Question

我突然从从Firebase下载用户目录的函数中得到了一些奇怪的结果。在控制台打印中，我看到该函数打印了5次，这意味着该函数正在循环播放。我正在从Firebase Storage异步下载产品图片。我首先登录时以级联调用的方式调用此函数，以获取用户的详细信息，订单，销售等。我想到此选项是因为用户需要更换iPad。

您能看到函数在哪里循环5次吗？

这是功能：

javax.servlet.http.HttpServletRequest

这是级联调用：

static func downloadProducts(completed: @escaping (Bool) -> (), vendor: String) {
        print("Firebase.downloadProducts() : Query for products in Firebase started")
        let ref = Database.database().reference()
        ref.child("Continent").child("Europe").child("Country").child("\(UserDetails.country!)").child("Catalog").queryOrdered(byChild: "Product Vendor").queryEqual(toValue: String(describing: UserDetails.fullName!)).observeSingleEvent(of: .value) { (snapshot) in
            print("downloadProducts() : snapshot is: \(snapshot)")
            guard let data = snapshot.value as? [String: [String : String]] else { print("downloadProducts() : data is not [String: [String : String]] "); return}
            //            guard let data = snapshot.value as? [String : String] else { print("downloadProducts() : wrong type of data"); return}
            for (_, value) in data {

                let productId = value["Product Id"]!
                let vendor = value["Product Vendor"]!
                let availableQuantity = value["Available Quantity"]!
                let soldQuantity = value["Sold Quantity"]!
                let minimumStock = value["Minimum Stock"]!
                let name = value["Product Name"]!
                let price = value["Product Price"]!
                let category = value["Product Category"]!
                let description = value["Product Description"]!
                let pictureUrl = value["Product Picture Url"]!
                let url = URL(string: pictureUrl)!
                DispatchQueue.global().async {
                    let data = try? Data(contentsOf : url)
                    DispatchQueue.main.async {

                        let productImage = UIImage(data: data!)!
                        do{
                            try Product.saveProduct(completed: { (true) in
                                print("Firebase.downloadProducts() : Product from Firebase added to CoreData")
                            }, productImage: productImage, productId: productId, category: category, name: name, price: price, vendor: vendor, availableQuantity: availableQuantity, soldQuantity: soldQuantity, minimumStock: minimumStock, description: description)
                        } catch {
                            print("Firebase.downloadProducts() : Error saving product to CoreData: \(error)")
                        }
                    }
                    print("downloadProducts() : User inventory found in Firebase and saved to CoreData")
                    print("downloadProducts() : Query for products in Firebase ended")
                    completed(true)
                }
            }
        }
    }

Answer 1

您在这里多次致电完成

DispatchQueue.global().async {
    let data = try? Data(contentsOf : url)
    DispatchQueue.main.async {

        let productImage = UIImage(data: data!)!
        do{
            try Product.saveProduct(completed: { (true) in
                print("Firebase.downloadProducts() : Product from Firebase added to CoreData")
            }, productImage: productImage, productId: productId, category: category, name: name, price: price, vendor: vendor, availableQuantity: availableQuantity, soldQuantity: soldQuantity, minimumStock: minimumStock, description: description)
        } catch {
            print("Firebase.downloadProducts() : Error saving product to CoreData: \(error)")
        }
    }
    print("downloadProducts() : User inventory found in Firebase and saved to CoreData")
    print("downloadProducts() : Query for products in Firebase ended")
    completed(true)
}

但您应该

let g = DispatchGroup() 
for (_, value) in data {    
    ...
    ...
    ...
    DispatchQueue.global().async {
        g.enter()
        let data = try? Data(contentsOf : url)
        DispatchQueue.main.async {

            let productImage = UIImage(data: data!)!
            do{
                try Product.saveProduct(completed: { (true) in
                    print("Firebase.downloadProducts() : Product from Firebase added to CoreData")
                }, productImage: productImage, productId: productId, category: category, name: name, price: price, vendor: vendor, availableQuantity: availableQuantity, soldQuantity: soldQuantity, minimumStock: minimumStock, description: description)
            } catch {
                print("Firebase.downloadProducts() : Error saving product to CoreData: \(error)")
            }
        }
        print("downloadProducts() : User inventory found in Firebase and saved to CoreData")
        print("downloadProducts() : Query for products in Firebase ended")
        g.leave() 
    }

}
g.notify(queue:.main) {
  completed(true)
}

Answer 2

我终于找到了问题所在。在Firebase.downloadOrders()中，我打电话给complete(true)是为了将订单保存到Core Data，因此对于保存的每个订单，都会调用Firebase.downloadProduct（）。将其放在for in循环之后，现在可以正确下载产品。

从Firebase Swift下载时功能循环

2 个答案: