Swift:无法链接 DispatchGroup

Question

我正在尝试链接两个调度组，但我的代码总是在随机位置崩溃。

我的功能是这样的:

func getAllActivities(userUID: String, _ completionHandler: @escaping (_ activities: [ActivitiesForUIStruct]?, _ error: Error?) -> Void) {
    let downloadGroup = DispatchGroup()
    let detailGroup = DispatchGroup()

    let _ = DispatchQueue.global(qos: .userInitiated)

    var activities = [ActivitiesForUIStruct]()

    getActivities(userUID: userUID) { (results, error) in
        //Handle error

        downloadGroup.enter()
        if let results = results {
            var groupTitle = ""
            var originatorName = ""
            var sourceName = ""

            results.forEach({ (activity) in
                let originatorUID = activity.originatorUserUID ?? ""
                let groupID = activity.inspectionGroupID ?? ""
                let sourceID = activity.sourceID ?? ""

                detailGroup.enter()
                self.getTitle(key: "inspectionGroups", value: groupID, { (title, error) in
                    ...

                    if let title = title {
                        groupTitle = title
                        detailGroup.leave()
                    }
                })

                detailGroup.enter()
                self.getName(userUID: originatorUID, { (name, error) in
                    ...

                    if let name = name {
                        originatorName = name
                        detailGroup.leave()
                    }
                })

                detailGroup.enter()
                self.getTitle(key: "users", value: sourceID, { (title, error) in
                    ... 

                    if let title = title {
                        sourceName = title
                        detailGroup.leave()
                    }
                })

                detailGroup.notify(queue: .global(qos: .userInitiated), execute: {
                    let activityUI = ActivitiesForUIStruct(activities: activity, originatorName: originatorName, sourceName: sourceName, inspectionGroupTitle: groupTitle)
                    activities.append(activityUI) //crash here
                    downloadGroup.leave() //crash here
                })
            })
        }

        downloadGroup.notify(queue: .main, execute: {
            completionHandler(activities, nil)
        })
    }
}

我的代码的目的是从 UID 中获取名称和标题，然后将其附加到自定义的 struct 中。但是，我的实现往往会在 detailGroup.notify 中标记的区域崩溃。有人会建议我哪里出了问题吗？

注意:我的函数调用是正确的，即我能够打印出 originatorName、sourceName 和 groupTitle。

Answer 1

首先，

你需要平衡enter()和leave() 。您的代码调用 downloadGroup.enter()仅一次出results.forEach ，但是downloadGroup.leave()在循环内被调用多次。所以，如果results有两个或多个元素，downloadGroup.leave()被调用的次数多于 downloadGroup.enter() .

<小时/>

第二，

一些看似异步的方法可能会同步调用完成处理程序。 (例如，当缓存命中时。)因此，一些 detailGroup.leave()可能会在 detailGroup.enter() 之前调用.

detailGroup.enter() +1
detailGroup.leave() 0  --> detailGroup.notify fires!
detailGroup.enter() +1
detailGroup.leave() 0  --> detailGroup.notify fires!
detailGroup.enter() +1
detailGroup.leave() 0  --> detailGroup.notify fires!

( detailGroup.notify 每次内部计数获取 0 时触发。)

如上所示，这可能会导致detailGroup.notify多次开火。使detailGroup.notify所有任务完成后只触发一次，需要保证所有 detailGroup.enter()在任何 detailGroup.leave() 之前执行.

detailGroup.enter() +1
detailGroup.enter() +2
detailGroup.enter() +3
detailGroup.leave() +2
detailGroup.leave() +1
detailGroup.leave() 0  --> detailGroup.notify fires!

<小时/>

第三，

swift Array不是线程安全的。所以，如果你的activities可以在多线程中更新，这可能会导致意外行为，包括崩溃。最好在 UI 线程中更新这样的数组。

<小时/>

考虑到所有这些因素，您的代码将如下所示:

func getAllActivities(userUID: String, _ completionHandler: @escaping (_ activities: [ActivitiesForUIStruct]?, _ error: Error?) -> Void) {
    let downloadGroup = DispatchGroup()
    let detailGroup = DispatchGroup()

    var activities = [ActivitiesForUIStruct]()

    getActivities(userUID: userUID) { (results, error) in
        //Handle error

        if let results = results {
            var groupTitle = ""
            var originatorName = ""
            var sourceName = ""

            results.forEach({ (activity) in
                //### 1. Balance enter() and leave()
                downloadGroup.enter()

                let originatorUID = activity.originatorUserUID ?? ""
                let groupID = activity.inspectionGroupID ?? ""
                let sourceID = activity.sourceID ?? ""

                //### 2. Guarantee all enter() executed before leave()
                detailGroup.enter() //for getTitle(key: "inspectionGroups"...
                detailGroup.enter() //for getName(userUID: originatorUID...
                detailGroup.enter() //for getTitle(key: "users"...

                self.getTitle(key: "inspectionGroups", value: groupID, { (title, error) in
                    //...
                    if let title = title {
                        groupTitle = title
                    }
                    detailGroup.leave()
                })

                self.getName(userUID: originatorUID, { (name, error) in
                    //...
                    if let name = name {
                        originatorName = name
                    }
                    detailGroup.leave()
                })

                self.getTitle(key: "users", value: sourceID, { (title, error) in
                    //...
                    if let title = title {
                        sourceName = title
                    }
                    detailGroup.leave()
                })

                //### 3. Use main queue to safely modify thread-unsafe objects
                detailGroup.notify(queue: .main, execute: {
                    let activityUI = ActivitiesForUIStruct(activities: activity, originatorName: originatorName, sourceName: sourceName, inspectionGroupTitle: groupTitle)
                    activities.append(activityUI)
                    downloadGroup.leave()
                })
            })
        }

        downloadGroup.notify(queue: .main, execute: {
            completionHandler(activities, nil)
        })
    }
}

在某些情况下，可能不需要我的一些建议，但在使用 DispatchGroup 或异步任务时，您最好记住所有 3 件事(Code Different 建议的 +1 件事)。