如何等待主队列完成处理程序?

我更喜欢使用背景组,你可以在你的类中创建一个这样的实例: var group = DispatchGroup()

    DispatchQueue.global(qos: .background).async {
        self.group.wait()

       // this part will execute after the last one left
       // .. now, we iterate part
    }


    for index in 0..<count {
        var outcome: Any?
        let queue = DispatchQueue(label: "\(index) iteration")



        funcWithCompletionHandler() { [weak self] (result) in
            if let strongSelf = self {
                outcome = result
                strongSelf.group.enter() // group count = 1
            }

        }

        queue.sync {
            if let o = outcome {
                handleOutcome(outcome)
                self.group.leave()
                // right here group count will be 0 and the line after wait will execute
            }
        }
    }