ios - (Swift + Firebase)转换observeSingleEvent以观察.childAdded

我的viewDidLoad函数中有一个observeSingleEvent，我想将它改为observeswith.childAdded，这样它将不断侦听，并在对象添加到数据库时添加它们。
下面是从firebase读取的代码：

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

    parentRef.observeSingleEvent(of: .value, with: { snapshot in

        // PROCESSES VALUES RECEIVED FROM SERVER
        if ( snapshot.value is NSNull ) {

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

        } else {

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

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

                // DEFINE VARIABLES FOR LABELS
                let recipeName = dict["Name"] as? String
                let recipeDescription = dict["Description"] as? String

                food.append(Element(name: recipeName!, description: recipeDescription!))

                self.tableView.reloadData()
            }
        }
    })

我现在的另一个问题（除了它只加载一次）是，每当我转到应用程序中的另一个视图，然后回来时，它会再次读取所有内容，这样我的tableView中的所有内容都会重复，这种情况还会发生吗？
编辑：
以下是我的数据库：

Recipes
   -Kv7FAqgLtDrRoyGd-99
       Description: "food description"
       Name: "food name"
   -KvBuzMUnIQXn8gpG2WL
       Description: "food description2"
       Name: "food name2"
   -KvH6yYeJaThK7oP8xBj
       Description: "food description3"
       Name: "food name3"

最佳答案

将observeSingleEvent更改为observe。
观察新值时清空食物数组。
在for循环外重新加载食物数组，以便仅在将所有项目加载到数组中时才重新加载。

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

// 1. Change to observe
parentRef.observe(.value, with: { snapshot in

   // PROCESSES VALUES RECEIVED FROM SERVER
   if ( snapshot.value is NSNull ) {

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

   } else {

       // 2. Empty food array
       self.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?]

           // DEFINE VARIABLES FOR LABELS
           let recipeName = dict["Name"] as? String
           let recipeDescription = dict["Description"] as? String

           food.append(Element(name: recipeName!, description: recipeDescription!))

        }
        // 3. reload tableview outside loop
        self.tableView.reloadData()
    }
})

通过在观察新值时清空食物数组，可以确保只显示数据库中存在的值，因此不会有重复的元素。