Swift和Firebase：封闭竞赛条件

问题描述

我需要在闭包内调用闭包，并避免出现竞争状况。

当前，我有一个名为 FirebaseUtilities 的类，其中包括与调用Firebase和检索数据有关的所有代码。

应用逻辑

文章包含大量数据，例如标题，摘要，正文，主题等。主题使用以下约定链接到文章：

在此类中，我有一个获取文章的函数：

获取文章的时间段

func fetchArticles(completion: @escaping ([Article]?,Error?) -> Void) {

    var subjects = [Subject]()

    Database.database().reference().child("articles").observe(.value) { (snapshot) in
        // in here i get the value of the snapshot and from there i extract the key of each subject and i store it in an array of String (ex. ["-MD4tN_AjPuCvTUEyZyL"])

        // then I call another closure that is responsible for fetching the subjects based on the UID of each subject (see below)
    }
    
})

关闭主题

func fetchSubjects(subjectsUids: [String],completion: @escaping ([Subject]?,Error?) -> Void) {
    
    var subjects = [Subject]()
    
    subjectsUids.forEach { (subjectUid) in
        self.dbSubjectsRef.child(subjectUid).observe(.value) { (snapshot) in
            guard let subjectDictionary = snapshot.value as? [String: Any] else { return }
            
            let subject = Subject(dictionary: subjectDictionary)
            
            subjects.append(subject)
            
            completion(subjects,nil)
        }
    }
}

问题

主题数组将始终为空数组，因为它始终比从Firebase提取数据快：

func fetchArticles(completion: @escaping ([Article]?,Error?) -> Void) {

    var subjects = [Subject]()

    Database.database().reference().child("articles").observe(.value) { (snapshot) in
        // in here i get the value of the snapshot and from there i extract the key of each subject and i store it in an array of String (ex. ["-MD4tN_AjPuCvTUEyZyL"])

        // then I call another closure that is responsible for fetching the subjects based on the UID of each subject

        self.fetchSubjects(subjectsUids: subjectsUids) { (subjectsResults,error) in
            guard let sub = subjectsResults else { return }
            subjects = sub
            
            // (A) printing here **shows desired results**
            print(subjects)
         }

         // (B) printing here **does not show the results**
         // it returns an empty array []
         print(subjects)
    }    
})

我想要什么：

我需要在第二个闭包（self.fetchSubjects（....））之外获取主题数组。

关于如何解决这种比赛条件的任何想法？

解决方法

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