Question

我有一个转义功能，一旦满足条件就可以完成：

  private func xxxfastLoadLSecurityDescriptions(session: URLSession, mySymbols: [String]?, completion: @escaping(Bool) ->())    {
        
        var counter = mySymbols?.count ?? 0
        if counter == 0 { completion(false) }
        var doubleCount = 0
        // print("DESCRIPTION Starting Counter = \(counter)")
        
        for symbolIndex in 0..<(mySymbols?.count ?? 0) {
            guard let mySymbol = mySymbols?[symbolIndex] else { print("ERROR in fastLoadLSecurityDescriptions loop: No Symbol") ;  continue }
            guard let myGetDescriptionRequest = GenericDataRequest(dataToSend: [mySymbol], token: sessionToken, mgbRoute: MGBServerRoutes.retrieveSecurityDescriptionsRoute!)
            else { print("Error Getting Security Description Request for \(mySymbol)") ; return }
            
            mySessionSendMGBGenericRequest(session: session, request: myGetDescriptionRequest) { [weak self]  success, serverMessage, returnUNISecurityDescription in
                guard let self = self else { print("ERROR: self is nil") ; return }
                if returnUNISecurityDescription?.count == 0 { print("nil returnUniSecurityDescription for \(mySymbol)") }
                // print("DESCRIPTIONS COUNTER = \(counter)")
                counter -= 1
                var myDescription = UNISecurityDescription()
                if returnUNISecurityDescription != nil, returnUNISecurityDescription?.count != 0 { myDescription = returnUNISecurityDescription![0]  }
                if myDescription.name == nil || myDescription.name == "" { print("Error: No Name for \(String(describing: mySymbol))") }
                let myContainersIndices = self.myUNIList.singleContainer.indices.filter({ self.myUNIList.singleContainer[$0].ticker?.symbol == mySymbol })
                var myPathArray = [IndexPath]()
                for index in 0..<myContainersIndices.count {
                    self.myUNIList.singleContainer[myContainersIndices[index]].name = myDescription.name
                    self.myUNIList.singleContainer[myContainersIndices[index]].currencySymbol = myDescription.currencySymbol
                    self.myUNIList.singleContainer[myContainersIndices[index]].fillFundamentals() // --> Fills the outputs for sortdata
                    myPathArray.append(IndexPath(row: myContainersIndices[index], section: 0))
                }
                DispatchQueue.main.async {
                    self.filteredData = self.myUNIList.singleContainer
                    self.myCollection?.reloadItems(at: myPathArray)
                }
                if counter == 0     {   // THIS IS TRUE MORE THAN ONCE WHILE IT SHOULD NOT BE TRU MORE THAN ONCE
                    if doubleCount > 0 {    print("WHY!!!!")  }
                    doubleCount += 1
                    print("DESCRIPTIONS counter = \(counter) -> \(self.myUNIList.listName) - symbols: \(String(describing: mySymbols?.count)) \n==================================================\n")
                    DispatchQueue.main.async {   self.sortNormalTap("Load")  { _ in     self.displayAfterLoading()  } }
                    completion(true)
                    return
                }
            }
        }
    }

要满足的条件是counter ==0。一旦满足，函数将完成并退出DispatchGroup。问题是计数器== 0多次为true（退出DispatchGroup时明显崩溃）。我真的不明白为什么会多次满足这个条件。代码是线性的，我看不出是什么原因造成的。任何帮助都非常感谢。这让我发疯。

Answer 1

您的代码不是线程安全的，尤其是计数器。我使用相同的逻辑编写了一个示例，以说明这一点。如果运行几次，最终将达到发生问题的相同条件。

override func viewDidLoad() {
    super.viewDidLoad()

    let mySymbols: [Int] = Array(0...100)

    for _ in 0..<100 {
        xxxfastLoadLSecurityDescriptions(session: URLSession.shared, mySymbols: mySymbols) { (success, counter, doubleCount) in
            print("Completed: \(success), Counter: \(counter), Double Count: \(doubleCount)")
        }
    }
}

private func xxxfastLoadLSecurityDescriptions(session: URLSession, mySymbols: [Int]?, completion: @escaping(Bool, Int, Int) ->())    {

    var counter = mySymbols?.count ?? 0

    if counter == 0 {
        return completion(false, -1, -1)
    }
    var doubleCount = 0

    for symbolIndex in 0..<(mySymbols?.count ?? 0) {
        guard let _ = mySymbols?[symbolIndex] else {
            print("Error")
            continue
        }

        DispatchQueue.global().asyncAfter(deadline: .now() + .milliseconds(Int.random(in: 50..<900))) {

            counter -= 1

            DispatchQueue.main.async {
                self.view.layoutIfNeeded()
            }

            if counter == 0 {
                if doubleCount > 0 {
                    // This will eventually print even though logically it shouldn't
                    print("*****Counter: \(counter), Double Count: \(doubleCount), Symbol Index: \(symbolIndex)")
                }
                doubleCount += 1
                completion(true, counter, doubleCount)
                return
            }
        }
    }
}

输出：

Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
*******************************************************************
*****   Counter: 0, Double Count: 1, Symbol Index: 15   
*******************************************************************
Completed: true, Counter: 0, Double Count: 2
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
*******************************************************************
*****   Counter: 0, Double Count: 1, Symbol Index: 26   
*******************************************************************
Completed: true, Counter: 0, Double Count: 2
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
Completed: true, Counter: 0, Double Count: 1
*******************************************************************
*****   Counter: 0, Double Count: 1, Symbol Index: 57   
*******************************************************************
Completed: true, Counter: 0, Double Count: 2
*******************************************************************
*****   Counter: 0, Double Count: 1, Symbol Index: 3    
*******************************************************************
Completed: true, Counter: 0, Double Count: 2

Answer 2

最后，我通过使用DispatchGroup来解决此问题，如下所示（简化代码）：

private func xxxFastLoadLSecurityDescriptions(session: URLSession, mySymbols: [String]?, completion: @escaping(Bool) ->())    {
    
    let myGroup = DispatchGroup()
    
    for symbolIndex in 0..<(mySymbols?.count ?? 0) {
        
        myGroup.enter()
    
        mySessionSendMGBGenericRequest(...) { [weak self] returnValue in
            
            guard let self = self else { myGroup.leave() ; return }
         // do some stuff with returnValue
            
            myGroup.leave()
        }
    }
    
    myGroup.notify(queue: .main, execute: {
        
        completion(true)
    })
}

我的问题是：是否有可能遇到与以前相同的问题？例如，说我有2个项目的循环。第一个项目进入组，并说第二个项目进入循环之前，异步调用返回一个值，第一个项目退出组。此时，在第二个项目进入该组之前，应触发.notify，并且完成第二个项目之前，completion（true）存在该函数。这可能吗？

Swift代码两次运行，不应该运行

2 个答案: