我建议使用调度组：

func resetDevice(completion: () -> ()) {
    let dispatchGroup = DispatchGroup()

    for device in devices {

        dispatchGroup.enter()

        device.isValid = 0

        DeviceManager.instance.updateDevice(device).call { response in
            print("device reset")
            dispatchGroup.leave()
        }
    }

    dispatchGroup.notify(queue: DispatchQueue.main) {
        // Some code to execute when all devices have been reset
    }
}

每个设备都立即进入该组，但直到收到响应后才离开该组。在所有对象都离开组之前，不会调用最后的notify块。