我在这里找到了这段代码，它介绍了如何同时下载图像而没有任何破损，
func loadImageRobsAnswer(with urlString: String?) {
// cancel prior task, if any
weak var oldTask = currentTask
currentTask = nil
oldTask?.cancel()
// reset imageview's image
self.image = nil
// allow supplying of `nil` to remove old image and then return immediately
guard let urlString = urlString else { return }
// check cache
if let cachedImage = DataCache.shared.object(forKey: urlString) {
self.transition(toImage: cachedImage as? UIImage)
//self.image = cachedImage
return
}
// download
let url = URL(string: urlString)!
currentURL = url
let task = URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
self?.currentTask = nil
if let error = error {
if (error as NSError).domain == NSURLErrorDomain && (error as NSError).code == NSURLErrorCancelled {
return
}
print(error)
return
}
guard let data = data, let downloadedImage = UIImage(data: data) else {
print("unable to extract image")
return
}
DataCache.shared.saveObject(object: downloadedImage, forKey: urlString)
if url == self?.currentURL {
DispatchQueue.main.async {
self?.transition(toImage: downloadedImage)
}
}
}
// save and start new task
currentTask = task
task.resume()
}
但是此代码在UIImageView扩展中使用
public extension UIImageView {
private static var taskKey = 0
private static var urlKey = 0
private var currentTask: URLSessionTask? {
get { return objc_getAssociatedObject(self, &UIImageView.taskKey) as? URLSessionTask }
set { objc_setAssociatedObject(self, &UIImageView.taskKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
}
private var currentURL: URL? {
get { return objc_getAssociatedObject(self, &UIImageView.urlKey) as? URL }
set { objc_setAssociatedObject(self, &UIImageView.urlKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
}}}
这就是我试图使此代码动态化的方式，因此它不仅限于UIImageView，而且可以用于下载多个资源。
class DataRequest {
private static var taskKey = 0
private static var urlKey = 0
static let shared = DataRequest()
typealias ImageDataCompletion = (_ image: UIImage?, _ error: Error? ) -> Void
private var currentTask: URLSessionTask? {
get { return objc_getAssociatedObject(self, &DataRequest.taskKey) as? URLSessionTask }
set { objc_setAssociatedObject(self, &DataRequest.taskKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
}
private var currentURL: URL? {
get { return objc_getAssociatedObject(self, &DataRequest.urlKey) as? URL }
set { objc_setAssociatedObject(self, &DataRequest.urlKey, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC) }
}
func downloadImage(with urlString: String?, completion: @escaping ImageDataCompletion) {
weak var oldTask = currentTask
currentTask = nil
oldTask?.cancel()
guard let urlString = urlString else { return }
if let cachedImage = DataCache.shared.object(forKey: urlString) {
DispatchQueue.main.async {
completion(cachedImage as? UIImage ,nil)
}
// self.transition(toImage: cachedImage as? UIImage)
//self.image = cachedImage
return
}
// download
let url = URL(string: urlString)!
currentURL = url
let task = URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
self?.currentTask = nil
if let error = error {
if (error as NSError).domain == NSURLErrorDomain && (error as NSError).code == NSURLErrorCancelled {
return
}
completion(nil,nil)
return
}
guard let data = data, let downloadedImage = UIImage(data: data) else {
print("unable to extract image")
return
}
DataCache.shared.saveObject(object: downloadedImage, forKey: urlString)
if url == self?.currentURL {
DispatchQueue.main.async {
completion(downloadedImage ,nil)
}
}
}
// save and start new task
currentTask = task
task.resume()
}
这样我现在就可以在像这样的UIImageview扩展中使用它
extension UIImageView {
func setImage(url: String?) {
self.image = nil
DataRequest.shared.downloadImage(with: url) { (image, error) in
DispatchQueue.main.async {
self.image = image
}
}
}
}
在UICollectionView上使用我的方法的结论是将错误的图像显示到单元格中并复制，如何防止这种情况发生？
答案 0 :(得分：1)
您问：
有没有一种方法可以使用
URLSession.shared.dataTask并行请求多个不同的资源
默认情况下，它会并行执行请求。
让我们退后一步：在您之前的问题中，您正在询问如何实现类似于Kingfisher的UIImageView扩展。在my answer中，我提到使用objc_getAssociatedObject和objc_setAssociatedObject来实现这一点。但是在这里的问题中，您已经采用了关联的对象逻辑并将其放在您的DataRequest对象中。
您的思想过程中，将异步图像检索逻辑从UIImageView中抽出是一个好主意：您可能需要请求按钮的图像。您可能会使用一个通用的“异步获取图像”例程，该例程与任何UIKit对象完全分离。因此，从扩展中抽象网络层代码是一个好主意。
但是，异步图像检索UIImageView / UIButton扩展背后的整个想法是，我们需要一个UIKit控件，它不仅可以执行异步请求，而且可以重用带有控件的单元格，它将在开始下一个异步请求之前取消先前的异步请求（如果有）。这样，如果我们快速向下滚动到图像80至99，则对单元0至79的请求将被取消，可见图像也不会积压在所有这些旧图像请求之后。
但是要实现这一点，这意味着控件需要某种方式来以某种方式跟踪对该重用单元格的先前请求。而且由于我们无法在UIImageView扩展名中添加存储的属性，所以这就是我们使用objc_getAssociatedObject和objc_setAssociatedObject模式的原因。但这必须在图像视图中。
不幸的是，在上面的代码中，关联的对象位于您的DataRequest对象中。首先，正如我试图概述的那样，整个想法是图像视图必须跟踪对该控件的先前请求。将这种“跟踪先前的请求”放在DataRequest对象中会破坏该目的。其次，值得注意的是，您不需要自己类型中的关联对象，例如DataRequest。您只会拥有一个存储的财产。扩展另一种类型（例如UIImageView）时，只需要遍历此关联的对象无聊即可。
下面是一个简短的示例，我迅速展示了一个UIImageView扩展名，用于异步图像检索。请注意，这并没有从扩展名中提取网络代码，但是请注意，用于跟踪先前请求的关联对象逻辑必须保留在扩展名中。
private var taskKey: Void?
