GCD_group

 //创建一个并发队列        var queueb = dispatch_queue_create("queueb", DISPATCH_QUEUE_CONCURRENT)                //创建一个组        var group = dispatch_group_create()                //任务一        dispatch_group_async(group, queueb) { () -> Void in            for(var i = 0;i<100;i++){                        println("a\(i)")                            }        }                //任务二        dispatch_group_async(group, queueb) { () -> Void in            for(var i = 0;i<100000;i++){                                println("b\(i)")                           }             sleep(4)        }        //分支任务执行完，汇总结果        dispatch_group_notify(group, queueb) { () -> Void in            println("OKOKOKOOK")        }

notify会等待任务分支执行完毕才开始执行