How to decode a nested JSON struct with Swift Decodable protocol? - json

Question

Here is my JSON
{
"id": 1,
"user": {
"user_name": "Tester",
"real_info": {
"full_name":"Jon Doe"
}
},
"reviews_count": [
{
"count": 4
}
]
}
Here is the structure I want it saved to (incomplete)
struct ServerResponse: Decodable {
var id: String
var username: String
var fullName: String
var reviewCount: Int
enum CodingKeys: String, CodingKey {
case id,
// How do i get nested values?
}
}
I have looked at Apple's Documentation on decoding nested structs, but I still do not understand how to do the different levels of the JSON properly. Any help will be much appreciated.

Answer 1

Another approach is to create an intermediate model that closely matches the JSON (with the help of a tool like quicktype.io), let Swift generate the methods to decode it, and then pick off the pieces that you want in your final data model:
// snake_case to match the JSON and hence no need to write CodingKey enums
fileprivate struct RawServerResponse: Decodable {
struct User: Decodable {
var user_name: String
var real_info: UserRealInfo
}
struct UserRealInfo: Decodable {
var full_name: String
}
struct Review: Decodable {
var count: Int
}
var id: Int
var user: User
var reviews_count: [Review]
}
struct ServerResponse: Decodable {
var id: String
var username: String
var fullName: String
var reviewCount: Int
init(from decoder: Decoder) throws {
let rawResponse = try RawServerResponse(from: decoder)
// Now you can pick items that are important to your data model,
// conveniently decoded into a Swift structure
id = String(rawResponse.id)
username = rawResponse.user.user_name
fullName = rawResponse.user.real_info.full_name
reviewCount = rawResponse.reviews_count.first!.count
}
}
This also allows you to easily iterate through reviews_count, should it contain more than 1 value in the future.

Answer 2

In order to solve your problem, you can split your RawServerResponse implementation into several logic parts (using Swift 5).
#1. Implement the properties and required coding keys
import Foundation
struct RawServerResponse {
enum RootKeys: String, CodingKey {
case id, user, reviewCount = "reviews_count"
}
enum UserKeys: String, CodingKey {
case userName = "user_name", realInfo = "real_info"
}
enum RealInfoKeys: String, CodingKey {
case fullName = "full_name"
}
enum ReviewCountKeys: String, CodingKey {
case count
}
let id: Int
let userName: String
let fullName: String
let reviewCount: Int
}
#2. Set the decoding strategy for id property
extension RawServerResponse: Decodable {
init(from decoder: Decoder) throws {
// id
let container = try decoder.container(keyedBy: RootKeys.self)
id = try container.decode(Int.self, forKey: .id)
/* ... */
}
}
#3. Set the decoding strategy for userName property
extension RawServerResponse: Decodable {
init(from decoder: Decoder) throws {
/* ... */
// userName
let userContainer = try container.nestedContainer(keyedBy: UserKeys.self, forKey: .user)
userName = try userContainer.decode(String.self, forKey: .userName)
/* ... */
}
}
#4. Set the decoding strategy for fullName property
extension RawServerResponse: Decodable {
init(from decoder: Decoder) throws {
/* ... */
// fullName
let realInfoKeysContainer = try userContainer.nestedContainer(keyedBy: RealInfoKeys.self, forKey: .realInfo)
fullName = try realInfoKeysContainer.decode(String.self, forKey: .fullName)
/* ... */
}
}
#5. Set the decoding strategy for reviewCount property
extension RawServerResponse: Decodable {
init(from decoder: Decoder) throws {
/* ...*/
// reviewCount
var reviewUnkeyedContainer = try container.nestedUnkeyedContainer(forKey: .reviewCount)
var reviewCountArray = [Int]()
while !reviewUnkeyedContainer.isAtEnd {
let reviewCountContainer = try reviewUnkeyedContainer.nestedContainer(keyedBy: ReviewCountKeys.self)
reviewCountArray.append(try reviewCountContainer.decode(Int.self, forKey: .count))
}
guard let reviewCount = reviewCountArray.first else {
throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: container.codingPath + [RootKeys.reviewCount], debugDescription: "reviews_count cannot be empty"))
}
self.reviewCount = reviewCount
}
}
Complete implementation
import Foundation
struct RawServerResponse {
enum RootKeys: String, CodingKey {
case id, user, reviewCount = "reviews_count"
}
enum UserKeys: String, CodingKey {
case userName = "user_name", realInfo = "real_info"
}
enum RealInfoKeys: String, CodingKey {
case fullName = "full_name"
}
enum ReviewCountKeys: String, CodingKey {
case count
}
let id: Int
let userName: String
let fullName: String
let reviewCount: Int
}
extension RawServerResponse: Decodable {
init(from decoder: Decoder) throws {
// id
let container = try decoder.container(keyedBy: RootKeys.self)
id = try container.decode(Int.self, forKey: .id)
// userName
let userContainer = try container.nestedContainer(keyedBy: UserKeys.self, forKey: .user)
userName = try userContainer.decode(String.self, forKey: .userName)
// fullName
let realInfoKeysContainer = try userContainer.nestedContainer(keyedBy: RealInfoKeys.self, forKey: .realInfo)
fullName = try realInfoKeysContainer.decode(String.self, forKey: .fullName)
// reviewCount
var reviewUnkeyedContainer = try container.nestedUnkeyedContainer(forKey: .reviewCount)
var reviewCountArray = [Int]()
while !reviewUnkeyedContainer.isAtEnd {
let reviewCountContainer = try reviewUnkeyedContainer.nestedContainer(keyedBy: ReviewCountKeys.self)
reviewCountArray.append(try reviewCountContainer.decode(Int.self, forKey: .count))
}
guard let reviewCount = reviewCountArray.first else {
throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: container.codingPath + [RootKeys.reviewCount], debugDescription: "reviews_count cannot be empty"))
}
self.reviewCount = reviewCount
}
}
Usage
let jsonString = """
{
"id": 1,
"user": {
"user_name": "Tester",
"real_info": {
"full_name":"Jon Doe"
}
},
"reviews_count": [
{
"count": 4
}
]
}
"""
let jsonData = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()
let serverResponse = try! decoder.decode(RawServerResponse.self, from: jsonData)
dump(serverResponse)
/*
prints:
▿ RawServerResponse #1 in __lldb_expr_389
- id: 1
- user: "Tester"
- fullName: "Jon Doe"
- reviewCount: 4
*/

