ios - 捕获Firebase实时observeSingleEvent数据在首次调用时失败

Question

我与Firebase实时数据库有正常的连接。通过点击IBAction按钮触发watchSingleEvent。首次调用以获取数据失败。第二次按下按钮将获取数据。第一次调用时如何获取数据？我试过多次调用observeSingleEvent，以编程方式点击按钮并添加等待以使observeSingleEvent完成。甚至在绝望时添加了完成块。我做错了什么提示将不胜感激。

-编辑-

修复了@rob建议的以下代码。 Rob回答的唯一补充是，在完成块的定义中需要@escaping。现在工作精美。

var prodNo: String = "XXX000000"
var productDataLocal: [String: String] = ["ProdNo": "", "Location": ""]

@IBAction func findTapped(_ sender: Any) {
        fetchFirebaseData { () -> () in
            if productDataLocal["ProdNo"] == prodNo {  // check that the data matches the product
                // launch a segue (that is detached from button)
                self.performSegue(withIdentifier: "moveToDetail", sender: nil)
            }
            else {
                showError("Product \(prodNo) can't be found.")
            }
        }
}   // findTapped

func fetchFirebaseData(completion: @escaping () -> ()) {  // --- EDIT ---@escaping addedd
    // check if this product can be found in Firebase!    
    self.ref.child("Id").child("\(prodNo)").observeSingleEvent(of: .value, with:
        { (snapshot) in
            // try if this product can be found in Firebase
            let tempData = snapshot.value as? Dictionary<String, Any>
            if let actualData = tempData {
                self.copyData(actualData)
            }
            completion()  // --- EDIT --- completion moved here
        })  { (error) in
            print(error.localizedDescription)
            completion()  // --- EDIT ---and completion moved here
            }
    // --- EDIT -- completion() removed from here
}  // fetchFirebaseData

func copyData(_ actualData: Dictionary<String, Any>) {
    dump(actualData)  // for debugging what "actualData" actually contains
                      // this is always OK when execution gets here
    self.productDataLocal["ProdNo"] = actualData["ProdNo"] as? String
    self.productDataLocal["Location"] = actualData["Location"] as? String
}

执行时，“实际”数据的转储起作用并产生正确的数据。问题是我无法找到任何方法来执行此代码，而是第二次按下按钮。 (是的，在JSON结构中重复了“ProdNo”，以便我可以检查它是否与请求的产品编号匹配。)---编辑---上面的代码现在可以解决问题

▿ 2 key/value pairs
  ▿ (2 elements)
    - key: "ProdNo"
    - value: XXX000000 #0
      - super: NSMutableString
        - super: NSString
          - super: NSObject
  ▿ (2 elements)
    - key: "Location"
    - value: "Warehouse 1" #1
      - super: NSMutableString
        - super: NSString
          - super: NSObject

Answer 1

您正在同步调用完成块。在observeSingleEvent完成之前，您不应调用它。如果将完成块移至observeSingleEvent处理程序，则应获得所需的结果:

func fetchFirebaseData(completion: @escaping () -> ()) { // --- EDIT --- added "@escaping
    // check if this product can be found in Firebase!
    self.ref.child("Id").child("\(prodNo)").observeSingleEvent(of: .value, with: { (snapshot) in
            // try if this product can be found in Firebase
        let tempData = snapshot.value as? Dictionary<String, Any>
        if let actualData = tempData {
            self.copyData(actualData)
        }
        completion()
        }) { (error) in
            print(error.localizedDescription)
            completion()
        }
}  // fetchFirebaseData