KeyPath to get/set from a Swift JSON object ([String: Any] or [Any])

Dictionary+KeyPath.swift

import Foundation

extension Dictionary where Key == String, Value == Any {
    
    public subscript(keyPath: JSON.Path.Index...) -> Value? {
        get { self[keyPath: .init(path: keyPath)] }
        set { self[keyPath: .init(path: keyPath)] = newValue }
    }
    
    public subscript(keyPath keyPath: JSON.Path) -> Value? {
        get {
            switch keyPath.headAndTail() {
            case nil: return nil
            case let (head, remaining)? where remaining.isEmpty:
                return self[head.stringValue] ?? nil
            case let (head, remaining)?:
                switch self[head] {
                case let array as [Value]:
                    return array[keyPath: remaining]
                case let dictionary as [Key: Value]:
                    return dictionary[keyPath: remaining]
                default: return nil
                }
            }
        }
        set {
            switch keyPath.headAndTail() {
            case nil: return
            case let (head, remaining)? where remaining.isEmpty:
                self[head.stringValue] = newValue
            case let (head, remaining)?:
                let key = head.stringValue
                let value = self[key]
                switch value {
                case var array as [Value]:
                    array[keyPath: remaining] = newValue
                    self[key] = array
                case var nested as [Key: Value]:
                    nested[keyPath: remaining] = newValue
                    self[key] = nested
                default:
                    guard let next = remaining.path.first else {
                        return (self[key] = newValue)
                    }
                    if let idx = next.intValue, idx >= 0 {
                        var array = [Value]()
                        array[keyPath: remaining] = newValue
                        self[key] = array
                    } else {
                        var dictionary = [String: Value]()
                        dictionary[keyPath: remaining] = newValue
                        self[key] = dictionary
                    }
                }
            }
        }
    }
    
}

extension Array where Element == Any {
    
    public subscript(keyPath: JSON.Path.Index...) -> Element? {
        get { self[keyPath: .init(path: keyPath)] }
        set { self[keyPath: .init(path: keyPath)] = newValue }
    }
    
    public subscript(keyPath keyPath: JSON.Path) -> Element? {
        get {
            guard let (head, remaining) = keyPath.headAndTail() else { return nil }
            guard let idx = head.intValue else { return nil }
            switch (idx, remaining) {
            case nil: return nil
            case let (idx, remaining) where remaining.isEmpty:
                return indices.contains(idx) ? self[idx] : nil
            case let (idx, remaining):
                switch self[idx] {
                case let array as [Element]:
                    return array[keyPath: remaining]
                case let dictionary as [String: Element]:
                    return dictionary[keyPath: remaining]
                default: return nil
                }
            }
        }
        set {
            guard let (head, remaining) = keyPath.headAndTail() else { return }
            guard let idx = head.intValue, idx >= 0 else { return }
            padded(to: idx, with: NSNull())
            switch (idx, remaining) {
            case nil: return
            case let (idx, remaining) where remaining.isEmpty:
                self[idx] = newValue ?? NSNull()
            case let (idx, remaining):
                let value = indices.contains(idx) ? self[idx] : nil
                switch value {
                case var .some(array as [Element]):
                    array[keyPath: remaining] = newValue
                    self[idx] = array
                case var .some(dictionary as [String: Element]):
                    dictionary[keyPath: remaining] = newValue
                    self[idx] = dictionary
                default:
                    guard let next = remaining.path.first else {
                        return self[idx] = newValue ?? NSNull()
                    }
                    if let idx = next.intValue, idx >= 0 {
                        var array = [Element]()
                        array[keyPath: remaining] = newValue
                        self[idx] = array
                    } else {
                        var dictionary = [String: Element]()
                        dictionary[keyPath: remaining] = newValue
                        self[idx] = dictionary
                    }
                }
            }
        }
    }
    
}

extension RangeReplaceableCollection where Self: BidirectionalCollection {
    public mutating func padded(to size: Int, with value: @autoclosure () -> Element) {
        guard !indices.contains(index(startIndex, offsetBy: size)) else { return }
        append(contentsOf: (0..<(1 + size - count)).map { _ in value() })
    }
}

// TODO 
public enum JSON { }

extension JSON {
    
    public struct Path {
        
        public enum Index {
            case int(Int)
            case string(String)
        }
        
        public var path: [CodingKey]
        public var isEmpty: Bool { path.isEmpty }
        
        func headAndTail() -> (head: Index, tail: Path)? {
            guard !isEmpty else { return nil }
            var tail = path
            let head = tail.removeFirst()
            return (head.intValue.map(Index.int) ?? .string(head.stringValue), Path(path: tail))
        }
    }
}

extension JSON.Path.Index: CodingKey, ExpressibleByStringLiteral, ExpressibleByIntegerLiteral {
    
    public init(stringLiteral value: String) {
        self = .string(value)
    }
    
    public init(integerLiteral value: Int) {
        self = .int(value)
    }
    
    public var stringValue: String {
        switch self {
        case let .int(o): return "\(o)"
        case let .string(o): return o
        }
    }
    
    public init?(stringValue: String) {
        self = .string(stringValue)
    }
    
    public var intValue: Int? {
        switch self {
        case let .int(o): return o
        case .string: return nil
        }
    }
    
    public init?(intValue: Int) {
        self = .int(intValue)
    }
    
}

Combine:

import Combine

extension Publisher {
    
    subscript<T>(path: JSON.Path.Index..., as type: T.Type = T.self) -> AnyPublisher<T, Failure> where T: Equatable {
        self[.init(path: path)]
    }
    
    subscript<T>(path: JSON.Path, as type: T.Type = T.self) -> AnyPublisher<T, Failure> where T: Equatable {
        compactMap{
            switch $0 {
            case let fragment as T: return fragment
            case let array as [Any]: return array[keyPath: path] as? T
            case let dictionary as [String: Any]: return dictionary[keyPath: path] as? T
            default: return nil
            }
        }
        .scanNewValues()
        .eraseToAnyPublisher()
    }
    
}

extension Publisher {
    subscript<Value>(keyPath: KeyPath<Output, Value>) -> AnyPublisher<Value, Failure> where Value: Equatable {
        map(keyPath)
            .scanNewValues()
            .eraseToAnyPublisher()
    }
}

extension Publisher where Output: Equatable {
    func scanNewValues() -> AnyPublisher<Output, Failure> {
        scan((nil, nil), { ($1, $0.0) })
            .compactMap{ old, new -> (newValue: Output, oldValue: Output)? in
                guard let a = old else { return nil }
                guard let b = new else { return (a, a) }
                guard a != b else { return nil }
                return (a, b)
            }
            .map(\.newValue)
            .eraseToAnyPublisher()
    }
}

class Test {
    @Published var json: [String: Any] = [
        "string": "hello world",
        "int": 1,
        "structure": [
            "is": [
                "good": [
                    true,
                    [
                        "and": [
                            "i": [
                                "like": [
                                    "pie",
                                    "programming",
                                    "dogs"
                                ]
                            ]
                        ]
                    ]
                ]
            ]
        ]
    ]
}

let test = Test()
var bag = Set<AnyCancellable>()

test.$json["structure", "is", "good", 0, as: Bool.self].sink { any in
    print("🌍", any)
}.store(in: &bag)

test.json["structure", "is", "good", 0] = false
test.json["structure", "is", "good", 0] = true

// 🌍 true
// 🌍 false
// 🌍 true