变量没有快速更新[重复]

Question

class ProductDecoer: ObservableObject {

  @Published var productName: String = ""

  func checkBarCode(barcode: String) -> String {

    let api = <url>
    let barcode = barcode
    let format = ".json"
    let url = URL(string: api + barcode + format)

    URLSession.shared.dataTask(with: url!) { [self] data, response, error in
      if let data = data {
        if let jsonString = String(data: data, encoding: .utf8) {
          productName = self.getProductInfo(json: jsonString)
          print("checkBarCode: " + self.productName) // --> print correctly
        }
      }
    }.resume()

    self.productName = productName
    print("return: " + self.productName) // --> is empty
    return self.productName
  }

  func getProductInfo(json: String) -> String {
    let jsonData = json.data(using: .utf8)!
    let productInfo: ProductInfo = try! JSONDecoder().decode(ProductInfo.self, from: jsonData)
    self.productName = productInfo.product_name
    print("AAA: " + self.productName)
    print("Prodotto: " + productInfo.product_name)

    productName = productInfo.product_name
    print("CC: " + productName)

    return productName
  }

  struct ProductInfo: Codable {

    var code: String
    var product_name: String

    enum CodingKeys: String, CodingKey {
      case code
      case product

      enum ProductCodingKeys: String, CodingKey {
        case product_name
      }
    }

    init(from decoder: Decoder) throws {
      let rootContainer = try decoder.container(keyedBy: CodingKeys.self)
      let productContainer = try rootContainer.nestedContainer(keyedBy: CodingKeys.ProductCodingKeys.self, forKey: .product)

      code = try rootContainer.decode(String.self, forKey: .code)
      product_name = try productContainer.decode(String.self, forKey: .product_name)

      print("BB: " + product_name)
    }

    func encode(to encoder: Encoder) throws {
      var rootContainer = encoder.container(keyedBy: CodingKeys.self)
      var productContainer = rootContainer.nestedContainer(keyedBy: CodingKeys.ProductCodingKeys.self, forKey: .product)

      try rootContainer.encode(code, forKey: .code)
      try productContainer.encode(product_name, forKey: .product_name)
    }
  }

}

Answer 1

urlsession.shared.datatask是一个异步函数。这意味着它独立执行，并在将来完成一个不确定的时间。

在它后面的{}中向它提供的内容称为完成处理程序或回调函数--这是datatask在执行结束后调用的代码，因此依赖于返回值的任何事情都需要在闭包中进行。

不幸的是，这意味着您不能直接从checkBarcode函数返回String。但是，您也可以为它创建一个回调函数：

func checkBarCode(barcode: String, completion: @escaping (String) -> Void) {
        
        let api = "URL"
        let barcode = barcode
        let format = ".json"
        let url = URL(string: api + barcode + format)
        
        URLSession.shared.dataTask(with: url!) { [self] data, response, error in
            if let data = data {
                if let jsonString = String(data: data, encoding: .utf8) {
                    self.productName = self.getProductInfo(json: jsonString)
                    print("checkBarCode: " + self.productName)
                    completion(self.productName)
                }
            }
        }.resume()
    }

checkBarCode(barcode: "") { productName in 
  //do something that depends on the value
}