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'dispatch_once_t' is unavailable in Swift: Use lazily initialized globals instead [duplicate]

Tags:

swift

swift3

I am having trouble with dispatch_once_t when migrating to Swift 3.

According to Apple's migration guide:

The free function dispatch_once is no longer available in Swift. In Swift, you can use lazily initialized globals or static properties and get the same thread-safety and called-once guarantees as dispatch_once provided. Example:

let myGlobal = { … global contains initialization in a call to a closure … }()

_ = myGlobal // using myGlobal will invoke the initialization code only the first time it is used.

So I wanted to migrate this code. So it was before migration:

class var sharedInstance: CarsConfigurator
{
    struct Static {
        static var instance: CarsConfigurator?
        static var token: dispatch_once_t = 0
    }

    dispatch_once(&Static.token) {
        Static.instance = CarsConfigurator()
    }

    return Static.instance!
}

After the migration, following the Apple's guidelines (manual migration), the code looks like this:

class var sharedInstance: CarsConfigurator
{
    struct Static {
        static var instance: CarsConfigurator?
        static var token = {0}()
    }

    _ = Static.token

    return Static.instance!
}

But when I run this I get the following error when accessing return Static.instance!:

fatal error: unexpectedly found nil while unwrapping an Optional value

I see from this error that the instance member is nil, but why is it? Is it something wrong with my migration?

like image 466
Vladimir Nul Avatar asked Jul 22 '16 01:07

Vladimir Nul


1 Answers

That code was overly verbose even though it was valid in Swift 2. In Swift 3, Apple forces you to use lazy initialization through closure:

class CarsConfigurator {
    static let sharedInstance: CarsConfigurator = { CarsConfigurator() }()
}
like image 128
Code Different Avatar answered Oct 22 '22 14:10

Code Different