Tantan Fu

Oct 03, 2015

深入理解 Swift 单例

Swift 中的单例是如何工作的呢
在 OC 中,我们常常这样来写一个单利
@implementation MyManager + (id)sharedManager { static MyManager *staticInstance = nil; static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ staticInstance = [[self alloc] init]; }); return staticInstance; } @end
使用 dispatch_once_t 来保证代码只被调用一次。而在 Swift 中,如下的方式,被认为是最佳实践:
private let sharedInstance = MyManager() class MyManager { class var sharedManager : MyManager { return sharedInstance } }
为什么这种方式是可行的呢?
private let sharedInstance = MyManager()
是如何保证只调用了一次,没有冲突的呢?
The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic. This enables a cool way to use dispatch_once in your code: just declare a global variable with an initializer and mark it private.
其中的奥秘在于,Swift 中的全局变量,在初始化的时候,都会使用 dispatch_once 来保证原子性。所以,说 Swift 天生支持单例模式,也不过分。 https://www.futantan.com/blog/dive-into-swift-singleton

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