滚动时现有的CollectionView图像更改
我已经查看了很多问题，我不相信这是重新使用单元格的结果，因为新的单元格图像是正确的，但现有的单元格图像不正确并且过去是正确的。我会先发布图像，这样问题就更容易理解了。
我有一个图像单元格的集合视图（类似于Instagrams用户页面）。我从Firebase获取所有数据。我在屏幕的初始加载时得到了前12个帖子。但是，如果快速向下滚动，则现有单元格图像将更改为新获取的图像。我不确定为什么会发生这种情况......也许这是一个缓存问题？该问题仅在您第一次加载屏幕时出现。我已经尝试将图像设置为nil，如下所示：
override func prepareForReuse() {
super.prepareForReuse()
self.imageView.image = UIImage()
}
但这对这个问题没有帮助。
这是我的cellForItemAt：
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "imageCell", for: indexPath) as! ImageCell
cell.indexPath = indexPath
cell.imageView.downloadImage(from: currentTablePosts[indexPath.row].pathToImage)
cell.layer.borderWidth = 1
cell.layer.borderColor = UIColor.black.cgColor
return cell
}
图像下载和缓存：
let imageCache = NSCache<NSString, UIImage>()
extension UIImageView {
func downloadImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = imageCache.object(forKey: imgURL! as NSString) {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
// create UIImage
let imageToCache = UIImage(data: data!)
// add image to cache
imageCache.setObject(imageToCache!, forKey: imgURL! as NSString)
self.image = imageToCache
}
}
task.resume()
}
}
Firebase查询：
static func getInitialTablesPosts(tableNumber: String) {
tableReference.child(tableNumber).queryLimited(toLast: 12).observeSingleEvent(of: .value, with: { snap in
for child in snap.children {
let child = child as? DataSnapshot
if let post = child?.value as? [String: AnyObject] {
let posst = Post()
if let author = post["author"] as? String, let likes = post["likes"] as? Int, let pathToImage = post["pathToImage"] as? String, let postID = post["postID"] as? String, let postDescription = post["postDescription"] as? String, let timestamp = post["timestamp"] as? Double, let category = post["category"] as? String, let table = post["group"] as? String, let userID = post["userID"] as? String, let numberOfComments = post["numberOfComments"] as? Int, let region = post["region"] as? String {
posst.author = author
posst.likes = likes
posst.pathToImage = pathToImage
posst.postID = postID
posst.userID = userID
posst.fancyPostDescription = Helper.createAttributedString(author: author, postText: postDescription)
posst.postDescription = author + ": " + postDescription
posst.timestamp = timestamp
posst.table = table
posst.region = region
posst.category = category
posst.numberOfComments = numberOfComments
posst.userWhoPostedLabel = Helper.createAttributedPostLabel(username: author, table: table, region: region, category: category)
if let people = post["peopleWhoLike"] as? [String: AnyObject] {
for(_, person) in people {
posst.peopleWhoLike.append(person as! String)
}
}
currentTablePosts.insert(posst, at: 0)
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "reloadTableCollectionView"), object: nil)
} // end of if let
}
}
})
tableReference.removeAllObservers()
}
static func getMoreTablePosts(tableNumber: String, lastVisibleKey: String) {
print("FIRED...")
