如何访问类常量和变量

Question

Why can I not perform operations with a variable or constant at the class level? Is this not allowed or is there a keyword I need to use to access them? Is this bad practice?

Controller:

class ViewController: UIViewController {
    let one = 1
    let two = 2
    var sum = one + two
}

Error:

ViewController.Type does not have a member named 'one'

Answer 1

Class variables and constants must be static , eg, static let one = 1 .

Here it suffices for the two let constants to be static for them to be usable in initializing both class and instance variables. The following works for me:

class MyClass {
    static let one = 1
    static let two = 2
    static var sum = one + two
    var instanceProduct = one * two
}

MyClass.one
MyClass.sum
MyClass().instanceProduct

Note that in the above example you can do MyClass.sum = 5 . If you meant the sum to be constant as well, simply change it to static let sum = one + two .

The requirement is that the constants you use outside of any functions and closures be declared static let . The implication of this is that they are truly constant for the entire class. If you need instance-specific constants, you cannot use them outside of functions or closures (as you've noticed) – as a workaround for a variable sum I would suggest a lazy variable initialized by a closure:

class MyClass {
    let one: Int // value given in `init`
    let two = 2
    lazy var sum: Int = { self.one + self.two }()

    init(one: Int = 1) {
        self.one = one
    }
}
MyClass().sum // 3
MyClass(one: 2).sum // 4

Answer 2

Simple fix

   override func viewDidLoad() {
        super.viewDidLoad()
        var result = one + two
    }