为什么我的功能下载数千次称为数千次的化身

Question

我的打印语句显示该功能在大约15秒内称为4771次，显然导致崩溃。

这是功能：

override func collectionView(_ collectionView: JSQMessagesCollectionView!, avatarImageDataForItemAt indexPath: IndexPath!) -> JSQMessageAvatarImageDataSource! {
    count += 1
    print("nnAvatar func called (count)n")
    let databaseRef = FIRDatabase.database().reference()
    let message = messages[indexPath.item]
    let placeHolderImage = UIImage(named: "Logo")
    let avatarImage = JSQMessagesAvatarImage(avatarImage: nil, highlightedImage: nil, placeholderImage: placeHolderImage)
    if let messageID = message.senderId {
        // Check cache for avatar
        if imageCache.object(forKey: messageID as NSString) != nil {
            DispatchQueue.main.async {
                avatarImage!.avatarImage = imageCache.object(forKey: messageID as NSString)
                avatarImage!.avatarHighlightedImage = imageCache.object(forKey: messageID as NSString)
                self.collectionView.reloadData()
            }
        } else {
            // If avatar isn't cached, fire off a new download
            databaseRef.child("users").child(messageID).observe(.value, with: { (snapshot) in
                if let profilePic = (snapshot.value as AnyObject!)!["profilePicture"] as! String! {
                    let profilePicURL: URL = URL(string: profilePic)!
                    Alamofire.request(profilePicURL)
                        .responseImage { response in
                            if let downloadedImage = response.result.value {
                                imageCache.setObject(downloadedImage, forKey: message.senderId as NSString)
                                DispatchQueue.main.async {
                                    avatarImage!.avatarImage = imageCache.object(forKey: message.senderId as NSString)
                                    avatarImage!.avatarHighlightedImage = imageCache.object(forKey: message.senderId as NSString)
                                    self.collectionView.reloadData()
                                }
                            }
                    }
                }
            })
        }
    }
    return avatarImage
}

是什么导致循环？无论如何，只有一个用户(我(可以得到一个化身。我对编程有些陌生，并试图弄清楚如何使用缓存...我的目的是检查用户的化身是否被缓存，如果是的话，请使用它。如果没有，请从Firebase开发新下载。但是我显然很难弄乱 - 我该如何编写，以便有效地检查缓存和/或下载图像，并且不会陷入循环？

Answer 1

您在功能中调用 reloadData，这会导致此函数再次调用，该功能调用 reloadData等；您创建了一个无限的循环。

您只需要在最初返回占位符然后随后从网络检索阿凡达的情况下重新加载任何内容即可。在这种情况下，重新加载整个收集视图是非常浪费的。您只需要重新加载受影响的项目：

override func collectionView(_ collectionView: JSQMessagesCollectionView!, avatarImageDataForItemAt indexPath: IndexPath!) -> JSQMessageAvatarImageDataSource! {
    count += 1
    print("nnAvatar func called (count)n")
    let databaseRef = FIRDatabase.database().reference()
    let message = messages[indexPath.item]
    let placeHolderImage = UIImage(named: "Logo")
    let avatarImage = JSQMessagesAvatarImage(avatarImage: nil, highlightedImage: nil, placeholderImage: placeHolderImage)
    if let messageID = message.senderId {
        // Check cache for avatar
        if let cacheObject = imageCache.object(forKey: messageID as NSString) {
                avatarImage!.avatarImage = cacheObject
                avatarImage!.avatarHighlightedImage = cacheObject
        } else {
            // If avatar isn't cached, fire off a new download
            databaseRef.child("users").child(messageID).observe(.value, with: { (snapshot) in
                if let profilePic = (snapshot.value as AnyObject!)!["profilePicture"] as! String! {
                    let profilePicURL: URL = URL(string: profilePic)!
                    Alamofire.request(profilePicURL)
                        .responseImage { response in
                            if let downloadedImage = response.result.value {
                                imageCache.setObject(downloadedImage, forKey: message.senderId as NSString)
                                DispatchQueue.main.async {
                                    self.collectionView.reloadItems(at:[indexPath])
                                }
                            }
                    }
                }
            })
        }
    }
    return avatarImage
}