早在Swift 3的時候,dispatch_once
就被蘋果廢棄了,並且推薦使用懶初始化全局變量方案代替。
官方推薦的解決辦法:
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.
下面來介紹4種實現dispatch_once
的只調用一次的方法
一. 帶立即執行閉包初始化器的全局變量
let dispatchOnce: () = {
print("dispatchOnce-------")
}()
/// 使用Aspects做交換: 用一個全局的常量來實現的dispatchOnce效果
let doSwizzles: () = {
print("doSwizzles----")
let oriSel1 = #selector(UIViewController.viewWillAppear(_:))
let wrappedBlock: @convention(block) (AspectInfo, Bool) -> Void = { aspectInfo, _ in
print("wrappedBlock---")
}
_ = try? UIViewController.aspect_hook(oriSel1, with: AspectOptions.positionBefore, usingBlock: wrappedBlock)
}()
// 之後在合適的時機使用變量,比如:
_ = dispatchOnce
二、全局常量
let foo = ViewController()
三、類,結構,枚舉中的靜態屬性
class Person22 {
static var dog = Dog()
}
四、封裝一個once函數
public extension DispatchQueue {
private static var _onceTracker = [String]()
/// 函數只被執行一次
static func once(token: String, block: () -> ()) {
// objc_sync_enter + objc_sync_exit 相當於OC中的@sychronize() {}
objc_sync_enter(self)
defer {
objc_sync_exit(self)
}
if _onceTracker.contains(token) {
return;
}
_onceTracker.append(token)
block()
}
}