等待2个回调实例化对象

Question

之前，我想从火力下载：从组配置文件发出

• 数据（火力地堡实时DB） 包括...从发行
  • 数据组管理轮廓（火力地堡实时DB）
  • 一组配置文件图像（火力地堡存储）

然后我可以实例化其数据的一组对象及其图像

第一方法中，我使用的3嵌套闭合，让我获取数据，然后，以获得图像。

它确实有效，但从Firebase获取所有这些东西的时间相当长。

因此，我尝试使用GCD，以便在同一时间（而不是一个接一个地）推送我最近的2个Firebase查询（用户数据+组图像），并等待最后一个回调开始实例化我的组。

这是一个正确的方法吗？

如果是的话，我觉得有些困难来实现它... 我的问题：returnedUser和returnedGroupImage总是零

这里是我的一堆代码：

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

我相信你使用DispatchGroup的方式是不正确的。

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) 
}