(Swift Firebase)转换观察levent以观察.Childadded

Question

我在查看didload函数中有一个观察力，我想将其更改为使用.ChildAdded的观察，以便它会不断地聆听并在将对象添加到数据库中。<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

这是从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()
            }
        }
    })

我现在遇到的另一个问题（此外，只有一次加载一次）是，每当我转到应用程序中的另一个视图然后再回来时，它将再次读取所有内容与观察？

编辑：

这是我的数据库的样子：

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

Answer 1

1. 改变观察的观察。
2. 每当观察新值时，空食物阵列。

3. 在循环外面重新加载食物阵列，以便只有在将所有物品加载到阵列中时才能重新加载。

   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()
       }
   })
   

每当观察新值时清空食物阵列，您可以确保仅显示数据库中存在的值，从而永远不会具有重复性元素。