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附加问题

我正式被困住了，也陷入了回调地狱。 我打电话给Firebase，检索FireStore中的所有文章。 每个文章对象内部都有一个图像文件名，该文件名转换为存储引用位置，需要将其传递给函数才能返回绝对 URL。 我会将 URL 存储在数据中，但它可能会更改。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(，我无法弄清楚我错过了什么。 在我添加self.getURL代码之前，这工作正常，但现在它将数组返回为空，然后完成所有工作。

如果有人在这里有一些关于在不诉诸 PromiseKit 或 GCD 的情况下将方法链接在一起的额外提示，那就太好了，但对所有建议持开放态度，以使其按原样工作 和/或重构以提高效率/可读性！

使用 GCD 和更新的示例提出的解决方案

这是在创建文章后调用作者 init。 我正在尝试转换 dataDict 字典，以便在作者初始化键 ["author"] 期间使用它。 我想我很接近，但不是 100% 确定我的 GCD 进入/离开电话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 = $0.data() //mutable copy of the dictionary data
let id = $0.documentID
//NEW EXAMPLE WITH ADDITIONAL TASK HERE
if let author = $0.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 = $0.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)
}

原始代码

调用设置代码

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

文章侦听器

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 = $0.data() //mutable copy of the dictionary data
let id = $0.documentID
if let imageURL = $0.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)
}

获取网址

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))
}
}
}
}

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 = $0.data() //mutable copy of the dictionary data
let id = $0.documentID
if let imageURL = $0.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)
}