascent: [])]
} catch {
    print(error)
}

Nested JSON

What if the JSON contains nested fields? No Problem! Swifth handles it as well! Create a typed struct that conforms to the Codable protocol and reference that type in the parent struct

import Foundation

var json = """
{
        "climber id": 42,
        "name": "Jess",
        "type_of_climb": "boulder",
        "grade": 6,
        "ascent": {
            "date": "01/05/2020",
            "type": "redpoint"
        }
    }
""".data(using: .utf8)!

struct Ascent: Codable {
    let date: String
    let type: String
}

struct Climber: Codable {
    let climberId: Int
    let name: String
    let type: String
    let grade: Int
    let ascent: Ascent

    // add CodingKeys enum here
    enum CodingKeys: String, CodingKey {
        case climberId = "climber id"
        case name = "name"
        case type = "type_of_climb"
        case grade = "grade"
        case ascent
    }
}

// create a JSON decoder
let decoder = JSONDecoder()

// decode the JSON into your model object and print the result
do {
    let climber = try decoder.decode(Climber.self, from: json)
    print(climber) // prints Climber(climberId: 42, name: "Jess", type: "boulder", grade: 6, ascent: workspace.Ascent(date: "01/05/2020", type: "redpoint"))
} catch {
    print(error)
}

Generic Function for GET Requests

class func taskForGETRequest<ResponseType: Codable>(url: URL, responseType: ResponseType.Type, completion: @escaping (ResponseType?, Error?) -> Void) {
    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data else {
            DispatchQueue.main.async {
                completion(nil, error)
            }
            return
        }
        let decoder = JSONDecoder()
        do {
            let responseObject = try decoder.decode(ResponseType.self, from: data)
            DispatchQueue.main.async {
                completion(responseObject, error)
            }
        } catch {
            DispatchQueue.main.async {
                completion(nil, error)
            }
        }
    }
    task.resume()
}

Example

Here's an example of how one would call the above method:

class func getFavorites(completion: @escaping ([Movie], Error?) -> Void) {
    taskForGETRequest(url: Endpoints.getFavorites.url, responseType: MovieResults.self) { (response, error) in
        if let response = response {
            completion(response.results, nil)
        }
        else {
            completion([], error)
        }
    }
}