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为什么我的完成处理程序不处理我的URLSession?

urlsession xcode swift

JamMan9 · 技术社区 · 6 年前

我的应用程序遇到了一个小问题。在粘贴任何代码之前,让我提供一点上下文。我有一个带有动态单元格的表格视图。每个单元格包含一个按钮。当我按下单元格内的按钮时,会调用一个函数,它发出一个URLSession数据任务请求。我使用一个可解码的类来存储JSON结果,我所要做的就是打印结果数组的计数。我遇到的问题;打印在返回结果之前发生。

为了解决这个问题,我尝试使用完成处理程序,并在DispatchGroup内运行urlsession,直到DispatchGroup结束后才打印。

我希望有人能快速查看我的代码并指出我的错误。

以下是我的函数,我在其中提出会话请求:

func loadLocalPrice(selectedItemName: String, completion: @escaping (_ result: [PriceModel])-> Void) {

    var localPrice = [PriceModel]()

    let urlApi = "http://my.url.com/category/"+self.selectedCategory+"/"+selectedItemName

    guard let url = URL(string: urlApi) else {return completion(localPrice)}

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else {return}
        self.dispatchGroupLoadItems.enter()
        do {
            localPrice = try JSONDecoder().decode([PriceModel].self, from: data)
            print("The result inside the function is: \(localPrice.count)")
        } catch let JSONerror {
            print("error decoding JSON", JSONerror)
        }
        self.dispatchGroupLoadItems.leave()

        }.resume()
        return completion(localPrice)
}

下面是调用包含urlsession的上述函数的函数:

    func addToBasket(sender: UIButton, name: String?, category: String?) {
    var localPrice = [PriceModel]()

    DispatchQueue.main.async {
        self.loadLocalPrice(selectedItemName: name!) {
            (result: [PriceModel]) in
            print("got back inside the dispatchGroup: \(result.count)")

            localPrice = result
        }


        self.dispatchGroupLoadItems.notify(queue: .main) {
            print("Got back outside the dispatchGroup: \(localPrice.count)")
        }
    }
}

下面是我的控制台输出,因此您可以看到返回结果的顺序:

got back inside the dispatchGroup: 0
Got back outside the dispatchGroup: 0
The result inside the function is: 1

3 回复 | 直到 6 年前

Kamran 6 年前

你只是不需要 dispatchGroup 当您已经有一个完成处理程序时。您的问题可以通过使用 completion 以如下正确的方式处理,

func loadLocalPrice(selectedItemName: String, completion: @escaping (_ result: [PriceModel])-> Void) {

    var localPrice = [PriceModel]()

    let urlApi = "http://my.url.com/category/"+self.selectedCategory+"/"+selectedItemName

    guard let url = URL(string: urlApi) else { 
              completion(localPrice)
              return 
            }

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else {
               completion(localPrice)
               return
             }
        do {
            localPrice = try JSONDecoder().decode([PriceModel].self, from: data)
        } catch let JSONerror {
            print("error decoding JSON", JSONerror)
        }
        DispatchQueue.main.async {
          completion(localPrice)
        }
     }.resume() 
}

func addToBasket(sender: UIButton, name: String?, category: String?) {
    var localPrice = [PriceModel]()

    self.loadLocalPrice(selectedItemName: name!) {
        (result: [PriceModel]) in
        print("Count: \(result.count)")
        localPrice = result
    }
}

whoover 6 年前

你不需要派遣组就可以返回这里。

 func loadLocalPrice(selectedItemName: String, completion: @escaping (_ result: [PriceModel])-> Void) {

      var localPrice = [PriceModel]()

      let urlApi = "http://my.url.com/category/"+self.selectedCategory+"/"+selectedItemName

      guard let url = URL(string: urlApi) else {
           completion(localPrice)
           return
      }

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else {
            completion(localPrice)
            return
        }
        do {
            localPrice = try JSONDecoder().decode([PriceModel].self, from: data)
            print("The result inside the function is: \(localPrice.count)"  
        } catch let JSONerror {
            print("error decoding JSON", JSONerror)
        }
        completion(localPrice)
    }.resume()
}

Ido 6 年前

问题是你试图 return completion(localPrice) . 首先, URLSession.shared.dataTask(with: url) 正在后台运行,因此不需要使用 dispatchGroupLoadItems .

你要做的就是给你打电话 completion block 在主线程上 urlsession.shared.datatask(带:url) 完井区块 就像这样:

func loadLocalPrice(selectedItemName: String, completion: @escaping (_ result: [PriceModel])-> Void) {
    var localPrice = [PriceModel]()
    let urlApi = "http://my.url.com/category/"+self.selectedCategory+"/"+selectedItemName
    guard let url = URL(string: urlApi) else {
        completion(localPrice)
        return
    }

    URLSession.shared.dataTask(with: url) { (data, response, error) in
        guard let data = data else {return}
        do {
            localPrice = try JSONDecoder().decode([PriceModel].self, from: data)
            print("The result inside the function is: \(localPrice.count)")
        } catch let JSONerror {
            print("error decoding JSON", JSONerror)
        }

        DispatchQueue.main.async {
            completion(localPrice)
        }
    }.resume()
}

以及 addToBasket 功能变为:

func addToBasket(sender: UIButton, name: String?, category: String?) {
    var localPrice = [PriceModel]()

    self.loadLocalPrice(selectedItemName: name!) {(result: [PriceModel]) in
        print("The result from completion block: \(result.count)")

        localPrice = result
    }
}