struct Coord: ExCodable {
    private(set) var y: Int = 0
    private(set) var x: Int = 0
    private(set) var w: Int = 0
    private(set) var h: Int = 0
    
    static var keyMapping: [KeyMap<Coord>] {
        [KeyMap(\.y, to: "y")]
    }
    init(from decoder: Decoder) throws {
        try decode(from: decoder, with: Self.keyMapping)
    }
}

static var keyMapping: [KeyMap<Coord>] {
      [KeyMap(\.y, to: "y"),
       ....
      ]
}