Swift3中的GCD

public extension DispatchQueue {
    
    private static var _onceTracker = [String]()
    
    /// 实现Swift3中取消的dispatch_once
    ///
    /// - Parameters:
    ///   - token: 一个唯一化的字符串，可以自定义也可以使用UUID. eg. NSUUID().uuidString
    ///   - block: 只需执行一次的代码
    public class func once(token: String, block:(Void)->Void) {
        objc_sync_enter(self); defer { objc_sync_exit(self) }
        
        if _onceTracker.contains(token) {
            return
        }
        
        _onceTracker.append(token)
        block()
    }
    
    /*
    上述方法有一个官方的解决方案：
     Dispatch 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:
     
     static let doOnce = { … global contains initialization in a call to a closure … }()
     _ = doOnce // using myGlobal will invoke the initialization code only the first time it is used.
     
    */
}

//这是官方文档的初始化方法
convenience init(label: String, qos: DispatchQoS = default,
                attributes: DispatchQueue.Attributes = default,
                autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency = default,
                target: DispatchQueue? = default)
// 没有传参数的时候默认创建串行队列，async异步执行，所以他会开一条线程
DispatchQueue(label: "my_queue").async {
    // do something     
}
//传入attributes为.concurrent时创建并行队列，异步执行，所以他会开多条线程
DispatchQueue(label: "concurrent_queue", attributes: .concurrent).async {
    // do something        
}