How does a block with curly braces and parentheses work?

Question

let loginRegisterButton:UIButton = {
    let button = UIButton(type: .system)
    button.backgroundColor = .white
    button.setTitle("Register", for: .normal)
    button.translatesAutoresizingMaskIntoConstraints = false
    button.setTitleColor(.white, for: .normal)
    return button
}()

is this variable or function , why does it return value? why do we call it? it does not work without parenthesis,why?

Answer 1

This is a closure created and used in the same spot. You use it for initialization when you cannot put everything in a single expression. This feature is helpful when creating read-only (let, as opposed to var) fields.

In the case above, the code creates a button, and then performs additional configuration on it before returning the result. This is a good way to move code from init into code blocks near the place of initialization.

One way to visualize what's going on is to think of a named function that does the same thing:

func makeWhiteButton() -> UIButton {
    let button = UIButton(type: .system)
    button.backgroundColor = UIColor.White
    button.setTitle("Register", for: .normal)
    button.translatesAutoresizingMaskIntoConstraints = false
    button.setTitleColor(.white, for: .normal)
    return button
}

Now you can use it in an initializer

let loginRegisterButton:UIButton = makeWhiteButton()

The code from your post does the same thing with an anonymous "closure" function. The parentheses after the closure block are there for the same reason as the parentheses after makeWhiteButton above.