Answer 3

Rather than having one big CodingKeys enumeration with all the keys you'll need for decoding the JSON, I would advise splitting the keys up for each of your nested JSON objects, using nested enumerations to preserve the hierarchy:
// top-level JSON object keys
private enum CodingKeys : String, CodingKey {
// using camelCase case names, with snake_case raw values where necessary.
// the raw values are what's used as the actual keys for the JSON object,
// and default to the case name unless otherwise specified.
case id, user, reviewsCount = "reviews_count"
// "user" JSON object keys
enum User : String, CodingKey {
case username = "user_name", realInfo = "real_info"
// "real_info" JSON object keys
enum RealInfo : String, CodingKey {
case fullName = "full_name"
}
}
// nested JSON objects in "reviews" keys
enum ReviewsCount : String, CodingKey {
case count
}
}
This will make it easier to keep track of the keys at each level in your JSON.
Now, bearing in mind that:
A keyed container is used to decode a JSON object, and is decoded with a CodingKey conforming type (such as the ones we've defined above).
An unkeyed container is used to decode a JSON array, and is decoded sequentially (i.e each time you call a decode or nested container method on it, it advances to the next element in the array). See the second part of the answer for how you can iterate through one.
After getting your top-level keyed container from the decoder with container(keyedBy:) (as you have a JSON object at the top-level), you can repeatedly use the methods:
nestedContainer(keyedBy:forKey:) to get a nested object from an object for a given key
nestedUnkeyedContainer(forKey:) to get a nested array from an object for a given key
nestedContainer(keyedBy:) to get the next nested object from an array
nestedUnkeyedContainer() to get the next nested array from an array
For example:
struct ServerResponse : Decodable {
var id: Int, username: String, fullName: String, reviewCount: Int
private enum CodingKeys : String, CodingKey { /* see above definition in answer */ }
init(from decoder: Decoder) throws {
// top-level container
let container = try decoder.container(keyedBy: CodingKeys.self)
self.id = try container.decode(Int.self, forKey: .id)
// container for { "user_name": "Tester", "real_info": { "full_name": "Jon Doe" } }
let userContainer =
try container.nestedContainer(keyedBy: CodingKeys.User.self, forKey: .user)
self.username = try userContainer.decode(String.self, forKey: .username)
// container for { "full_name": "Jon Doe" }
let realInfoContainer =
try userContainer.nestedContainer(keyedBy: CodingKeys.User.RealInfo.self,
forKey: .realInfo)
self.fullName = try realInfoContainer.decode(String.self, forKey: .fullName)
// container for [{ "count": 4 }] – must be a var, as calling a nested container
// method on it advances it to the next element.
var reviewCountContainer =
try container.nestedUnkeyedContainer(forKey: .reviewsCount)
// container for { "count" : 4 }
// (note that we're only considering the first element of the array)
let firstReviewCountContainer =
try reviewCountContainer.nestedContainer(keyedBy: CodingKeys.ReviewsCount.self)
self.reviewCount = try firstReviewCountContainer.decode(Int.self, forKey: .count)
}
}
Example decoding:
let jsonData = """
{
"id": 1,
"user": {
"user_name": "Tester",
"real_info": {
"full_name":"Jon Doe"
}
},
"reviews_count": [
{
"count": 4
}
]
}
""".data(using: .utf8)!
do {
let response = try JSONDecoder().decode(ServerResponse.self, from: jsonData)
print(response)
} catch {
print(error)
}
// ServerResponse(id: 1, username: "Tester", fullName: "Jon Doe", reviewCount: 4)
Iterating through an unkeyed container
Considering the case where you want reviewCount to be an [Int], where each element represents the value for the "count" key in the nested JSON:
"reviews_count": [
{
"count": 4
},
{
"count": 5
}
]
You'll need to iterate through the nested unkeyed container, getting the nested keyed container at each iteration, and decoding the value for the "count" key. You can use the count property of the unkeyed container in order to pre-allocate the resultant array, and then the isAtEnd property to iterate through it.
For example:
struct ServerResponse : Decodable {
var id: Int
var username: String
var fullName: String
var reviewCounts = [Int]()
// ...
init(from decoder: Decoder) throws {
// ...
// container for [{ "count": 4 }, { "count": 5 }]
var reviewCountContainer =
try container.nestedUnkeyedContainer(forKey: .reviewsCount)
// pre-allocate the reviewCounts array if we can
if let count = reviewCountContainer.count {
self.reviewCounts.reserveCapacity(count)
}
// iterate through each of the nested keyed containers, getting the
// value for the "count" key, and appending to the array.
while !reviewCountContainer.isAtEnd {
// container for a single nested object in the array, e.g { "count": 4 }
let nestedReviewCountContainer = try reviewCountContainer.nestedContainer(
keyedBy: CodingKeys.ReviewsCount.self)
self.reviewCounts.append(
try nestedReviewCountContainer.decode(Int.self, forKey: .count)
)
}
}
}

