GCD具有嵌套的解析查询 [英] GCD with nested Parse Queries

问题描述

func getPosts(skip: Int){
    var query = PFQuery(className: self.parseClassName!)
    query.includeKey("posted_by")
    query.findObjectsInBackgroundWithBlock {
        (objects: [AnyObject]?, error: NSError?) -> Void in

        if error == nil && objects != nil {
            if let objects = objects as? [PFObject] {

                var requestGroup = dispatch_group_create()

                for post in objects
                {
                    dispatch_group_enter(requestGroup)
                    let queryKommentar1 = PFQuery(className:"Comment")
                    queryKommentar1.whereKey("posted_to", equalTo: post)
                    queryKommentar1.limit = 3
                    queryKommentar1.includeKey("written_by")
                    queryKommentar1.findObjectsInBackgroundWithBlock() {
                        (commentObjects: [AnyObject]?, error: NSError?) -> Void in

                      //Creating UITableViewCells from data and store in array 
                      dispatch_group_leave(requestGroup)

                    }   
                }

                println("Successfully retrieved \(objects.count) posts.")
                dispatch_group_notify(requestGroup, dispatch_get_main_queue()) {
                    println("All done")

                }  
            }
        }
    }
}

因此，我不确定是否误解了调度组，但我的意图是针对不同的类进行两个解析查询，并根据提供的数据创建TableViewCells.这可以正常工作，但是由于我不希望在用户滚动表时加载数据，因此我想预加载数据并创建单元格，然后将它们存储在Array中.由于我想删除任何活动指示并重新加载表，因此在获取完成之前，我(在Som Googleing之后)认为调度组可能是一个很好的解决方案.但是，所有完成"永远不会在控制台中打印.

So, I'm not sure if I misunderstood dispatch groups, but my intention is to make two Parse Queries, targeting different classes, and create TableViewCells from the data provided. This works fine, but since I don't want the data to load when the user is scrolling the table, I want to preload the data and create the cells, and store them in an Array. Since I would like to remove any Activity Indication, and reload the table, by the time the fetch is complete I though (after som Googleing) that dispatch groups might be a good solution for this. However, "All done" is never printed in the console.

当我围绕外部查询创建一个调度组(在查询之前进入该组，并在代码块的最后一行保留)时，它运行良好.我究竟做错了什么?嵌套异步调用时不可能使用此功能吗?

When I made a dispatch group around the outer query (entering the group just before the query, and leaving as the last line in the block) that worked fine. What am I doing wrong? Is it impossible to use this when nesting asynchronous calls?

仅供参考，我删除了很多代码，例如创建单元格并使用了Parse中的数据，因为我想避免您阅读这些混乱的信息.

FYI, I removed a lot of code, like creating the cells and using the data from Parse, since I would like to spare you from reading that mess.

推荐答案

此调度组模式基本上是正确的.

This dispatch group pattern is basically right.

我建议在dispatch_group_enter和dispatch_group_leave处记录一些消息，看看它是否按照您认为的方式被调用，并且每个enter都被一个leave偏移.

I would suggest logging some message at dispatch_group_enter and at dispatch_group_leave and see if it's getting called as you think it should and that every enter is offset by a leave.

如果dispatch_group_leave的出现次数少于对dispatch_group_enter的调用次数，则不会调用dispatch_group_notify块.

If the number of occurrences of dispatch_group_leave are less than the number of calls to dispatch_group_enter, the dispatch_group_notify block will not get called.

也许在内部findObjectsInBackgroundWithBlock闭包中有一些路径阻止了它触发dispatch_group_leave调用.

Perhaps you have some path in that inner findObjectsInBackgroundWithBlock closure that is preventing it from hitting the dispatch_group_leave call.

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