将观察 .value 转换为 .child 快速添加

Question

目前，我在将 Firebase 数据库中的数据加载到数组中的这段代码中遇到了一些问题。由于这是在 viewDidLoad 内部，我必须在将数据加载到其中之前清空我的数组food = []，如果我不这样做，它将复制所有对象，并且第二次加载时我将重复两次，第三次加载三次等等...但是由于多种原因，这不是一个好的解决方法，所以我想要的是它只会使用.childAdded 从数据库中添加新对象但是如果我只是用.childAdded 切换出.value 它会崩溃，我在这条线上得到一个Thread 1: signal SIGABRT：let dict = user_snap.value as! [String: String?]。我对 swift 很陌生，不知道如何解决这个问题，非常感谢一些帮助。

let parentRef = Database.database().reference().child("Recipes")
let storage = Storage.storage()

parentRef.observe(.value, with: { snapshot in

        if ( snapshot.value is NSNull ) {

            // DATA WAS NOT FOUND
            print("– – – Data was not found – – –")

        } else {

            //Clears array so that it does not load duplicates
            food = []

            // DATA WAS FOUND
            for user_child in (snapshot.children) {

                let user_snap = user_child as! DataSnapshot
                let dict = user_snap.value as! [String: String?]

                //Defines variables for labels
                let recipeName = dict["Name"] as? String
                let recipeDescription = dict["Description"] as? String
                let downloadURL = dict["Image"] as? String

                let storageRef = storage.reference(forURL: downloadURL!)

                storageRef.getData(maxSize: 1 * 1024 * 1024) { (data, error) -> Void in

                    let recipeImage = UIImage(data: data!)

                    food.append(Element(name: recipeName!, description: recipeDescription!, image: recipeImage!))
                    self.tableView.reloadData()
                }
            }
        }
    })

Answer 1

let dict = user_snap.value as! [String: String?]

代替

let dict = snapshot.value as! Dictionary<String, String>

也许你可以做空测试：

let dict = snapshot.value as! Dictionary<String, String>

if let recipeName = dict["Name"] as String!, let recipeDescription = dict["Description"] as String!, let downloadURL = dict["Image"] as String! {
    let storageRef = storage.reference(forURL: downloadURL)

    storageRef.getData(maxSize: 1 * 1024 * 1024) { (data, error) -> Void in

        let recipeImage = UIImage(data: data!)

        food.append(Element(name: recipeName, description: recipeDescription, image: recipeImage!, downloadURL: downloadURL))
        self.tableView.reloadData()                        
    }                    
}else {
    print("Error! Could not decode data")                    
}

Answer 2

试试这个。它应该工作

for child in snapshot.children.allObjects as! [FIRDataSnapshot] {

let dict = child.value as! Dictionary<String, Any>

//.....
}