Swift - 预期'{'开始设置器定义

Question

我正在关注高级 swift WWDC 2014 视频，使用类上的下标来委托给类属性，但使用与视频中相同的语法时出现错误

enum Direction {
    case North, South, East, West
}

class Place: Thing {
    init (_ location: Thing?, _ name: String, _ longDescription: String) {
        self.exits = Dictionary<Direction, Place> ()
        super.init(location, name, longDescription)
    }
    var exits: Dictionary<Direction, Place>
}

extension Place {
    subscript(direction: Direction) -> Place? {
        get {
            return exits[direction]
        }
        set (destination: Place?) { // error here
            exits[direction] = destination
        }
    }
}

我得到的错误是

Expected '{' to start setter definition

Answer 1

省略 setter 中的类型，它会自动推断：

subscript(direction: Direction) -> Place? {
    get {
        return exits[direction]
    }
    set (destination) {
        exits[direction] = destination
    }
}

您也可以完全跳过该参数，在这种情况下，它被隐式命名为newValue：

subscript(direction: Direction) -> Place? {
    get {
        return exits[direction]
    }
    set {
        exits[direction] = newValue
    }
}

Answer 2

不要传递参数来设置

set  { 
    exits[direction] = newValue
}