下载Firebase Storage数据时DispatchGroup执行异常，photoImage始终为nil怎么解决？

核心问题点 #

• 下载图片的错误分支未调用group.leave()：当图片下载出错时直接return，没有触发离开组的逻辑，导致group.wait()无限等待，后续赋值和登录逻辑永远不会执行
• DispatchGroup定义为全局实例属性：如果多次调用fetchUserData会导致组计数混乱，应该改为方法内的局部变量
• 下载成功后未保证在主队列赋值UI相关属性：UIImage赋值属于UI相关操作，Firebase Storage的回调默认不是主队列，直接赋值可能引发异常甚至赋值失败
• photoImage未标记为@Published：如果需要在SwiftUI视图中监听这个属性的变化，没有@Published修饰的话视图不会收到更新通知，也会出现看起来赋值失败的假象

修复后的代码 #

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

    var downloadImageTask: StorageReference?
    // 移除全局的DispatchGroup，改为方法内局部变量
    
    func fetchUserData(){
        db.collection("Users").document("\(userIdRef)").getDocument { [self] document, error in
            // 合并空值判断逻辑简化代码
            if error == nil, let document = document, 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)
                }
                
                let group = DispatchGroup() // 局部DispatchGroup，避免多次调用冲突
                guard let downloadTask = downloadImageTask else {
                    // 没有下载任务直接走登录逻辑
                    DispatchQueue.main.async {
                        withAnimation {
                            self.signedIn = true
                        }
                    }
                    return
                }
                
                group.enter()
                downloadTask.getData(maxSize: 6 * 1024 * 1024) { data, error in
                    defer { group.leave() } // 用defer保证不管是否出错都调用leave
                    if let error = error {
                        print("Got an error fetching data: \(error.localizedDescription)")
                        return
                    }
                    guard let data = data, let image = UIImage(data: data) else {
                        print("图片数据无效")
                        return
                    }
                    DispatchQueue.main.async { // 主队列赋值UI属性保证生效
                        self.photoImage = image
                    }
                }
                
                group.notify(queue: .main) { // 用notify替代wait，不会阻塞线程
                    if self.photoImage != nil {
                        print("Photo Image Is Set")
                    }
                    withAnimation {
                        self.signedIn = true
                    }
                }
            }
        }
    }
}

额外优化说明 #

• 用group.notify替代group.wait：wait会阻塞当前线程，notify是任务全部完成后自动回调到指定队列，性能更好也不会有死锁风险
• 新增空值判断避免强制解包 crash：去掉了data!的强制解包，增加有效性校验
• 用defer统一管理group.leave：避免漏写离开逻辑的问题

内容的提问来源于stack exchange，提问作者RapsIn4