我想从我的应用程序中的URL加载图像，所以我首先尝试了Objective-C和它的工作，但是，与Swift，我有一个编译错误:
'imageWithData'不可用:使用对象构造'UIImage(data:)'
我的函数:
@IBOutlet var imageView : UIImageView
override func viewDidLoad() {
super.viewDidLoad()
var url:NSURL = NSURL.URLWithString("http://myURL/ios8.png")
var data:NSData = NSData.dataWithContentsOfURL(url, options: nil, error: nil)
imageView.image = UIImage.imageWithData(data)// Error here
}
在objective - c中:
- (void)viewDidLoad {
[super viewDidLoad];
NSURL *url = [NSURL URLWithString:(@"http://myURL/ios8.png")];
NSData *data = [NSData dataWithContentsOfURL:url];
_imageView.image = [UIImage imageWithData: data];
_labelURL.text = @"http://www.quentinroussat.fr/assets/img/iOS%20icon's%20Style/ios8.png";
}
有人能解释一下为什么imageWithData:不能与Swift一起工作，我该如何解决这个问题。
编辑为最新更改2021年9月
// It's better to use extension
extension UIImageView {
func downloadImage(from URLString: String, with completion: @escaping (_ response: (status: Bool, image: UIImage? ) ) -> Void) {
guard let url = URL(string: URLString) else {
completion((status: false, image: nil))
return
}
URLSession.shared.dataTask(with: url) { data, response, error in
guard error == nil else {
completion((status: false, image: nil))
return
}
guard let httpURLResponse = response as? HTTPURLResponse,
httpURLResponse.statusCode == 200,
let data = data else {
completion((status: false, image: nil))
return
}
let image = UIImage(data: data)
completion((status: true, image: image))
}.resume()
}
}
快乐的鳕鱼。欢呼:)
斯威夫特2。x答案下载图像到文件(与Leo Dabus的答案相反，它将图像存储在内存中)。根据Leo Dabus的回答和Rob的回答，从完成处理程序的NSURLSession DownloadTaskWithRequest中获取数据:
// Set download vars
let downloadURL = NSURL() // URL to download from
let localFilename = "foobar.png" // Filename for storing locally
// Create download request
let task = NSURLSession.sharedSession().downloadTaskWithURL(downloadURL) { location, response, error in
guard location != nil && error == nil else {
print("Error downloading message: \(error)")
return
}
// If here, no errors so save message to permanent location
let fileManager = NSFileManager.defaultManager()
do {
let documents = try fileManager.URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false)
let fileURL = documents.URLByAppendingPathComponent(localFilename)
try fileManager.moveItemAtURL(location!, toURL: fileURL)
self.doFileDownloaded(fileURL, localFilename: localFilename)
print("Downloaded message @ \(localFilename)")
} catch {
print("Error downloading message: \(error)")
}
}
// Start download
print("Starting download @ \(downloadURL)")
task.resume()
// Helper function called after file successfully downloaded
private func doFileDownloaded(fileURL: NSURL, localFilename: String) {
// Do stuff with downloaded image
}
(Swift 4更新)
为了直接回答最初的问题，下面是发布的Objective-C代码片段的快速等效版本。
let url = URL(string: image.url)
let data = try? Data(contentsOf: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check / try-catch
imageView.image = UIImage(data: data!)
免责声明:
需要注意的是，Data(contentsOf:)方法将在执行代码的同一线程中同步下载url的内容，因此不要在应用程序的主线程中调用此方法。
让相同的代码异步运行，而不阻塞UI的简单方法是使用GCD:
let url = URL(string: image.url)
DispatchQueue.global().async {
let data = try? Data(contentsOf: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check / try-catch
DispatchQueue.main.async {
imageView.image = UIImage(data: data!)
}
}
也就是说，在现实生活中的应用程序中，如果您希望获得最佳的用户体验并避免同一映像的多次下载，您可能还希望不仅下载它们，而且缓存它们。已经有相当多的库可以无缝地做到这一点，而且它们都非常易于使用。我个人推荐翠鸟:
import Kingfisher
let url = URL(string: "url_of_your_image")
// this downloads the image asynchronously if it's not cached yet
imageView.kf.setImage(with: url)
就是这样
