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Whither dispatch_once in Swift 3?6
> Using a dispatch_once singleton model in Swift25个
迁移到Swift 3时,dispatch_once_t有问题.
> Using a dispatch_once singleton model in Swift25个
迁移到Swift 3时,dispatch_once_t有问题.
根据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.
所以我想迁移这个代码.所以在迁移之前：
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!
}
迁移后,按照Apple的指南(手动迁移),代码如下所示：
class var sharedInstance: CarsConfigurator
{
struct Static {
static var instance: CarsConfigurator?
static var token = {0}()
}
_ = Static.token
return Static.instance!
}
但是当我运行这个访问返回时,我得到以下错误Static.instance !:
Fatal error: unexpectedly found nil while unwrapping an Optional value
我从这个错误中看到,实例成员为零,但为什么呢？我的迁移有问题吗？
即使Swift 2中有效,该代码过于冗长.在Swift 3中,Apple强制您通过关闭来使用延迟初始化：
class CarsConfigurator {
static let sharedInstance: CarsConfigurator = { CarsConfigurator() }()
}