How do you parse recursive JSON in Swift? - json

Question

I am receiving JSON from a server where the data is recursive. What is the best way to parse this into a convenient Swift data structure?
Defining a Swift Codable data structure to parse it into fails because the recursive properties are not allowed.
The Swift compiler reports: "Value type 'FamilyTree.Person' cannot have a stored property that recursively contains it"
{
"familyTree": {
"rootPerson": {
"name": "Tom",
"parents": {
"mother": {
"name": "Ma",
"parents": {
"mother": {
"name": "GraMa",
"parents": {}
},
"father": {
"name": "GraPa",
"parents": {}
}
}
},
"father": {
"name": "Pa",
"parents": {}
}
}
}
}
}
Ideally the end result would be a bunch of person objects pointing to their mother and father objects starting from a rootPerson object.

Answer 1

The first thought is to create structs such as:
struct Person: Codable {
var name: String
var parents: Parents
}
struct Parents: Codable {
var mother: Person?
var father: Person?
}
But this doesn't work since you can't have recursive stored properties like this.
Here is one possible working solution:
let json = """
{
"familyTree": {
"rootPerson": {
"name": "Tom",
"parents": {
"mother": {
"name": "Ma",
"parents": {
"mother": {
"name": "GraMa",
"parents": {}
},
"father": {
"name": "GraPa",
"parents": {}
}
}
},
"father": {
"name": "Pa",
"parents": {}
}
}
}
}
}
"""
struct Person: Codable {
var name: String
var parents: [String: Person]
}
struct FamilyTree: Codable {
var rootPerson: Person
}
struct Root: Codable {
var familyTree: FamilyTree
}
let decoder = JSONDecoder()
let tree = try decoder.decode(Root.self, from: json.data(using: .utf8)!)
print(tree)
In a playground this will correctly parse the JSON.
The parents dictionary of Person will have keys such as "mother" and "father". This supports a person have any number of parents with any role.

Answer 2

Possible implementation using classes.
(Swift 5 synthesizes default initializers for classes. Dont remember if so for Swift 4)
import Foundation
var json: String = """
{
"familyTree": {
"rootPerson": {
"name": "Tom",
"parents": {
"mother": {
"name": "Ma",
"parents": {
"mother": {
"name": "GraMa",
"parents": {}
},
"father": {
"name": "GraPa",
"parents": {}
}
}
},
"father": {
"name": "Pa",
"parents": {}
}
}
}
}
}
"""
final class Parents: Codable{
let mother: Person?
let father: Person?
}
final class Person: Codable{
let name: String
let parents: Parents?
}
final class RootPerson: Codable {
var rootPerson: Person
}
final class Root: Codable {
var familyTree: RootPerson
}
var jsonData = json.data(using: .utf8)!
do{
let familyTree = try JSONDecoder().decode(Root.self, from: jsonData)
print("Ma •••>", familyTree.familyTree.rootPerson.parents?.mother?.name)
print("GraPa •••>", familyTree.familyTree.rootPerson.parents?.mother?.parents?.father?.name)
print("Shoud be empty •••>", familyTree.familyTree.rootPerson.parents?.mother?.parents?.father?.parents?.father?.name)
} catch {
print(error)
}