我正在创建一个简单的聊天应用程序，它有一个加载屏幕，如果用户未登录，则可以转到登录屏幕，如果用户已登录，则可以直接转到他的聊天。聊天显示在 UICollectionView 中。当我第一次测试时，我用我在类本身中声明的虚拟数据填充它，一切正常。现在我在加载屏幕中从在线数据库中获取用户的聊天记录，并将它们存储在一个名为 user_chats 的数组中，该数组是全局声明的。

我使用以下代码填充 UICollectionView:

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
   // getUserChats()
    return user_chats.count

}

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

let cell = collectionView.dequeueReusableCellWithReuseIdentifier("chat_cell" , forIndexPath: indexPath) as! SingleChat

cell.chatName?.text = user_chats[indexPath.row].chat_partner!.name
cell.chatTextPreview?.text = user_chats[indexPath.row].chat_messages!.last!.text
let profile_pic_URL = NSURL(string : user_chats[indexPath.row].chat_partner!.profile_pic!)
downloadImage(profile_pic_URL!, imageView: cell.chatProfilePic)
cell.chatProfilePic.layer.cornerRadius = 26.5
cell.chatProfilePic.layer.masksToBounds = true

    let dividerLineView: UIView = {
        let view = UIView()
        view.backgroundColor = UIColor(white: 0.5, alpha: 0.5)
        return view
    }()
    dividerLineView.translatesAutoresizingMaskIntoConstraints = false

    cell.addSubview(dividerLineView)
    cell.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-1-[v0]|", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": dividerLineView]))
    cell.addSubview(dividerLineView)
    cell.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:[v0(1)]|", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": dividerLineView]))
    return cell

}

func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {

    self.performSegueWithIdentifier("showChat", sender: self)
}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {

    if (segue.identifier == "showChat") {

        let IndexPaths = self.collectionView!.indexPathsForSelectedItems()!
        let IndexPath = IndexPaths[0] as NSIndexPath
        let vc = segue.destinationViewController as! SingleChatFull

        vc.title = user_chats[IndexPath.row].chat_partner!.name
    }

}

数据获取:

func getUserChats() {
    let scriptUrl = "*****"
    let userID = self.defaults.stringForKey("userId")
    let params = "user_id=" + userID!
    let myUrl = NSURL(string: scriptUrl);
    let request: NSMutableURLRequest = NSMutableURLRequest(URL: myUrl!)
    request.HTTPMethod = "POST"
    let data = params.dataUsingEncoding(NSUTF8StringEncoding)
    request.timeoutInterval = 10
    request.HTTPBody=data
    request.HTTPShouldHandleCookies=false
    UIApplication.sharedApplication().networkActivityIndicatorVisible = true
    let queue:NSOperationQueue = NSOperationQueue()
    NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler:{ (response: NSURLResponse?, data: NSData?, error: NSError?) -> Void in
        do {
            if (data != nil) {
                do {
                    var dataString = String(data: data!, encoding: NSUTF8StringEncoding)
                    var delimiter = "]"
                    var token = dataString!.componentsSeparatedByString(delimiter)
                    dataString = token[0] + "]"
                    print(dataString)
                    let data_fixed = dataString!.dataUsingEncoding(NSUTF8StringEncoding)
                    do {
                        let jsonArray = try NSJSONSerialization.JSONObjectWithData(data_fixed!, options:[])


                        // LOOP THROUGH JSON ARRAY AND FETCH VALUES
                        for anItem in jsonArray as! [Dictionary<String, AnyObject>] {
                            let curr_chat = Chat()
                            if let chatId = anItem["chatId"] as? String {
                                curr_chat.id = chatId
                            }
                            let friend = Friend()
                            let user1id = anItem["user1_id"] as! String
                            let user2id = anItem["user2_id"] as! String
                            if (user1id == userID) {
                                if let user2id = anItem["user2_id"] as? String {
                                    friend.id = user2id
                                }
                                if let user2name = anItem["user2_name"] as? String {
                                    friend.name = user2name
                                }
                                if let user2profilepic = anItem["user2_profile_pic"] as? String {
                                    friend.profile_pic = user2profilepic
                                }
                            }
                            else if (user2id == userID){
                                if let user1id = anItem["user1_id"] as? String {
                                    friend.id = user1id
                                }
                                if let user1name = anItem["user1_name"] as? String {
                                    friend.name = user1name
                                }
                                if let user1profilepic = anItem["user1_profile_pic"] as? String {
                                    friend.profile_pic = user1profilepic
                                }
                            }

