我正式陷入困境，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，该文件名会转换为存储引用位置，该位置需要传递给函数以获取绝对URL。我会将URL存储在数据中，但是可能会更改。问题是ArticleListener函数会在没有所有数据的情况下过早返回闭包（returnArray），我无法弄清丢失的内容。在添加 self.getURL 代码之前，此方法工作正常，但现在它将数组返回为空，然后完成所有工作。
如果有人在不使用PromiseKit或GCD的情况下将方法链接在一起有一些额外的提示，那将是不错的选择，但欢迎所有建议使它们按原样工作 和/或重构以提高效率/可读性！
带有GCD的建议解决方案和更新的示例
在创建文章后，这将调用Author初始化。我正在尝试转换dataDict词典，以便它在键[“ author”]的Author初始化期间使用。我想我已经接近了，但不能100％确定我的GCD进入/离开呼叫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)
}
呼叫设置代码
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 = $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)
}
GetURL
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))
}
}
}
}
答案 0 :(得分：2)
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)
}