使用 Grand Central Dispatch(GCD)

GCD 将保证你的单例只被实例化一次，即使从多个线程调用也是如此。将此插入到名为 shared 的单例实例的任何类中。

+ (instancetype)shared {

    // Variable that will point to the singleton instance. The `static`
    // modifier makes it behave like a global variable: the value assigned
    // to it will "survive" the method call.
    static id _shared;

    static dispatch_once_t _onceToken;
    dispatch_once(&_onceToken, ^{

        // This block is only executed once, in a thread-safe way.
        // Create the instance and assign it to the static variable.
        _shared = [self new];
    });

    return _shared;
}