ios - (Swift + Firebase) 将 observeSingleEvent 转换为观察 .childAdded

Question

我的 viewDidLoad 函数中有一个 observeSingleEvent，我想将其更改为带有 .childAdded 的 observe，以便它会持续监听并在对象添加到数据库时添加它们。

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(除了它只加载一次)是，每当我转到应用程序中的另一个 View 然后返回时，它会再次读取所有内容，所以我会在我的 tableView 中获得所有内容的副本，这是否仍然会发生用观察？

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. 更改 observeSingleEvent 以进行观察。
2. 每当观察到新值时清空食物数组。

3. 在 for 循环之外重新加载您的食物数组，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()
       }
   })
   

通过在每次观察到新值时清空食物数组，您可以确保只显示数据库中存在的值，因此绝不会出现重复元素。