在实例化对象之前等待 2 个回调

Question

我想从 firebase 下载：

从组配置文件发出的数据（Firebase 实时数据库）包括...
- 从组管理员配置文件（Firebase 实时数据库）发出的数据
- 一组个人资料图片（Firebase 存储）

然后我可以用它的数据和图像实例化一个组对象

第一种方法，我使用了 3 个嵌套闭包来获取数据，然后获取图像。

它确实有效，但是从 firebase 依次获取所有这些东西需要很长时间。

所以我尝试使用 GCD 同时推送我的 2 个最新的 Firebase 查询（用户数据 + 组图像）（而不是一个接一个），并等待最后一个回调开始实例化我的组。

这是正确的做法吗？

如果是，我发现实施起来有些困难... 我的问题：returnedUser 和 returnedGroupImage 总是 nil

这是我的一堆代码：

static func getGroup(_ groupID:String, completionBlock: @escaping (_ group: Group?) -> ()) { 
   dataRef.child("data").child("groups").child(groupID).observe(.value, with: { (snapshot) in  
        if let snapshotValue = snapshot.value {
            guard let name = (snapshotValue as AnyObject).object(forKey: "name") as? String else
            {
                completionBlock(nil)
                return
            }

            guard let adminID = (snapshotValue as AnyObject).object(forKey: "adminID") as? String else
            {
                completionBlock(nil)
                return
            }


            let queue = DispatchQueue(label: "asyncQueue", attributes: .concurrent, target: .main)
            let dispatch_group = DispatchGroup()


            var returnedUser: User?
            var returnedGroupImage: UIImage?


            queue.async (group: dispatch_group) {
            FireBaseHelper.getUser(adminID, completionBlock: { (user) in
                if user != nil {
                    returnedUser = user
                }
            })
            }

            queue.async (group: dispatch_group) {
            FireBaseHelper.getGroupImage(groupID, completionBlock: { (image) in
                if image != nil {
                    returnedGroupImage = image
                }
            })
            }

            dispatch_group.notify(queue: DispatchQueue.main) {
                // Single callback that is supposed to be executed after all tasks are complete.
                if (returnedUser == nil) || (returnedGroupImage == nil) {
                    // always true !
                    return
                }

                let returnedGroup = Group(knownID: (snapshotValue as AnyObject).key, named: name, createdByUser: currentUser!)

                returnedGroup.groupImage = returnedGroupImage
                completionBlock(returnedGroup)
            }
        }
    })
}

感谢您的帮助！

Answer 1

我认为您使用 DispatchGroup 的方式不正确。

let dispatch_group = DispatchGroup()

var returnedUser: User?
var returnedGroupImage: UIImage?

dispatch_group.enter()
FireBaseHelper.getUser(adminID, completionBlock: { (user) in
   if user != nil {
      returnedUser = user
   }
   dispatch_group.leave()
})

dispatch_group.enter()
FireBaseHelper.getGroupImage(groupID, completionBlock: { (image) in
   if image != nil {
      returnedGroupImage = image
   }
   dispatch_group.leave()
})

dispatch_group.notify(queue: DispatchQueue.main) {
// Single callback that is supposed to be executed after all tasks are complete.
   if (returnedUser == nil) || (returnedGroupImage == nil) {
      // always true !
      return
   }

   let returnedGroup = Group(knownID: (snapshotValue as AnyObject).key, named: name, createdByUser: currentUser!)

   returnedGroup.groupImage = returnedGroupImage
   completionBlock(returnedGroup)
}

在实例化对象之前等待 2 个回调

Wait for 2 callbacks before instantiating an object

grand-central-dispatch

firebase

firebase-realtime-database

swift3

firebase-storage