                            curr_chat.chat_partner = friend

                            var chat_messages = [Message]()
                            if let dataArray = anItem["message"] as? [String : AnyObject] {
                                for (_, messageDictionary) in dataArray {
                                    if let onemessage = messageDictionary as? [String : AnyObject] {                                        let curr_message = Message()
                                        if let messageid = onemessage["message_id"] as? String {
                                            curr_message.id =  messageid
                                        }
                                        if let messagedate = onemessage["timestamp"] as? String {
                                            let dateFormatter = NSDateFormatter()
                                            dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
                                            let date = dateFormatter.dateFromString(messagedate)
                                            curr_message.date = date
                                        }
                                        if let messagesender = onemessage["sender"] as? String {

                                            curr_message.sender = messagesender
                                        }
                                        if let messagetext = onemessage["text"] as? String {
                                            curr_message.text = messagetext
                                        }
                                        chat_messages.append(curr_message)
                                    }}
                            }

                            curr_chat.chat_messages = chat_messages
                            user_chats.append(curr_chat)
                        }

                    }
                    catch {
                        print("Error: \(error)")
                    }
                }
//                       NSUserDefaults.standardUserDefaults().setObject(user_chats, forKey: "userChats")

            }
            else {
                dispatch_async(dispatch_get_main_queue(), {
                    let uiAlert = UIAlertController(title: "No Internet Connection", message: "Please check your internet connection.", preferredStyle: UIAlertControllerStyle.Alert)

                    uiAlert.addAction(UIAlertAction(title: "Ok", style: .Default, handler: { action in
                        self.dismissViewControllerAnimated(true, completion:nil)
                    }))
                    self.presentViewController(uiAlert, animated: true, completion: nil)
                })
            }

        } catch _ {
            NSLog("error")
        }

    })
}

问题是 Collection View 现在总是空的。我做了一些调试并在第一个函数中放置了一个断点，我看到当加载屏幕仍然显示给用户并且甚至还没有加载聊天屏幕时调用了这个方法。我怀疑这是在加载屏幕中从互联网获取数据之前调用的，因此 user_chats 数组的大小为 0。我习惯于使用 Android 和 ListView 在父 View 显示在屏幕上之前永远不会填充 ListView，因此我很困惑。 从在线数据库中获取数据的方法工作正常，我已经广泛调试了它，所以问题不存在。

最佳答案

最好的选择是在您的函数中添加一个completionHandler，以便在返回数据和/或async 函数执行完毕时收到通知。下面的代码是带有 completionHandler 的 getUserCharts 函数的截断版本，它在加载数据时返回 true 或 false(您可以修改它以返回任何您想要的).您可以阅读更多关于闭包/完成处理程序的信息 https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html或谷歌。

函数

func getUserChats(completionHandler: (loaded: Bool, dataNil: Bool) -> ()) -> (){
        NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler:{ (response: NSURLResponse?, data: NSData?, error: NSError?) -> Void in
            do {
                if (data != nil) {
                    do {
                        var dataString = String(data: data!, encoding: NSUTF8StringEncoding)
                        var delimiter = "]"
                        var token = dataString!.componentsSeparatedByString(delimiter)
                        dataString = token[0] + "]"
                        print(dataString)
                        let data_fixed = dataString!.dataUsingEncoding(NSUTF8StringEncoding)
                        do {
                            let jsonArray = try NSJSONSerialization.JSONObjectWithData(data_fixed!, options:[])

                            // LOOP THROUGH JSON ARRAY AND FETCH VALUES
                            completionHandler(loaded: true, dataNil: false)
                        }
                        catch {
                            print("Error: \(error)")
                        }
                    }
                }
                else {
                    //Handle error or whatever you wish
                    completionHandler(loaded: true, dataNil: true)
                }

            } catch _ {
                NSLog("error")
            }

如何使用它

override func viewDidLoad() {
        getUserChats(){
            status in
            if status.loaded == true && status.dataNil == false{
                self.collectionView?.reloadData()
            }
        }
    }

关于ios - 从在线数据库填充 UICollectionView，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/37469441/