Answer 4

Copy the json file to https://app.quicktype.io
Select Swift (if you use Swift 5, check the compatibility switch for Swift 5)
Use the following code to decode the file
Voila!
let file = "data.json"
guard let url = Bundle.main.url(forResource: "data", withExtension: "json") else{
fatalError("Failed to locate \(file) in bundle.")
}
guard let data = try? Data(contentsOf: url) else{
fatalError("Failed to locate \(file) in bundle.")
}
let yourObject = try? JSONDecoder().decode(YourModel.self, from: data)

Answer 5

Many good answers have already been posted, but there is a simpler method not described yet IMO.
When the JSON field names are written using snake_case_notation you can still use the camelCaseNotation in your Swift file.
You just need to set
decoder.keyDecodingStrategy = .convertFromSnakeCase
After this ☝️ line Swift will automatically match all the snake_case fields from the JSON to the camelCase fields in the Swift model.
E.g.
user_name` -> userName
reviews_count -> `reviewsCount
...
Here's the full code
1. Writing the Model
struct Response: Codable {
let id: Int
let user: User
let reviewsCount: [ReviewCount]
struct User: Codable {
let userName: String
struct RealInfo: Codable {
let fullName: String
}
}
struct ReviewCount: Codable {
let count: Int
}
}
2. Setting the Decoder
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
3. Decoding
do {
let response = try? decoder.decode(Response.self, from: data)
print(response)
} catch {
debugPrint(error)
}

Answer 6

Also you can use library KeyedCodable I prepared. It will require less code. Let me know what you think about it.
struct ServerResponse: Decodable, Keyedable {
var id: String!
var username: String!
var fullName: String!
var reviewCount: Int!
private struct ReviewsCount: Codable {
var count: Int
}
mutating func map(map: KeyMap) throws {
var id: Int!
try id <<- map["id"]
self.id = String(id)
try username <<- map["user.user_name"]
try fullName <<- map["user.real_info.full_name"]
var reviewCount: [ReviewsCount]!
try reviewCount <<- map["reviews_count"]
self.reviewCount = reviewCount[0].count
}
init(from decoder: Decoder) throws {
try KeyedDecoder(with: decoder).decode(to: &self)
}
}