json - 如何在 Swift 中使用 OperationQueue 下载和解析 JSON

Question

我陷入了一个概念上简单的问题。发生的事情是解析操作在下载操作的完成处理程序完成之前执行。因此，没有要解析的数据。您可以将以下代码直接放入文件中并运行它。

如何确保在解析操作运行之前完成下载？

import UIKit

let search = "https://api.nal.usda.gov/ndb/search/?format=json&q=butter&sort=n&max=25&offset=0&api_key=DEMO_KEY"

class ViewController: UIViewController {



    override func viewDidLoad() {
        super.viewDidLoad()

        let fetch = FetchNBDNumbersOperation()
        let parse = NDBParseOperation()

        // 1
        let adapter = BlockOperation() { [unowned parse, unowned fetch] in
            parse.data = fetch.data
        }

        // 2
        adapter.addDependency(fetch)
        parse.addDependency(adapter)

        // 3
        let queue = OperationQueue()
        queue.addOperations([fetch, parse, adapter], waitUntilFinished: true)
    }
}

class FetchNBDNumbersOperation: Operation {

    var data: Data?

    override func main() {
        let url = URL(string: search)!
        let urlSession = URLSession.shared
        let dataTask = urlSession.dataTask(with: url) { (jsonData, response, error) in
            guard let jsonData = jsonData, let response = response else {
                debugPrint(error!.localizedDescription)
                return
            }
            self.data = jsonData
            print("Response URL: \(String(describing: response.url?.absoluteString))")
        }
        dataTask.resume()
    }
}

class NDBParseOperation: Operation {

    var data: Data?
    var nbdNumbers = [NBDNumber]()

    override func main() {
        let decoder = JSONDecoder()
        do {
            guard let jsonData = self.data else {
                fatalError("No Data")
            }
            let dictionary = try decoder.decode( [String: USDAFoodSearch].self, from: jsonData )
            for (_, foodlist) in dictionary {
                for food in foodlist.item {
                    print("\(food.name) \(food.ndbno) \(food.group)")
                    let nbdNumber = NBDNumber(name: food.name, nbdNo: food.ndbno)
                    nbdNumbers.append(nbdNumber)
                }
            }
        } catch {
            print(error.localizedDescription)
        }
    }
}

struct NBDNumber {
    var name: String
    var nbdNo: String
}

struct USDAFoodSearch: Decodable {
    let q: String
    let sr: String
    let ds: String
    let start: Int
    let end: Int
    let total: Int
    let group: String
    let sort: String
    let item: [USDAFood]

    struct USDAFood: Decodable {
        let offset: Int     //Position in Array
        let group: String
        let name: String
        let ndbno: String
        let ds: String
    }
}

Answer 1

在您的获取操作中，您恢复了 URLSessionDataTask。此时，该操作认为它已完成，因为它不知道其他线程上发生了什么，并且本质上不关心并发操作，并且它依赖于 (adapter) 启动。与此同时，URLSessionDataTask 仍在另一个线程上执行。

来自 Apple 关于 NSOperation 的文档

For non-concurrent operations, you typically override only one method:

• main()

URLSessionDataTask 同时运行，因此您需要做更多的工作将它们包装在 NSOperation 中。为了包装并发操作，您需要:

...override the following methods and properties at a minimum:

start()

isAsynchronous

isExecuting

isFinished

NSOperation docs对此进行了相当多的详细介绍，但总结一下:您需要覆盖 start() 而不是 main() 并让您的实现保持操作状态日期。