Swift iOS9 - dispatch_group - 需要代码按特定顺序执行

Question

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我的代码没有按预期工作。我需要代码按照键入的顺序操作。目前只有前两个部分正在运行。 请查看代码中的注释以获取更多详细信息。

tl; dr我如何按照我在评论中输入的顺序执行注释部分？

func getResults() {
    resultsFollowingArray.removeAll(keepCapacity: false)
    resultsDisplayNameArray.removeAll(keepCapacity: false)
    resultsProfilePicArray.removeAll(keepCapacity: false)
    resultsNoticeReasonArray.removeAll(keepCapacity: false)
    resultsNoticeDescriptionArray.removeAll(keepCapacity: false)
    resultsDatePostedArray.removeAll(keepCapacity: false)


    let followGroup: dispatch_group_t = dispatch_group_create()
    let profilePicGroup: dispatch_group_t = dispatch_group_create()
    let noticesGroup: dispatch_group_t = dispatch_group_create()

    /*** needs to execute first ***/
    dispatch_group_async(followGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
        let fQ = PFQuery(className: "Following")
        fQ.limit = 30
        fQ.whereKey("follower", equalTo: myEmail)

        do {
            let objs = try fQ.findObjects()
            for obj in objs {
                self.resultsFollowingArray.append(obj.valueForKey("followed") as! String)
            }
            print(self.resultsFollowingArray) //works fine
        } catch _ {

        }
    })

    /*** needs to execute second ***/
    dispatch_group_notify(followGroup, dispatch_get_main_queue(), {
        if (self.resultsFollowingArray.count > 0) {
            dispatch_group_async(profilePicGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
                let uQ = PFQuery(className: "_User")
                uQ.limit = 30
                for (var i = 0; i < self.resultsFollowingArray.count; i++) {
                    uQ.whereKey("username", equalTo: self.resultsFollowingArray[i])
                    do {
                        try self.resultsProfilePicArray.append(uQ.getFirstObject().valueForKey("profile_picture") as! PFFile)
                    } catch _ {

                    }
                }
                print(self.resultsProfilePicArray) //works fine
            })
        }
    })

    /*** needs to execute third ***/
    /*** this section seems to be the first section to execute, therefore it is not working as I need it ***/

    dispatch_group_notify(profilePicGroup, dispatch_get_main_queue(), {
        print(self.resultsFollowingArray) //prints empty array
        if (self.resultsFollowingArray.count > 0) {
            dispatch_group_async(noticesGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
                let nQ = PFQuery(className: "Notices")
                nQ.limit = 30
                for (var i = 0; i < self.resultsFollowingArray.count; i++) {
                    nQ.whereKey("poster", equalTo: self.resultsFollowingArray[i])
                    do {
                        let notices = try nQ.findObjects()
                        for notice in notices {
                            self.resultsDisplayNameArray.append(notice.valueForKey("poster_display_name") as! String)
                            self.resultsNoticeReasonArray.append(notice.valueForKey("reason") as! String)
                            self.resultsNoticeDescriptionArray.append(notice.valueForKey("notice_description") as! String)
                            self.resultsDatePostedArray.append(notice.valueForKey("date_posted") as! String)
                        }
                        self.tvMyFeed.reloadData()
                        print(self.resultsDisplayNameArray) //prints nothing
                    } catch _ {

                    }
                }
            })
        }
    })

    /*** needs to execute last ***/
    dispatch_group_notify(noticesGroup, dispatch_get_main_queue(), {
        self.tvMyFeed.reloadData()
    })
}

更新的代码：由于一些奇怪的原因，PFQuery“Notices”会被调用两次，它会搞砸所有内容。这是我想要做的，但我试图“在后台”，这段代码仍然冻结用户界面：

let group: dispatch_group_t = dispatch_group_create()

    dispatch_group_enter(group)
    let fQ = PFQuery(className: "Following")
    fQ.limit = 30
    fQ.whereKey("follower", equalTo: myEmail)
    do {
        let objs = try fQ.findObjects()
        for obj in objs {
            self.resultsFollowingArray.append(obj.valueForKey("followed") as! String)
        }
        dispatch_group_leave(group)
    } catch _ {
        dispatch_group_leave(group)
    }