extension UIImageView {
private static let imageProcessingQueue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".imageprocessing", attributes: .concurrent)
private var savedTask: URLSessionTask? {
get { return objc_getAssociatedObject(self, &taskKey) as? URLSessionTask }
set { objc_setAssociatedObject(self, &taskKey, newValue, .OBJC_ASSOCIATION_RETAIN) }
}
/// Set image asynchronously.
///
/// - Parameters:
/// - url: `URL` for image resource.
/// - placeholder: `UIImage` of placeholder image. If not supplied, `image` will be set to `nil` while request is underway.
/// - shouldResize: Whether the image should be scaled to the size of the image view. Defaults to `true`.
func setImage(_ url: URL, placeholder: UIImage? = nil, shouldResize: Bool = true) {
savedTask?.cancel()
savedTask = nil
image = placeholder
if let image = ImageCache.shared[url] {
DispatchQueue.main.async {
UIView.transition(with: self, duration: 0.1, options: .transitionCrossDissolve, animations: {
self.image = image
}, completion: nil)
}
return
}
var task: URLSessionTask!
let size = bounds.size * UIScreen.main.scale
task = URLSession.shared.dataTask(with: url) { [weak self] data, response, error in
guard
error == nil,
let httpResponse = response as? HTTPURLResponse,
(200..<300) ~= httpResponse.statusCode,
let data = data
else {
return
}
UIImageView.imageProcessingQueue.async { [weak self] in
var image = UIImage(data: data)
if shouldResize {
image = image?.scaledAspectFit(to: size)
}
ImageCache.shared[url] = image
DispatchQueue.main.async {
guard
let self = self,
let savedTask = self.savedTask,
savedTask.taskIdentifier == task.taskIdentifier
else {
return
}
self.savedTask = nil
UIView.transition(with: self, duration: 0.1, options: .transitionCrossDissolve, animations: {
self.image = image
}, completion: nil)
}
}
}
task.resume()
savedTask = task
}
}
class ImageCache {
static let shared = ImageCache()
private let cache = NSCache<NSURL, UIImage>()
private var observer: NSObjectProtocol?
init() {
observer = NotificationCenter.default.addObserver(forName: UIApplication.didReceiveMemoryWarningNotification, object: nil, queue: nil) { [weak self] _ in
self?.cache.removeAllObjects()
}
}
deinit {
NotificationCenter.default.removeObserver(observer!)
}
subscript(url: URL) -> UIImage? {
get {
return cache.object(forKey: url as NSURL)
}
set {
if let data = newValue {
cache.setObject(data, forKey: url as NSURL)
} else {
cache.removeObject(forKey: url as NSURL)
}
}
}
}
这是我调整大小的例程：
extension UIImage {
/// Resize the image to be the required size, stretching it as needed.
///
/// - parameter newSize: The new size of the image.
/// - parameter contentMode: The `UIView.ContentMode` to be applied when resizing image.
/// Either `.scaleToFill`, `.scaleAspectFill`, or `.scaleAspectFit`.
///
/// - returns: Return `UIImage` of resized image.
func scaled(to newSize: CGSize, contentMode: UIView.ContentMode = .scaleToFill) -> UIImage? {
switch contentMode {
case .scaleToFill:
return filled(to: newSize)
case .scaleAspectFill, .scaleAspectFit:
let horizontalRatio = size.width / newSize.width
let verticalRatio = size.height / newSize.height
let ratio: CGFloat!
if contentMode == .scaleAspectFill {
ratio = min(horizontalRatio, verticalRatio)
} else {
ratio = max(horizontalRatio, verticalRatio)
}
let sizeForAspectScale = CGSize(width: size.width / ratio, height: size.height / ratio)
let image = filled(to: sizeForAspectScale)
let doesAspectFitNeedCropping = contentMode == .scaleAspectFit && (newSize.width > sizeForAspectScale.width || newSize.height > sizeForAspectScale.height)
if contentMode == .scaleAspectFill || doesAspectFitNeedCropping {
let subRect = CGRect(
x: floor((sizeForAspectScale.width - newSize.width) / 2.0),
y: floor((sizeForAspectScale.height - newSize.height) / 2.0),
width: newSize.width,
height: newSize.height)
return image?.cropped(to: subRect)
}
return image
default:
return nil
}
}
/// Resize the image to be the required size, stretching it as needed.
