在将其分配给 Swift 中的变量之前等待网络请求的结果

Question

我想这个问题已经出现过很多次了，但我无法理解所有 GCD 和完成处理的内容。非常感谢提供一些指向有用文章的链接的快速修复。

我有一个函数可以从网络请求中获取和处理数据：

  func getTracklist(album id: String) -> String {

        //create a GET request

        let task = URLSession.shared.dataTask(with: request) { (data, response, error) -> Void in
            guard let data = data else { return }
            let decoder = JSONDecoder()
            if let jsonTracks = try? decoder.decode(TrackRoot.self, from: data) {
                tracks = jsonTracks.items!
            }
            //append the tracklist string
            for track in tracks {
                combinedTracks += "\(track.id)%2C"
            }
        }
            task.resume()

        return String(combinedTracks.dropLast(3))

    }

返回值在另一个函数中使用：

formattedAlbum.trackList = self.getTracklist(album: album.id!)

您可能已经猜到了，第一个函数未能及时获取数据，因此将一个空字符串分配给了曲目列表。

使用 Alexey 的回答：

for album in self.albums {
                let formattedAlbum = AlbumFormatted(context: self.persistenceManager.context)

                formattedAlbum.albumName = album.name
                self.getTracklist(album: album.id!) { (data) in
                    formattedAlbum.trackList = data
                }
                self.formattedAlbums.append(formattedAlbum)
                print(formattedAlbum)
            }
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + .seconds(5)) {
                print(self.formattedAlbums[1])
            }

func getTracklist(album id: String, completion: @escaping (String?)->()) {
   //URLSession stuff

   let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
            guard let data = data else { return }
            let decoder = JSONDecoder()
            if let jsonTracks = try? decoder.decode(TrackRoot.self, from: data) {
                tracks = jsonTracks.items!
                for track in tracks {
                    combinedTracks += "\(track.id),"
                }
                completion(String(combinedTracks.dropLast()))
            }
        }
            task.resume()
    }

打印：

(entity: AlbumFormatted; id: 0x6000018fb400 <x-coredata:///AlbumFormatted/tFD0D9588-2820-451B-9A00-99175026ED043>; data: {
    albumName = "Everyday Life";
    trackList = "6Tb7Zfo4PcSiS4TqQ4NnTh,1e8D1BCD2afT56Km7UahpB,45PqOIkZ9PdCjsCJQYzx9G,1cXXhzPnbrXjNQYbLdUJdy,3pcPPhPAiurm2Ior11SHrz,7jib2tJjQ82kTIZZATMvAK,0ZlVUhjO8c0bOx1D2Btznf,0UvUivL70eDwhTWBd8S38I,6VzRvCbolqcUswaS";
})

Answer 1

我看到了使用闭包的解决方案：只需在您的 ViewController 中测试以供参考。

override func viewDidLoad() {
    super.viewDidLoad()
    getTracklist { data in
        print(data)
    }
}

func getTracklist(completion: @escaping (Data?)->()) {
    let request = URLRequest(url: URL(string: "https://jsonplaceholder.typicode.com/todos/1")!)
    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        // process errors here
        completion(data) // here you return your decoded data (I omit your json, as we don't have a model)
    }
    task.resume()
}

UPD 阅读您的评论后：

for album in self.albums { 
    let formattedAlbum = AlbumFormatted(context: self.persistenceManager.context) 
    formattedAlbum.albumName = album.name 
    self.getTracklist(album: album.id!) { data in 
        print(data) 
        formattedAlbum.trackList = data 
    }
    print(formattedAlbum.trackList) // will return you nil
    // here you will not have results in formattedAlbum.trackList, because getTracklist method processes the data asynchronously   
}

检查 this, that and this 个 SO 线程。

Answer 2

这是处理回调的一种合法方式：

func getTrackList(album id: String, completion: @escaping(String)->()){
    //create a GET request
    let task = URLSession.shared.dataTask(with: request) { (data, response, error) -> Void in
        guard let data = data else { return }
        let decoder = JSONDecoder()
        if let jsonTracks = try? decoder.decode(TrackRoot.self, from: data) {
            tracks = jsonTracks.items!
        }
        //append the tracklist string
        for track in tracks {
            combinedTracks += "\(track.id)%2C"
        }
        completion(combinedTracks)
    }
    task.resume()
}

调用此函数时：

getTrackList(album: album.id) { [weak self] combinedTrack in
    guard let self = self else {return}
    self.formattedAlbum.trackList = combinedTrack
}

注意：如果请求失败或收到空 response/string

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