    dispatch_group_enter(group)
    let uQ = PFQuery(className: "_User")
    uQ.limit = 30
    for (var i = 0; i < self.resultsFollowingArray.count; i++) {
        uQ.whereKey("username", equalTo: self.resultsFollowingArray[i])
        do {
            try self.resultsProfilePicArray.append(uQ.getFirstObject().valueForKey("profile_picture") as! PFFile)
        } catch _ {
            dispatch_group_leave(group)
        }
    }
    dispatch_group_leave(group)

    dispatch_group_enter(group)
    let nQ = PFQuery(className: "Notices")
    nQ.limit = 30
    for (var i = 0; i < self.resultsFollowingArray.count; i++) {
        nQ.whereKey("poster", equalTo: self.resultsFollowingArray[i])
        do {
            let notices = try nQ.findObjects()
            for notice in notices {
                self.resultsDisplayNameArray.append(notice.valueForKey("poster_display_name") as! String)
                self.resultsNoticeReasonArray.append(notice.valueForKey("reason") as! String)
                self.resultsNoticeDescriptionArray.append(notice.valueForKey("notice_description") as! String)
                self.resultsDatePostedArray.append(notice.valueForKey("date_posted") as! String)
            }
            dispatch_group_leave(group)
        } catch _ {
            dispatch_group_leave(group)
        }
    }

    dispatch_group_notify(group, dispatch_get_main_queue(), {
        self.tvMyFeed.reloadData()
    })

Answer 1

我找到了解决方案。为了这个目的而奉献自己的人可以随意发布你的，我很乐意接受你的智慧，让它根据需要发挥作用。

这是我的最终解决方案，我正在寻找的那个（感谢上帝，我能够自己找到它！）：它很有效！

func getResults() {
    resultsFollowingArray.removeAll(keepCapacity: false)
    resultsDisplayNameArray.removeAll(keepCapacity: false)
    resultsProfilePicArray.removeAll(keepCapacity: false)
    resultsNoticeReasonArray.removeAll(keepCapacity: false)
    resultsNoticeDescriptionArray.removeAll(keepCapacity: false)
    resultsDatePostedArray.removeAll(keepCapacity: false)

    let followGroup: dispatch_group_t = dispatch_group_create()

    dispatch_group_async(followGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
        let fQ = PFQuery(className: "Following")
        fQ.limit = 30
        fQ.whereKey("follower", equalTo: myEmail)

        do {
            let objs = try fQ.findObjects()
            for obj in objs {
                if (obj.valueForKey("followed") as! String != myEmail) {
                    self.resultsFollowingArray.append(obj.valueForKey("followed") as! String)
                }
            }
            if (self.resultsFollowingArray.count > 0) {
                let nQ = PFQuery(className: "Notices")
                nQ.limit = 30
                for (var i = 0; i < self.resultsFollowingArray.count; i++) {
                    nQ.whereKey("poster", equalTo: self.resultsFollowingArray[i])
                    do {
                        let notices = try nQ.findObjects()
                        for notice in notices {
                            self.resultsDisplayNameArray.append(notice.valueForKey("poster_display_name") as! String)
                            self.resultsNoticeReasonArray.append(notice.valueForKey("reason") as! String)
                            self.resultsNoticeDescriptionArray.append(notice.valueForKey("notice_description") as! String)
                            self.resultsDatePostedArray.append(notice.valueForKey("date_posted") as! String)
                        }
                        self.tvMyFeed.reloadData()
                    } catch _ {

                    }
                }
            }
        } catch _ {

        }
    })

    dispatch_group_notify(followGroup, dispatch_get_main_queue(), {
        if (self.resultsFollowingArray.count > 0) {
            let uQ = PFQuery(className: "_User")
            uQ.limit = 30
            for (var i = 0; i < self.resultsFollowingArray.count; i++) {
                uQ.whereKey("username", equalTo: self.resultsFollowingArray[i])
                do {
                    try self.resultsProfilePicArray.append(uQ.getFirstObject().valueForKey("profile_picture") as! PFFile)
                } catch _ {

                }
            }
            self.tvMyFeed.reloadData()
        }
    })
}