为什么我的iOS应用程序无法从Firebase数据库读取？

Question

I am currently having troubling reading and displaying my firebase database into a table list. Below is the code I am using in its current View Controller.

import UIKit
import Firebase
import FirebaseDatabase

class Buy_1: UIViewController, UITableViewDataSource, 
 UITableViewDelegate   {

var ref: DatabaseReference!
var databaseHandle: DatabaseHandle!

var postItem: [String] = []

@IBOutlet weak var TableView: UITableView!

@IBAction func goBackToOneButtonTapped(_ sender: Any) {
    performSegue(withIdentifier: "unwindSegueToVC1", sender: self)
}

override func viewDidLoad() {
    super.viewDidLoad()

    if FirebaseApp.app() == nil {
        FirebaseApp.configure()
    }

    //Set Self as tableview's data source and delegate
    self.TableView.delegate = self
    self.TableView.dataSource = self

    //Set the firebase reference
    ref = Database.database().reference()

    //Retrieve and listen for changes
    databaseHandle = ref?.child ("items").observe(.childAdded, with: { (snapshot) in

        //Convert value of data into string
        if let item = snapshot.value as? String
        {
            self.postItem.append(item)
            self.TableView.reloadData()
            self.ref?.keepSynced(true)
        }

        /*if let actualItem = item {
            //Appends data to our Item Array
            self.postItem.append(actualItem)

            //Reload TableView
            self.TableView.reloadData()
        }*/

    })
    // Do any additional setup after loading the view.
}


override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

/*
// MARK: - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    // Get the new view controller using segue.destinationViewController.
    // Pass the selected object to the new view controller.
}
*/

//MARK: - UITableView Delegate Methods


func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return postItem.count
}

func tableView(_ TableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = TableView.dequeueReusableCell(withIdentifier: "BasicCell", for: indexPath)

    cell.textLabel?.text = postItem[indexPath.row]
    return cell

  }
}

I do not receive any errors in the code so I am not sure what is wrong. & I have already set the database rules to true.

Thank you.

Answer 1

You are observing .childAdded , which will trigger your block only when a child is being added to your observed tree of data. Change it to .value .

Answer 2

The issue in the code is that the children of items are not strings. They in themselves are childSnapshots or key:value pairs.

Here's some example code to read just the itemName from a structure similar to yours

items
   item_0
      itemName: "Josh"
   item_1
      itemName: "Screaming Eagle"
   item_2
      itemName: "Insignia"

and the code to print the key of each child node and the itemName

let ref = self.ref.child("items")
ref.observe(.childAdded, with: { snapshot in
    if snapshot.exists() {
        let dict = snapshot.value as! [String: Any]
        let itemName = dict["itemName"] as? String ?? "No Item Name"
        print(snapshot.key, itemName)
    }
})

and the output

item_0 Josh
item_1 Screaming Eagle
item_2 Insignia

Note that I used a nil coalescing operator (the ??) to populate the itemName in case it was not found.

I am treating the value portion of the snapshot as a dictionary to allow you to access the child nodes but you could also accomplish that with .childSnapshot like this

let i = snapshot.childSnapshot(forPath: "itemName").value as? String ?? "No Item"

The snapshot.exists is extraneous in this specific example but it should be used with reading by .value.