"initialize" class method for classes in Swift?

Question

I'm looking for behavior similar to Objective-C's +(void)initialize class method, in that the method is called once when the class is initialized, and never again thereafter.

A simple class init () {} in a class closure would be really sleek! And obviously when we get to use "class vars" instead of "static vars in a struct closure", this will all match really well!

Answer 1

If you have an Objective-C class, it's easiest to just override +initialize. However, make sure subclasses of your class also override +initialize or else your class's +initialize may get called more than once! If you want, you can use dispatch_once() (mentioned below) to safeguard against multiple calls.

class MyView : UIView {   override class func initialize () {     // Do stuff   } } 

 

If you have a Swift class, the best you can get is dispatch_once() inside the init() statement.

private var once = dispatch_once_t()  class MyObject {   init () {     dispatch_once(&once) {       // Do stuff     }   } } 

This solution differs from +initialize (which is called the first time an Objective-C class is messaged) and thus isn't a true answer to the question. But it works good enough, IMO.

Answer 2

There is no type initializer in Swift.

“Unlike stored instance properties, you must always give stored type properties a default value. This is because the type itself does not have an initializer that can assign a value to a stored type property at initialization time.”

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks.

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You could use a type property which default value is a closure. So the code in the closure would be executed when the type property (or class variable) is set.

class FirstClass {     class var someProperty = {      // you can init the class member with anything you like or perform any code         return SomeType     }() } 

But class stored properties not yet supported (tested in Xcode 8).

One answer is to use static, it is the same as class final.

Good link for that is

Setting a Default Property Value with a Closure or Function

Excerpt From: Apple Inc. “The Swift Programming Language.” iBooks.

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Code example:

class FirstClass {     static let someProperty = {         () -> [Bool] in         var temporaryBoard = [Bool]()         var isBlack = false         for i in 1...8 {             for j in 1...8 {                 temporaryBoard.append(isBlack)                 isBlack = !isBlack             }             isBlack = !isBlack         }          print("setting default property value with a closure")         return temporaryBoard     }() }  print("start") FirstClass.someProperty 

Prints

start

setting default property value with a closure

So it is lazy evaluated.