调度组：for循环和for循环内的异步调用

Question

我了解如何在简单的 for 循环中使用调度组。但是，我有一个更复杂的 for 循环，其中包含更多异步调用。我希望在执行组完成代码之前完成所有异步调用。我试图应用此答案中的原则无济于事- 。这是我的代码，基于我在 link:

中观察到的技术

let group = DispatchGroup()

for ref in self.notifsRefList {
    group.enter()
    self.db.fetch(withRecordID: ref.recordID) { notifRecord, notifErr in
        print("async call")
        if notifErr == nil {

            // do stuff

            if let ref = notifRecord?.object(forKey: POST) as! CKReference? {
                group.enter()
                self.db.fetch(withRecordID: ref.recordID) { postRecord, err in
                    print("async call")
                    if err == nil {
                        // do stuff
                        group.leave()
                    }
                    else {
                        print("\(err)")
                        group.leave()
                    }
                }
            }
            if let ref = notifRecord?.object(forKey: USER_NOTIF) as! CKReference? {
                self.db.fetch(withRecordID: ref.recordID) { userRecord, err2 in
                    group.enter()
                    print("async call")
                    if err2 == nil {
                        // do stuff
                        group.leave()
                    }
                    else {
                        print("\(err2)")
                        group.leave()
                    }
                }
            }
            if let ref = notifRecord?.object(forKey: LIBRARY_ITEM) as! CKReference? {
                self.db.fetch(withRecordID: ref.recordID) { libRecord, err3 in
                    group.enter()
                    print("async call")
                    if err3 == nil {
                        // do stuff                        
                        group.leave()
                    }
                    else {
                        print("\(err3)")
                        group.leave()
                    }
                }
            }

            group.leave()

        }
        else {
            print("\(notifErr)")
            group.leave()
        }
    }

}


group.notify(queue: .main, execute: { // executed after all async calls in for loop finish
    print("done with all async calls")
    // do stuff
})

从我包含的打印语句中，我知道我的异步调用是不正确的：有时 "done with all async calls" 在 "async call." 的所有实例之前打印任何关于如何让这个调度组正常工作的帮助将是非常感激。谢谢！

Answer 1

问题是第 2 和第 3 个内部异步调用。您在完成块内而不是在异步调用之前调用 group.enter()。

也不需要那么多次调用leave。

你需要移动两个如下：

let group = DispatchGroup()

for ref in self.notifsRefList {
    group.enter()
    self.db.fetch(withRecordID: ref.recordID) { notifRecord, notifErr in
        print("async call")
        if notifErr == nil {

            // do stuff

            if let ref = notifRecord?.object(forKey: POST) as! CKReference? {
                group.enter()
                self.db.fetch(withRecordID: ref.recordID) { postRecord, err in
                    print("async call")
                    if err == nil {
                        // do stuff
                    }
                    else {
                        print("\(err)")
                    }
                    group.leave()
                }
            }
            if let ref = notifRecord?.object(forKey: USER_NOTIF) as! CKReference? {
                group.enter()
                self.db.fetch(withRecordID: ref.recordID) { userRecord, err2 in
                    print("async call")
                    if err2 == nil {
                        // do stuff
                    }
                    else {
                        print("\(err2)")
                    }
                    group.leave()
                }
            }
            if let ref = notifRecord?.object(forKey: LIBRARY_ITEM) as! CKReference? {
                group.enter()
                self.db.fetch(withRecordID: ref.recordID) { libRecord, err3 in
                    print("async call")
                    if err3 == nil {
                        // do stuff                        
                    }
                    else {
                        print("\(err3)")
                    }
                    group.leave()
                }
            }
        }
        else {
            print("\(notifErr)")
        }
        group.leave()
    }
}

group.notify(queue: .main, execute: { // executed after all async calls in for loop finish
    print("done with all async calls")
    // do stuff
})

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