swift - 完成另一个 Alamofire 请求后运行 Alamofire 请求

Question

我想运行一个 Alamofire 请求，该请求使用先前 Alamofire 请求的结果作为参数。为了简单起见:

    //Code1
    Alamofire.request("URL", method: HTTPMethod.post, parameters: ["id": id as NSString)], encoding: URLEncoding.httpBody).response(completionHandler: { (response) in
        let json = response.data
        do {
            print("This should be First")
            let data = try JSON(data: json!)
            let alllastmessages = data["Messages"].arrayValue
            for  i in 0..<alllastmessages.count {
                self.List.append(alllastmessages[i]["message"].stringValue)
            }}
        catch _{}
    })
    //End Code1

    //Code2
    print("This should be Last")
    for  i in 0..<List.count {
        Alamofire.request("URL2", method: .post , parameters: ["id": id as NSString] , encoding: URLEncoding.httpBody).response(completionHandler: { (response) in
            //Do Something
        })
        self.tableView.reloadData()
    }
    //End Code2

(此代码已简化，我只是在寻找一种方法使 Code1 在 Code2 之前运行)

Answer 1

IMO 最简单的方法是使用 DispatchGroup，您可以在此处阅读更多相关信息:https://developer.apple.com/documentation/dispatch/dispatchgroup

这里有一些对我来说效果很好的代码:

    DispatchQueue.global(qos: .userInitiated).async {
        let group = DispatchGroup()
        group.enter()

        print("\(Date()) Start request 1!")
        Alamofire.request("https://github.com/CosmicMind/Material",
                          method: .get ,
                          parameters: [:] , encoding: URLEncoding.httpBody).response(completionHandler: { (response) in
                            print("\(Date()) Request 1 completed!")
                            group.leave()
                          })

        group.wait()

        print("\(Date()) Start request 2!")
        Alamofire.request("https://github.com/CosmicMind/Material",
                          method: .get ,
                          parameters: [:] , encoding: URLEncoding.httpBody).response(completionHandler: { (response) in
                            print("\(Date()) Request 2 completed!")
                          })
    }

打印:

2017-12-22 18:24:45 +0000 Start request 1!
2017-12-22 18:24:47 +0000 Request 1 completed!
2017-12-22 18:24:47 +0000 Start request 2!
2017-12-22 18:24:49 +0000 Request 2 completed!