///
/// - parameter newSize: The new size of the image.
///
/// - returns: Resized `UIImage` of resized image.
func filled(to newSize: CGSize) -> UIImage? {
let format = UIGraphicsImageRendererFormat()
format.opaque = false
format.scale = scale
return UIGraphicsImageRenderer(size: newSize, format: format).image { _ in
draw(in: CGRect(origin: .zero, size: newSize))
}
}
/// Crop the image to be the required size.
///
/// - parameter bounds: The bounds to which the new image should be cropped.
///
/// - returns: Cropped `UIImage`.
func cropped(to bounds: CGRect) -> UIImage? {
// if bounds is entirely within image, do simple CGImage `cropping` ...
if CGRect(origin: .zero, size: size).contains(bounds) {
return cgImage?.cropping(to: bounds * scale).flatMap {
UIImage(cgImage: $0, scale: scale, orientation: imageOrientation)
}
}
// ... otherwise, manually render whole image, only drawing what we need
let format = UIGraphicsImageRendererFormat()
format.opaque = false
format.scale = scale
return UIGraphicsImageRenderer(size: bounds.size, format: format).image { _ in
let origin = CGPoint(x: -bounds.minX, y: -bounds.minY)
draw(in: CGRect(origin: origin, size: size))
}
}
/// Resize the image to fill the rectange of the specified size, preserving the aspect ratio, trimming if needed.
///
/// - parameter newSize: The new size of the image.
///
/// - returns: Return `UIImage` of resized image.
func scaledAspectFill(to newSize: CGSize) -> UIImage? {
return scaled(to: newSize, contentMode: .scaleAspectFill)
}
/// Resize the image to fit within the required size, preserving the aspect ratio, with no trimming taking place.
///
/// - parameter newSize: The new size of the image.
///
/// - returns: Return `UIImage` of resized image.
func scaledAspectFit(to newSize: CGSize) -> UIImage? {
return scaled(to: newSize, contentMode: .scaleAspectFit)
}
/// Create smaller image from `Data`
///
/// - Parameters:
/// - data: The image `Data`.
/// - maxSize: The maximum edge size.
/// - scale: The scale of the image (defaults to device scale if 0 or omitted.
/// - Returns: The scaled `UIImage`.
class func thumbnail(from data: Data, maxSize: CGFloat, scale: CGFloat = 0) -> UIImage? {
guard let imageSource = CGImageSourceCreateWithData(data as CFData, nil) else {
return nil
}
return thumbnail(from: imageSource, maxSize: maxSize, scale: scale)
}
/// Create smaller image from `URL`
///
/// - Parameters:
/// - data: The image file URL.
/// - maxSize: The maximum edge size.
/// - scale: The scale of the image (defaults to device scale if 0 or omitted.
/// - Returns: The scaled `UIImage`.
class func thumbnail(from fileURL: URL, maxSize: CGFloat, scale: CGFloat = 0) -> UIImage? {
guard let imageSource = CGImageSourceCreateWithURL(fileURL as CFURL, nil) else {
return nil
}
return thumbnail(from: imageSource, maxSize: maxSize, scale: scale)
}
private class func thumbnail(from imageSource: CGImageSource, maxSize: CGFloat, scale: CGFloat) -> UIImage? {
let scale = scale == 0 ? UIScreen.main.scale : scale
let options: [NSString: Any] = [
kCGImageSourceThumbnailMaxPixelSize: maxSize * scale,
kCGImageSourceCreateThumbnailFromImageAlways: true
]
if let scaledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options as CFDictionary) {
return UIImage(cgImage: scaledImage, scale: scale, orientation: .up)
}
return nil
}
}
extension CGSize {
static func * (lhs: CGSize, rhs: CGFloat) -> CGSize {
return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
}
}
extension CGPoint {
static func * (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
return CGPoint(x: lhs.x * rhs, y: lhs.y * rhs)
}
}
extension CGRect {
static func * (lhs: CGRect, rhs: CGFloat) -> CGRect {
return CGRect(origin: lhs.origin * rhs, size: lhs.size * rhs)
}
}
话虽如此，我们确实应该将并发请求限制在合理的范围内（一次4-6），以使它们在尝试完成（或取消）之前不会尝试启动以避免超时。典型的解决方案是使用异步Operation子类包装请求，将其添加到操作队列中，并将maxConcurrentOperationCount约束为您选择的任何值。