在异步工作完成之前关闭返回数据

Question

更新了建议的解决方案和其他问题

我正式卡住了，也陷入了回调地狱。我打电话给 Firebase 以检索 FireStore 中的所有文章。在每个文章对象中都有一个图像文件名，该文件名转换为需要传递给函数以获取绝对 URL 返回的存储引用位置。我会将 URL 存储在数据中，但它可能会改变。问题是 ArticleListener 函数在没有所有数据的情况下过早地返回闭包 (returnArray)，我不知道我遗漏了什么。在我添加 self.getURL 代码之前，这工作正常，但现在它返回数组为空，然后完成所有工作。

如果有人在这里有一些关于将方法链接在一起而不求助于 PromiseKit 或 GCD 的奖励提示，那会很棒，但欢迎所有建议以使其按原样工作 and/or 重构以提高效率/可读性！

使用 GCD 和更新示例的建议解决方案

这是在创建文章后调用作者初始化。我正在尝试转换 dataDict 字典，以便它在 Author init 期间用于键 ["author"]。我想我已经接近了，但不能 100% 确定我的 GCD enter/leave 调用是否按正确的顺序进行

public func SetupArticleListener(completion: @escaping ([Article]) -> Void) {
        var returnArray = [Article]()
        let db = FIRdb.articles.reference()

        let listener = db.addSnapshotListener() { (querySnapshot, error) in
            returnArray = [] // nil this out every time
            if let error = error {
                print("Error in setting up snapshot listener - \(error)")
                } else {

                let fireStoreDispatchGrp = DispatchGroup() /// 1

                querySnapshot?.documents.forEach {
                    var dataDict = [=10=].data() //mutable copy of the dictionary data
                    let id = [=10=].documentID

//NEW EXAMPLE WITH ADDITIONAL TASK HERE
                    if let author = [=10=].data()["author"] as? DocumentReference {
                        author.getDocument() {(authorSnapshot, error) in
                            fireStoreDispatchGrp.enter() //1
                            if let error = error {
                                print("Error getting Author from snapshot inside Article getDocumentFunction - leaving dispatch group and returning early")
                                fireStoreDispatchGrp.leave()
                                return
                            }

                            if let newAuthor = authorSnapshot.flatMap(Author.init) {
                                print("Able to build new author \(newAuthor)")
                                dataDict["author"] = newAuthor
                                dataDict["authorId"] = authorSnapshot?.documentID
                                print("Data Dict successfully mutated \(dataDict)")
                            }
                            fireStoreDispatchGrp.leave() //2
                        }

                    }

///END OF NEW EXAMPLE

                    if let imageURL = [=10=].data()["image"] as? String {
                        let reference = FIRStorage.articles.referenceForFile(filename: imageURL)

                        fireStoreDispatchGrp.enter() /// 2

                        self.getURL(reference: reference){ result in
                            switch result {
                            case .success(let url) :
                                dataDict["image"] = url.absoluteString

                            case .failure(let error):
                                print("Error getting URL for author: \n Error: \(error) \n forReference: \(reference) \n forArticleID: \(id)")
                            }

                            if let newArticle = Article(id: id, dictionary: dataDict) {
                                returnArray.append(newArticle)
                            }

                            fireStoreDispatchGrp.leave() ///3
                        }
                    }
                }
                //Completion block
                print("Exiting dispatchGroup all data should be setup correctly")
                fireStoreDispatchGrp.notify(queue: .main) { ///4
                completion(returnArray)

                }
            }
        }
        updateListeners(for: listener)
    }

原码

调用设置代码

self.manager.SetupArticleListener() { [weak self] articles in
                    print("In closure function to update articles")
                    self?.articles = articles

                }

文章监听器

 public func SetupArticleListener(completion: @escaping ([Article]) -> Void) {
        var returnArray = [Article]()
        let db = FIRdb.articles.reference()

        let listener = db.addSnapshotListener() { (querySnapshot, error) in
            returnArray = [] // nil this out every time
            if let error = error {
                printLog("Error retrieving documents while adding snapshotlistener, Error: \(error.localizedDescription)")
            } else {

                querySnapshot?.documents.forEach {
                    var dataDict = [=12=].data() //mutable copy of the dictionary data
                    let id = [=12=].documentID
                    if let imageURL = [=12=].data()["image"] as? String {
                        let reference = FIRStorage.articles.referenceForFile(filename: imageURL)
                        self.getURL(reference: reference){ result in
                            switch result {
                            case .success(let url) :
                                print("Success in getting url from reference \(url)")
                                dataDict["image"] = url.absoluteString
                                print("Dictionary XFORM")

                            case .failure(let error):
                                print("Error retrieving URL from reference \(error)")
                            }

                            if let newArticle = Article(id: id, dictionary: dataDict) {
                                printLog("Success in creating Article with xformed url")
                                returnArray.append(newArticle)
                            }
                        }
                    }
                }
                print(" sending back completion array \(returnArray)")
                completion(returnArray)
            }
        }
        updateListeners(for: listener)
    }

得到URL

private func getURL(reference: StorageReference, _ result: @escaping (Result<URL, Error>) -> Void) {
        reference.downloadURL() { (url, error) in
            if let url = url {
                result(.success(url))
            } else {
                if let error = error {
                    print("error")
                    result(.failure(error))
                }
            }

        }
    }

Answer 1

您需要调度组，因为 for 循环包含多个异步调用

public func SetupArticleListener(completion: @escaping ([Article]) -> Void) {
    var returnArray = [Article]()
    let db = FIRdb.articles.reference()

    let listener = db.addSnapshotListener() { (querySnapshot, error) in
        returnArray = [] // nil this out every time
        if let error = error {
            printLog("Error retrieving documents while adding snapshotlistener, Error: \(error.localizedDescription)")
        } else {

            let g = DispatchGroup() /// 1
            querySnapshot?.documents.forEach {
                var dataDict = [=10=].data() //mutable copy of the dictionary data
                let id = [=10=].documentID
                if let imageURL = [=10=].data()["image"] as? String {
                    let reference = FIRStorage.articles.referenceForFile(filename: imageURL)
                    g.enter() /// 2
                    self.getURL(reference: reference){ result in
                        switch result {
                        case .success(let url) :
                            print("Success in getting url from reference \(url)")
                            dataDict["image"] = url.absoluteString
                            print("Dictionary XFORM")

                        case .failure(let error):
                            print("Error retrieving URL from reference \(error)")
                        }

                        if let newArticle = Article(id: id, dictionary: dataDict) {
                            printLog("Success in creating Article with xformed url")
                            returnArray.append(newArticle)
                        }

                        g.leave() /// 3
                    }
                }
            }
            g.notify(queue:.main) {   /// 4
              print(" sending back completion array \(returnArray)")
              completion(returnArray)
            }    
        }
    }
    updateListeners(for: listener)
}