Swift Tree Structure, encoding and accessing nodes - json

Question

I am having trouble figuring out why the following does not run. The goal is to create a tree structure with reference types that can be saved to a json file, but the following playground code does not run.
The root node has a nil parent, but I thought the encoder ignored nil values. In my app I get an EXC_BAD_ACCESS. Does this need to be done with structs instead of classes, and if so, is there a way to access a particular node without traversing the entire tree? Any help appreciated.
import Cocoa
final class Node: Codable {
var id: UUID
var data: [MyData]
var children: [Node]
var parent: Node? = nil
init() {
self.id = UUID()
self.data = []
self.children = []
}
func add(data: MyData) {
data.parent = self
self.data.append(data)
}
func add(child: Node) {
child.parent = self
self.children.append(child)
}
}
final class MyData: Codable {
var id: UUID
var label: String
var value: String
var parent: Node? = nil
init(label: String, value: String) {
self.id = UUID()
self.label = label
self.value = value
}
}
var root = Node()
root.add(data: MyData(label: "label 1", value: "value 1"))
root.add(data: MyData(label: "label 2", value: "value 2"))
var child = Node()
child.add(data: MyData(label: "label 3", value: "value 3"))
root.add(child: child)
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let json = try encoder.encode(root)
print(String(data: json, encoding: .utf8)!)

Answer 1

The problem is that by default Codable tries to encode/decode all properties of a conforming type. This means that the children try to encode/decode their parent, which parent contains the children as well, hence leading to an infinite loop.
You need to manually specify which properties to encode/decode by providing a CodingKey conformant type. By leaving out the parent property from both Node.CodingKeys and MyData.CodingKeys, you resolve the infinite loop.
import Foundation
final class Node: Codable {
let id = UUID()
var data: [MyData]
var children: [Node]
weak var parent: Node? = nil
init() {
self.data = []
self.children = []
}
func add(data: MyData) {
data.parent = self
self.data.append(data)
}
func add(child: Node) {
child.parent = self
self.children.append(child)
}
private enum CodingKeys: String, CodingKey {
case data
case children
}
}
final class MyData: Codable {
let id = UUID()
var label: String
var value: String
weak var parent: Node? = nil
init(label: String, value: String) {
self.label = label
self.value = value
}
private enum CodingKeys: String, CodingKey {
case label
case value
}
}
var root = Node()
root.add(data: MyData(label: "label 1", value: "value 1"))
root.add(data: MyData(label: "label 2", value: "value 2"))
var child = Node()
child.add(data: MyData(label: "label 3", value: "value 3"))
root.add(child: child)
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let json = try encoder.encode(root)
print(String(data: json, encoding: .utf8)!)