Decodes multiple layers of nested containers using a String array for key path. (It's actually cleaner to declare another struct for the nested data. But this is to show how this can be done.)

DynamicCodingKey.swift

import Foundation

// Allows defining CodingKey from String
struct DynamicCodingKey: CodingKey {
  var intValue: Int?
  var stringValue: String

  init?(intValue: Int) {
    assertionFailure("Not implemented")
    return nil
  }
  init?(stringValue: String) {
    self.stringValue = stringValue
  }
}

extension KeyedDecodingContainer where Key == DynamicCodingKey {
  // Decodes multiple layers of nested containers using a String array for key path.
  func decode<T: Decodable>(_ type: T.Type, forKeyPath keys: [String]) throws -> T? {
    var container: KeyedDecodingContainer = self
    for key in keys {
      guard let realKey = container.allKeys.first(where: { $0.stringValue == key }) else { return nil }
      if key == keys.last {
        return try container.decode(T.self, forKey: realKey)
      }
      else {
        container = try container.nestedContainer(keyedBy: Key.self, forKey: realKey)
      }
    }
    return nil
  }
}

DynamicCodingKeyTests.swift

import Foundation

@testable import Viki
import XCTest

class DynamicCodingKeyTests: XCTestCase {

  private struct Foo: Decodable {
    let teamName: String?

    init(from decoder: Decoder) throws {
      let container = try decoder.container(keyedBy: DynamicCodingKey.self)
      self.teamName = try container.decode(String.self, forKeyPath: ["team", "name"])
    }
  }

  func testDefaultSettingsFileNotFound() throws {
    let data = """
    {
      "team": { "name": "foo" }
    }
    """.data(using: .utf8)!

    let output = try JSONDecoder().decode(Foo.self, from: data)
    XCTAssertEqual(output.teamName, "foo")
  }

}