观察调用不更新字典值。异步的问题

Question

我正在尝试从我的 firebase 数据库中获取图像列表。在观察方法中，如果我打印帖子的数量，它可以正常工作。如果我在观察函数之外打印帖子数，但仍在 fetchPosts() 函数内，我得到 0。如果我在 fetchPosts() 调用（使用观察的函数）之后打印帖子数，我得到 0。

如何在此异步调用中将值保存到我的字典帖子中？我已经尝试过完成和调度组。我可能没有正确实现它们，所以如果你看到一个简单的方法，那么请帮助我。代码如下：

import UIKit
import SwiftUI
import Firebase
import FirebaseUI
import SwiftKeychainWrapper

class FeedViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource{


    @IBOutlet weak var collectionview: UICollectionView!

   //var posts = [Post]()


    var posts1 = [String](){
    didSet{
    collectionview.reloadData()
    }
    }
    var following = [String]()
    //var posts1 = [String]()
    var userStorage: StorageReference!
    var ref : DatabaseReference!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        fetchPosts()


    }

//    func lengthyTask(completionHandler: (Int) -> Int)
//    {
//        let result = completionHandler(42)
//        print(result)
//    }
//
//    lengthyTask(completionHandler: { number in
//        print(number)
//        return 101
//    })
//
    func  fetchPosts() {
        let uid = Auth.auth().currentUser!.uid
        let ref = Database.database().reference().child("posts")
        let uids = Database.database().reference().child("users")
        uids.observe(DataEventType.value, with: { (snapshot) in
            let dict = snapshot.value as! [String:NSDictionary]
            for (_,value) in dict {
                if let uid = value["uid"] as? String{
                    self.following.append(uid)
                }
            }
        ref.observe(DataEventType.value, with: { (snapshot2) in
            let dict2 = snapshot2.value as! [String:NSDictionary]
            for(key, value) in dict{
                for uid2 in self.following{
                    if (uid2 == key){
                        for (key2,value2) in value as! [String:String]{
                            //print(key2 + "this is key2")
                            if(key2 == "urlToImage"){
                                let urlimage = value2
                                //print(urlimage)
                                self.posts1.append(urlimage)
                                self.collectionview.reloadData()
                                print(self.posts1.count)

                            }
                        }

                    }
                }
            }
            })
            self.collectionview.reloadData()
        })
    //ref.removeAllObservers()
    //uids.removeAllObservers()
    print("before return")
    print(self.posts1.count)


    //return self.posts1
    }
    func numberOfSections(in collectionView: UICollectionView) ->Int {
        return 1
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return posts1.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "PostCell", for: indexPath) as! PostCell
        cell.postImage.sd_setImage(with: URL(string: posts1[indexPath.row]))
        //creating the cell
        //cell.postImage.downloadImage(from: self.posts[indexPath.row])
//        let storageRef = Storage.storage().reference(forURL: self.posts[indexPath.row].pathToImage)
//
//
        print("im trying")

        //let stickitinme = URL(fileURLWithPath: posts1[0])
        //cell.postImage.sd_setImage(with: stickitinme)

        //cell.authorLabel.text = self.posts[indexPath.row].author
        //cell.likeLabel.text = "\(self.posts[indexPath.row].likes) Likes"

        return cell
    }


    @IBAction func signOutPressed(_sender: Any){
           signOut()
           self.performSegue(withIdentifier: "toSignIn", sender: nil)
       }

    @objc func signOut(){
           KeychainWrapper.standard.removeObject(forKey:"uid")

           do{
               try Auth.auth().signOut()
           } catch let signOutError as NSError{
               print("Error signing out: %@", signOutError)
           }
           dismiss(animated: true, completion: nil)
       }


    /*
    // 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.destination.
        // Pass the selected object to the new view controller.
    }
    */

}

Answer 1

您只需要稍作改动

1. 简单地声明posts1

   var posts1 = [String]()
   

   并删除属性观察者didSet

2. 在self.posts1.append(..之后删除self.collectionview.reloadData()这一行

3. 将最后出现的self.collectionview.reloadData() 上移一级，将其包装在DispatchQueue 块中以更新主线程上的集合视图并删除外部闭包后的print 行

              }
              DispatchQueue.main.async {
                  self.collectionview.reloadData()
              }
           })
       })
   }
   

4. 而且第二个闭包中有一个错字。应该是

   let dict2 = snapshot2.value as! [String:NSDictionary]
   for(key, value) in dict2 {
   

   带有尾随索引的变量名很容易出错，最好是 userDict 和 postDict

编辑：

这是执行顺序的代码

override func viewDidLoad() {
    super.viewDidLoad()
    collectionview.dataSource = self
    collectionview.delegate = self
    // 1
    fetchPosts()
    // 5 
}

func fetchPosts() {
    // 2       
    let uid = Auth.auth().currentUser!.uid
    let ref = Database.database().reference().child("posts")
    let uids = Database.database().reference().child("users")
    // 3
    uids.observe(DataEventType.value, with: { (snapshot) in
        // 6
        let dict = snapshot.value as! [String:NSDictionary]
        for (_,value) in dict {
            if let uid = value["uid"] as? String{
                self.following.append(uid)
            }
        }
        // 7
        ref.observe(DataEventType.value, with: { (snapshot2) in
            // 9
            let dict2 = snapshot2.value as! [String:NSDictionary]
            for(key, value) in dict2 { // TYPO!!!!
                for uid2 in self.following{
                    if (uid2 == key){
                       for (key2,value2) in value as! [String:String]{
                            //print(key2 + "this is key2")
                            if(key2 == "urlToImage"){
                                let urlimage = value2
                                //print(urlimage)
                                self.posts1.append(urlimage)
                                print(self.posts1.count)

                            }
                       }

                    }
                }
            }
            DispatchQueue.main.async {
               // 11
               self.collectionview.reloadData()
            }
            // 10
        })
        // 8
    })
    // 4
}