我有一个UICollectionView,显示全屏的单元格(即画廊)。 有时,在向左或向右滑动以获取新图像时,会短暂地显示错误的图像,然后才会更新为正确的图像。 其他情况下,可能会显示错误的图像,但如果您向左滑动再向右滑动(即图像消失并重新出现),则正确的图像会出现。 我不明白这种行为的原因。 当集合视图需要它们时,图像会异步下载。 以下是代码:
let blank = UIImage(named: "blank.png")
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as! UICollectionViewCell
let image = images[indexPath.row]
if let imageView = cell.viewWithTag(5) as? UIImageView {
imageView.image = blank
if let cachedImage = cache.objectForKey(image.url) as? UIImage {
imageView.image = cachedImage
} else {
UIImage.asyncDownloadImageWithUrl(image.url, completionBlock: { (succeded, dimage) -> Void in
if succeded {
self.cache.setObject(dimage!, forKey: image.url)
imageView.image = dimage
}
})
}
}
return cell
}
UIImage.asyncDownloadImageWithUrl是什么:
extension UIImage {
static func asyncDownloadImageWithUrl(url: NSURL, completionBlock: (succeeded: Bool, image: UIImage?) -> Void) {
let request = NSMutableURLRequest(URL: url)
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue.mainQueue(), completionHandler: { (response, data, error) in
if error == nil {
if let image = UIImage(data: data) {
completionBlock(succeeded: true, image: image)
}
} else {
completionBlock(succeeded: false, image: nil)
}
})
}
}
对于第一张显示的图片:
func collectionView(collectionView: UICollectionView, willDisplayCell cell: UICollectionViewCell, forItemAtIndexPath indexPath: NSIndexPath) {
if let index = self.index {
let newIndexPath = NSIndexPath(forItem: index, inSection: 0)
self.collectionView.scrollToItemAtIndexPath(newIndexPath, atScrollPosition: UICollectionViewScrollPosition.Left, animated: false)
self.index = nil
}
}