A technique to avoid having optional array properties in your models when decoding JSON using Swift 4

KeyedDecodingContainer+EmptyRepresentable.swift

/// A type that has an "empty" representation.
public protocol EmptyRepresentable {
  static func empty() -> Self
}

extension Array: EmptyRepresentable {
  public static func empty() -> Array<Element> {
    return Array()
  }
}

extension KeyedDecodingContainer {
  public func decode<T>(_ type: T.Type, forKey key: KeyedDecodingContainer<K>.Key) throws -> T where T: Decodable & EmptyRepresentable {
    if let result = try decodeIfPresent(T.self, forKey: key) {
      return result
    } else {
      return T.empty()
    }
  }
}

struct Model: Decodable {
  let id: Int
  
  // Instead of making this property optional because it may be not present
  // in the JSON, we leverage the `KeyedDecodingContainer` extension above
  // to have an empty array in that case.
  let items: [String]
}

let json = """
{
  "id": 42
}
""".data(using: .utf8)!

let decoder = JSONDecoder()

do {
  let model = try decoder.decode(Model.self, from: json)
  print(model)
} catch {
  print(error)
}