图像下载和缓存问题
图像下载和缓存问题
提问于 2019-07-29 09:45:49
我正在从服务器下载图像并在collectionView中显示。我正在缓存图像，这样用户就可以得到快速的服务器响应，并且不会出现UI中的故障。在没有下载图像之前，我也添加了占位符图像。
但是在我的输出中，图像是在其他单元格中复制的，图像没有正确地在NSCache中缓存。
下面是代码
class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
@IBOutlet weak var colView: UICollectionView!
var imageCache = NSCache<NSString, UIImage>()
var arrURLs = [
"https://homepages.cae.wisc.edu/~ece533/images/airplane.png",
"https://homepages.cae.wisc.edu/~ece533/images/arctichare.png",
"https://homepages.cae.wisc.edu/~ece533/images/baboon.png",
"https://homepages.cae.wisc.edu/~ece533/images/barbara.png",
"https://homepages.cae.wisc.edu/~ece533/images/boat.png",
"https://homepages.cae.wisc.edu/~ece533/images/cat.png",
"https://homepages.cae.wisc.edu/~ece533/images/fruits.png",
"https://homepages.cae.wisc.edu/~ece533/images/frymire.png",
"https://homepages.cae.wisc.edu/~ece533/images/girl.png",
"https://homepages.cae.wisc.edu/~ece533/images/goldhill.png",
"https://homepages.cae.wisc.edu/~ece533/images/lena.png",
"https://homepages.cae.wisc.edu/~ece533/images/monarch.png",
"https://homepages.cae.wisc.edu/~ece533/images/mountain.png",
"https://homepages.cae.wisc.edu/~ece533/images/peppers.png",
"https://homepages.cae.wisc.edu/~ece533/images/pool.png",
"https://homepages.cae.wisc.edu/~ece533/images/sails.png",
"https://homepages.cae.wisc.edu/~ece533/images/serrano.png",
"https://homepages.cae.wisc.edu/~ece533/images/tulips.png",
"https://homepages.cae.wisc.edu/~ece533/images/watch.png",
"https://homepages.cae.wisc.edu/~ece533/images/zelda.png"
]
func downloadImage(url: URL, imageView: UIImageView, placeholder : UIImage) {
imageView.image = placeholder // Set default placeholder..
// Image is set if cache is available
if let cachedImage = imageCache.object(forKey: url.absoluteString as NSString) {
imageView.image = cachedImage
} else {
// Reset the image to placeholder as the URLSession fetches the new image
imageView.image = placeholder
URLSession.shared.dataTask(with: url) { (data, response, error) in
guard error == nil else {
// You should be giving an option to retry the image here
imageView.image = placeholder
return
}
if let respo = response as? HTTPURLResponse {
print("Status Code : ", respo.statusCode)
if let imageData = data, let image = UIImage(data: imageData) {
self.imageCache.setObject(image, forKey: url.absoluteString as NSString)
// Update the imageview with new data
DispatchQueue.main.async {
imageView.image = image
}
} else {
// You should be giving an option to retry the image here
imageView.image = placeholder
}
}
}.resume()
}
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let w = self.view.bounds.width - 30
return CGSize(width: w, height: w + 60)
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return arrURLs.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "DummyCollectionViewCell", for: indexPath) as! DummyCollectionViewCell
let str = arrURLs[indexPath.item]
let url = URL(string: str)
downloadImage(url: url!) { (img) in
DispatchQueue.main.async {
cell.imgView.image = img ?? UIImage(named: "placeholder")
}
}
return cell
}
}输出GIF
由于堆栈上的大小限制，上面的gif质量很低。如果您需要检查gif的全部大小，请参阅：https://imgur.com/tcjMWgc
回答 5
Stack Overflow用户
回答已采纳
发布于 2019-07-29 11:41:24
我认为问题在您的响应处理程序中，您正在为您请求的url设置缓存，而不是从响应中为url设置缓存，我稍微修改了您的代码，尝试，希望它能帮助您。
func downloadImage(url: URL, imageView: UIImageView, placeholder: UIImage? = nil, row: Int) {
imageView.image = placeholder
imageView.cacheUrl = url.absoluteString + "\(row)"
if let cachedImage = imageCache.object(forKey: url.absoluteString as NSString) {
imageView.image = cachedImage
} else {
URLSession.shared.dataTask(with: url) { (data, response, error) in
guard
let response = response as? HTTPURLResponse,
let imageData = data,
let image = UIImage(data: imageData),
let cacheKey = response.url?.absoluteString,
let index = self.arrURLs.firstIndex(of: cacheKey)
else { return }
DispatchQueue.main.async {
if cacheKey + "\(index)" != imageView.cacheUrl { return }
imageView.image = image
self.imageCache.setObject(image, forKey: cacheKey as NSString)
}
}.resume()
}
}和
var associateObjectValue: Int = 0
extension UIImageView {
fileprivate var cacheUrl: String? {
get {
return objc_getAssociatedObject(self, &associateObjectValue) as? String
}
set {
return objc_setAssociatedObject(self, &associateObjectValue, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
}
}
}更新：
Stack Overflow用户
发布于 2019-07-30 09:44:33
USE THIS IMAGE LOADER EXTENSION
let imageCache = NSCache<AnyObject, AnyObject>()
class ImageLoader: UIImageView {
var imageURL: URL?
let activityIndicator = UIActivityIndicatorView()
func loadImageWithUrl(_ url: URL) {
// setup activityIndicator...
activityIndicator.color = .darkGray
addSubview(activityIndicator)
activityIndicator.translatesAutoresizingMaskIntoConstraints = false
activityIndicator.centerXAnchor.constraint(equalTo: centerXAnchor).isActive = true
activityIndicator.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true
imageURL = url
image = nil
activityIndicator.startAnimating()
// retrieves image if already available in cache
if let imageFromCache = imageCache.object(forKey: url as AnyObject) as? UIImage {
self.image = imageFromCache
activityIndicator.stopAnimating()
return
}
// image does not available in cache.. so retrieving it from url...
URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in
if error != nil {
print(error as Any)
self.activityIndicator.stopAnimating()
return
}
DispatchQueue.main.async(execute: {
if let unwrappedData = data, let imageToCache = UIImage(data: unwrappedData) {
if self.imageURL == url {
self.image = imageToCache
}
imageCache.setObject(imageToCache, forKey: url as AnyObject)
}
self.activityIndicator.stopAnimating()
})
}).resume()
}
}
** design controller **
import UIKit
class ImageController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
private let cellId = "cellId"
lazy var imagesSliderCV: UICollectionView = {
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .vertical
let cv = UICollectionView(frame: .zero, collectionViewLayout: layout)
cv.translatesAutoresizingMaskIntoConstraints = false
cv.backgroundColor = .white
cv.showsHorizontalScrollIndicator = false
cv.delegate = self
cv.dataSource = self
cv.isPagingEnabled = true
cv.register(ImageSliderCell.self, forCellWithReuseIdentifier: self.cellId)
return cv
}()
//
// Mark:- CollectionView Methods........
//
var arrURLs = [
"https://homepages.cae.wisc.edu/~ece533/images/airplane.png",
"https://homepages.cae.wisc.edu/~ece533/images/arctichare.png",
"https://homepages.cae.wisc.edu/~ece533/images/baboon.png",
"https://homepages.cae.wisc.edu/~ece533/images/barbara.png",
"https://homepages.cae.wisc.edu/~ece533/images/boat.png",
"https://homepages.cae.wisc.edu/~ece533/images/cat.png",
"https://homepages.cae.wisc.edu/~ece533/images/fruits.png",
"https://homepages.cae.wisc.edu/~ece533/images/frymire.png",
"https://homepages.cae.wisc.edu/~ece533/images/girl.png",
"https://homepages.cae.wisc.edu/~ece533/images/goldhill.png",
"https://homepages.cae.wisc.edu/~ece533/images/lena.png",
"https://homepages.cae.wisc.edu/~ece533/images/monarch.png",
"https://homepages.cae.wisc.edu/~ece533/images/mountain.png",
"https://homepages.cae.wisc.edu/~ece533/images/peppers.png",
"https://homepages.cae.wisc.edu/~ece533/images/pool.png",
"https://homepages.cae.wisc.edu/~ece533/images/sails.png",
"https://homepages.cae.wisc.edu/~ece533/images/serrano.png",
"https://homepages.cae.wisc.edu/~ece533/images/tulips.png",
"https://homepages.cae.wisc.edu/~ece533/images/watch.png",
"https://homepages.cae.wisc.edu/~ece533/images/zelda.png"
]
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return arrURLs.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! ImageSliderCell
let ImagePath = arrURLs[indexPath.item]
if let strUrl = ImagePath.addingPercentEncoding(withAllowedCharacters: .urlFragmentAllowed),
let imgUrl = URL(string: strUrl) {
cell.frontImg.loadImageWithUrl(imgUrl)
}
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return CGSize(width: screenWidth, height: 288)
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
return 0
}
func setupAutoLayout(){
NSLayoutConstraint.activate([
imagesSliderCV.leftAnchor.constraint(equalTo: view.leftAnchor),
imagesSliderCV.rightAnchor.constraint(equalTo: view.rightAnchor),
imagesSliderCV.topAnchor.constraint(equalTo: view.topAnchor),
imagesSliderCV.bottomAnchor.constraint(equalTo: view.bottomAnchor),
])
}
}
**collectionView cell **
import UIKit
class ImageSliderCell: UICollectionViewCell {
//
let frontImg: ImageLoader = {
let img = ImageLoader()
img.translatesAutoresizingMaskIntoConstraints = false
img.contentMode = .scaleAspectFill
img.clipsToBounds = true
return img
}()
//
override init(frame: CGRect) {
super.init(frame: frame)
addSubview(frontImg)
setupAutolayout()
}
func setupAutolayout(){
frontImg.leftAnchor.constraint(equalTo: leftAnchor, constant: 8).isActive = true
frontImg.rightAnchor.constraint(equalTo: rightAnchor, constant: -8).isActive = true
frontImg.topAnchor.constraint(equalTo: topAnchor, constant: 8).isActive = true
frontImg.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8).isActive = true
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}产出：-
Stack Overflow用户
发布于 2019-07-29 10:00:46
那是因为细胞是可重用的。
上单元格被重用，但是单元格没有更新图像，因为单元格的图像已经设置好了。
您应该扩展UIImage以更新单元格的图像
就像这样：
扩展UIImageView {
func loadImageNone(_ urlString: String) {
if let cacheImage = imageCache.object(forKey: urlString as NSString) {
self.run(with: cacheImage)
return
} else {
URLSession.shared.dataTask(with: url) { (data, response, error) in
guard error == nil else {
completion(nil)
return
}
if let respo = response as? HTTPURLResponse {
if let imageData = data, let image = UIImage(data: imageData) {
imageCache.setObject(image, forKey: urlString as NSString)
DispatchQueue.main.async {
self.image = image
}
}
}
}.resume()
}
func run(with image: UIImage) {
UIView.transition(with: self,
duration: 0.5,
options: [],
animations: { self.image = image },
completion: nil)
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/57251174
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