Lazy Var vs Let

Question

I want to use Lazy initialization for some of my properties in Swift. My current code looks like this:

lazy var fontSize : CGFloat = {
  if (someCase) {
    return CGFloat(30)
  } else {
    return CGFloat(17)
  }
}()

The thing is that once the fontSize is set it will NEVER change. So I wanted to do something like this:

lazy let fontSize : CGFloat = {
  if (someCase) {
    return CGFloat(30)
  } else {
    return CGFloat(17)
  }
}()

Which is impossible.

Only this works:

let fontSize : CGFloat = {
  if (someCase) {
    return CGFloat(30)
  } else {
    return CGFloat(17)
  }
}()

So - I want a property that will be lazy loaded but will never change. What is the correct way to do that? using let and forget about the lazy init? Or should I use lazy var and forget about the constant nature of the property?

Answer 1

This is the latest scripture from the Xcode 6.3 Beta / Swift 1.2 release notes:

let constants have been generalized to no longer require immediate initialization. The new rule is that a let constant must be initialized before use (like a var), and that it may only be initialized: not reassigned or mutated after initialization.

This enables patterns like:

let x: SomeThing
if condition {
    x = foo()
} else {
    x = bar()
}

use(x)

which formerly required the use of a var, even though there is no mutation taking place. (16181314)

Evidently you were not the only person frustrated by this.