Swift Singleton Theory

在OC中我们经常这样写单例：

@implemention MyManager 

+ (id)sharedManager {

    static MyManager *shareInstance = nil;

    static dispatch_once_t onceToken;

    dispatch_once(&onceTokec, ^{

        shareInstance = [[self alloc]  init];

    });

    return shareInstance;

}

使用dispatch_once_t来保证单例代码在生命周期内只被调用一次。

然而在Swift中，如下方式是最佳的单例实现方式：

private let  sharedInstance = MyManager()

为什么简单的一个全局变量就可行呢？如何保证只调用了一次没有冲突呢？

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.

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Swift天生支持单例模式，简单方便。

developer.apple.com/swift/blog/