let currentNumberOfPosts = currentTablePosts.count
print("Number of posts before fetiching ", currentNumberOfPosts)
print("Oldest post key ", oldestTableKeys[tableNumber] ?? "not set yet", "***********")
tableReference.child(tableNumber).queryOrderedByKey().queryEnding(atValue: lastVisibleKey).queryLimited(toLast: 12).observeSingleEvent(of: .value, with: { snap in
for child in snap.children {
let child = child as? DataSnapshot
if let post = child?.value as? [String: AnyObject] {
if let id = post["postID"] as? String {
if id == lastVisibleKey {
return
}
}
let posst = Post()
if let author = post["author"] as? String, let likes = post["likes"] as? Int, let pathToImage = post["pathToImage"] as? String, let postID = post["postID"] as? String, let postDescription = post["postDescription"] as? String, let timestamp = post["timestamp"] as? Double, let category = post["category"] as? String, let table = post["group"] as? String, let userID = post["userID"] as? String, let numberOfComments = post["numberOfComments"] as? Int, let region = post["region"] as? String {
posst.author = author
posst.likes = likes
posst.pathToImage = pathToImage
posst.postID = postID
posst.userID = userID
posst.fancyPostDescription = Helper.createAttributedString(author: author, postText: postDescription)
posst.postDescription = author + ": " + postDescription
posst.timestamp = timestamp
posst.table = table
posst.region = region
posst.category = category
posst.numberOfComments = numberOfComments
posst.userWhoPostedLabel = Helper.createAttributedPostLabel(username: author, table: table, region: region, category: category)
if let people = post["peopleWhoLike"] as? [String: AnyObject] {
for(_, person) in people {
posst.peopleWhoLike.append(person as! String)
}
}
currentTablePosts.insert(posst, at: currentNumberOfPosts)
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "reloadTableCollectionView"), object: nil)
if let oldestTableKey = oldestTableKeys[tableNumber] {
if postID == oldestTableKey {
print("returning")
print("number of posts on return \(currentTablePosts.count)")
return
}
}
} // end if let
}
}
})
tableReference.removeAllObservers()
}
1 个答案:
答案 0 :(得分：0)
*更新*
此处使用的图像缓存在我的体验中仍然存在一些问题。我已经开始使用Kingfisher，这非常容易设置和使用。
*旧解决方案*
根据评论中建议的答案找到solution。
我修改了用于缓存图片的扩展程序。虽然将来我想我会继承UIImageView。这是我的代码的修改版本。
import UIKit
let userImageCache = NSCache<NSString, UIImage>()
let imageCache = NSCache<AnyObject, AnyObject>()
var imageURLString: String?
extension UIImageView {
public func imageFromServerURL(urlString: String, collectionView: UICollectionView, indexpath : IndexPath) {
imageURLString = urlString
if let url = URL(string: urlString) {
image = nil
if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {
self.image = imageFromCache
return
}
URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
if error != nil{
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async(execute: {
if let imgaeToCache = UIImage(data: data!){
if imageURLString == urlString {
self.image = imgaeToCache
}
imageCache.setObject(imgaeToCache, forKey: urlString as AnyObject)// calls when scrolling
collectionView.reloadItems(at: [indexpath])
}
})
}) .resume()
}
}
func downloadImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = imageCache.object(forKey: imgURL! as AnyObject) as? UIImage {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
let imageToCache = UIImage(data: data!)
imageCache.setObject(imageToCache!, forKey: imgURL! as AnyObject)
self.image = imageToCache
}
}
task.resume()
}
func downloadUserImage(from imgURL: String!) {
let url = URLRequest(url: URL(string: imgURL)!)
// set initial image to nil so it doesn't use the image from a reused cell
image = nil
// check if the image is already in the cache
if let imageToCache = userImageCache.object(forKey: imgURL! as NSString) {
self.image = imageToCache
return
}
// download the image asynchronously
let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
if error != nil {
// user an alert to display the error
if let topController = UIApplication.topViewController() {
Helper.showAlertMessage(vc: topController, title: "Error Downloading Image", message: error as! String)
}
return
}
DispatchQueue.main.async {
// create UIImage
let imageToCache = UIImage(data: data!)
// add image to cache
userImageCache.setObject(imageToCache!, forKey: imgURL! as NSString)
self.image = imageToCache
}
}
task.resume()
}
}
我创建了第一个缓存collectionView图像的方法，其他方法仍然用于tableViews。缓存也已从let imageCache = NSCache<NSString, UIImage>()更改为let imageCache = NSCache<AnyObject, AnyObject>()
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