下载 Firebase 存储数据时，DispatchGroup 未正确执行答案

【问题标题】：DispatchGroup is not executing correctly when downloading Firebase Storage Data下载 Firebase 存储数据时，DispatchGroup 未正确执行
【发布时间】：2021-11-07 02:08:08
【问题描述】：

我有一个从 Firebase 获取用户数据的闭包，在其中我有另一个从 Firebase 存储下载图像的闭包。

在用户登录之前，我需要能够从存储中下载图像。我想我会尝试一个 DispatchGroup()，但我无法将图像数据分配给 self.photoImage，它总是返回 nil。

self.photoImage = UIImage(data: data!)

这是我第一次使用 Storage 和 GroupDispatch()，因此我们将不胜感激。

相关代码：

class AppViewModel: ObservableObject {
    private var db = Firestore.firestore()
    
    @Published var userInfo: User?
    @Published var signedIn: Bool = false
    
    var handle: AuthStateDidChangeListenerHandle?
    let authRef = Auth.auth()
    
    var authHandle : AuthStateDidChangeListenerHandle?
    var rootInfoCollection : CollectionReference!
    var userIdRef = ""

    var photoImage: UIImage?
    var downloadImageTask: StorageReference?
    
    var group = DispatchGroup()
    
    func fetchUserData(){
        db.collection("Users").document("\(userIdRef)").getDocument { [self] document, error in
            // Check for error
            if error == nil {
                // Check that this document exists
                if document != nil && document!.exists {
                                        
                    self.userInfo = document.map { (documentSnapshot) -> User in
                        let data = documentSnapshot.data()
                        
                        let uid = data?["uid"] as? UUID ?? UUID()
                        let company = data?["company"] as? String ?? ""
                        let name = data?["name"] as? String ?? ""
                        let admin = data?["admin"] as? Bool ?? false
                        let photoRef = data?["photoRef"] as? String ?? ""
                        
                        self.downloadImageTask = Storage.storage().reference().child(photoRef)
                        
                        return User(uid: uid, company: company, name: name, admin: admin, photoRef: photoRef, photoImage: nil)
                    }
                    
                    group.enter()
                    downloadImageTask?.getData(maxSize: 6 * 1024 * 1024) { data, error in
                        if let error = error {
                            print("Got an error fetching data: \(error.localizedDescription)")
                            return
                        } else {
                            self.photoImage = UIImage(data: data!)
                        }
                        self.group.leave()
                    }
                    
                    DispatchQueue.global(qos: .default).async {
                        
                        self.group.wait()
                        
                        DispatchQueue.main.async {
                            
                            if photoImage != nil {
                                print("Photo Image Is Set")
                            }
                            
                            withAnimation {
                                self.signedIn = true
                            }
                        }
                    }
                    
                    
                    
                }
            }
        }
    }

【问题讨论】：

你的 DispatchGroup 的整个结构是错误的，无论如何这不是 DispatchGroup 合适或需要的情况。
我建议你去读一些关于 GCD 的东西，Ray Wenderlich 的教程很不错raywenderlich.com/…

标签： swift firebase firebase-storage

【解决方案1】：

虽然我没有测试过，但也许这对你有用：

class AppViewModel: ObservableObject {

    private var db = Firestore.firestore()

    @Published var userInfo: User?
    @Published var signedIn: Bool = false

    var handle: AuthStateDidChangeListenerHandle?
    let authRef = Auth.auth()

    var authHandle : AuthStateDidChangeListenerHandle?
    var rootInfoCollection : CollectionReference!
    var userIdRef = ""

    var photoImage: UIImage?
    var downloadImageTask: StorageReference?

    var group = DispatchGroup()

    func fetchUserData(){
        db.collection("Users").document("\(userIdRef)").getDocument { [self] document, error in
            // Check for error
            if error == nil {
                // Check that this document exists
                if document != nil && document!.exists {
                                    
                    self.userInfo = document.map { (documentSnapshot) -> User in
                        let data = documentSnapshot.data()
                    
                        let uid = data?["uid"] as? UUID ?? UUID()
                        let company = data?["company"] as? String ?? ""
                        let name = data?["name"] as? String ?? ""
                        let admin = data?["admin"] as? Bool ?? false
                        let photoRef = data?["photoRef"] as? String ?? ""
                    
                        self.downloadImageTask = Storage.storage().reference().child(photoRef)
                    
                        return User(uid: uid, company: company, name: name, admin: admin, photoRef: photoRef, photoImage: nil)
                    }
                
                    group.enter()
                    downloadImageTask?.getData(maxSize: 6 * 1024 * 1024) { data, error in
                        if let error = error {
                            print("Got an error fetching data: \(error.localizedDescription)")
                            return
                        } else {
                            self.photoImage = UIImage(data: data!)
                        }
                        self.group.leave()
                    }
                    group.notify(queue: .main) { [self] in
                        if photoImage != nil {
                            print("Photo Image Is Set")
                        }
                        
                        withAnimation {
                            self.signedIn = true
                        }
                    }
                }
            }
        }
    }

【讨论】：