Getters 和 Setters 示例

struct Square {
    private(set) var area = 0

    var side: Int = 0 {
        didSet {
            area = side*side
        }
    }
}

public struct Square {
    public private(set) var area = 0
    public var side: Int = 0 {
        didSet {
            area = side*side
        }
    }
    public init() {}
}