在 Swift 3 中，我试图从互联网上捕获图像，并使用以下代码行：
var catPictureURL = NSURL(fileURLWithPath: "http://i.imgur.com/w5rkSIj.jpg")
var catPictureData = NSData(contentsOf: catPictureURL as URL) // nil
var catPicture = UIImage(data: catPictureData as! Data)
我在这里做错了什么？
【问题讨论】：
标签： swift3
您的代码存在一些问题：
我们要做的第一件事是将你的变量声明为let，因为我们以后不会修改它。
let catPictureURL = URL(string: "http://i.imgur.com/w5rkSIj.jpg")! // We can force unwrap because we are 100% certain the constructor will not return nil in this case.
然后我们需要下载该 URL 的内容。我们可以使用URLSession 对象来做到这一点。当调用完成处理程序时，我们会从网上下载一个UIImage。
// Creating a session object with the default configuration.
// You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
let session = URLSession(configuration: .default)
// Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.
let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
// The download has finished.
if let e = error {
print("Error downloading cat picture: \(e)")
} else {
// No errors found.
// It would be weird if we didn't have a response, so check for that too.
if let res = response as? HTTPURLResponse {
print("Downloaded cat picture with response code \(res.statusCode)")
if let imageData = data {
// Finally convert that Data into an image and do what you wish with it.
let image = UIImage(data: imageData)
// Do something with your image.
} else {
print("Couldn't get image: Image is nil")
}
} else {
print("Couldn't get response code for some reason")
}
}
}
最后你需要在下载任务上调用resume，否则你的任务永远不会启动：
downloadPicTask.resume().
所有这些代码一开始可能看起来有点吓人，但 URLSession API 是基于块的，因此它们可以异步工作 - 如果您将 UI 线程阻塞几秒钟，操作系统将终止您的应用。
您的完整代码应如下所示：
let catPictureURL = URL(string: "http://i.imgur.com/w5rkSIj.jpg")!
// Creating a session object with the default configuration.
// You can read more about it here https://developer.apple.com/reference/foundation/urlsessionconfiguration
let session = URLSession(configuration: .default)
// Define a download task. The download task will download the contents of the URL as a Data object and then you can do what you wish with that data.
let downloadPicTask = session.dataTask(with: catPictureURL) { (data, response, error) in
// The download has finished.
if let e = error {
print("Error downloading cat picture: \(e)")
} else {
// No errors found.
// It would be weird if we didn't have a response, so check for that too.
if let res = response as? HTTPURLResponse {
print("Downloaded cat picture with response code \(res.statusCode)")
if let imageData = data {
// Finally convert that Data into an image and do what you wish with it.
let image = UIImage(data: imageData)
// Do something with your image.
} else {
print("Couldn't get image: Image is nil")
}
} else {
print("Couldn't get response code for some reason")
}
}
}
downloadPicTask.resume()
【讨论】：
let url = URL(string: "http://i.imgur.com/w5rkSIj.jpg")
let data = try? Data(contentsOf: url)
if let imageData = data {
let image = UIImage(data: imageData)
}
【讨论】：
data!
您也可以使用 Alamofire\AlmofireImage 来完成该任务： https://github.com/Alamofire/AlamofireImage
代码应该看起来像这样（基于上面链接上的第一个示例）：
import AlamofireImage
Alamofire.request("http://i.imgur.com/w5rkSIj.jpg").responseImage { response in
if let catPicture = response.result.value {
print("image downloaded: \(image)")
}
}
虽然它既整洁又安全，但您应该考虑是否值得 Pod 开销。 如果您要使用更多图像并想添加过滤器和转换，我会考虑使用 AlamofireImage
【讨论】：
使用此扩展程序并更快地下载图像。
extension UIImageView {
public func imageFromURL(urlString: String) {
let activityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .gray)
activityIndicator.frame = CGRect.init(x: 0, y: 0, width: self.frame.size.width, height: self.frame.size.height)
activityIndicator.startAnimating()
if self.image == nil{
self.addSubview(activityIndicator)
}
URLSession.shared.dataTask(with: NSURL(string: urlString)! as URL, completionHandler: { (data, response, error) -> Void in
if error != nil {
print(error ?? "No Error")
return
}
DispatchQueue.main.async(execute: { () -> Void in
let image = UIImage(data: data!)
activityIndicator.removeFromSuperview()
self.image = image
})
}).resume()
}
}
【讨论】：
通过扩展
扩展本机功能的良好解决方案import UIKit
extension UIImage {
convenience init?(url: URL?) {
guard let url = url else { return nil }
do {
self.init(data: try Data(contentsOf: url))
} catch {
print("Cannot load image from url: \(url) with error: \(error)")
return nil
}
}
}
便利初始化程序是失败的，并接受可选的URL - 方法是安全的。
imageView.image = UIImage(url: URL(string: "some_url.png"))
【讨论】：
在 Swift 3 上使用 Alamofire 对我很有帮助：
第 1 步：
使用 pod 进行集成。
pod 'Alamofire', '~> 4.4'
pod 'AlamofireImage', '~> 3.3'
第 2 步：
导入 AlamofireImage
导入 Alamofire
第 3 步：
Alamofire.request("https://httpbin.org/image/png").responseImage { response in
if let image = response.result.value {
print("image downloaded: \(image)")
self.myImageview.image = image
}
}
【讨论】：
我认为最简单的方法是使用 SDWebImage
将此添加到您的 pod 文件中
pod 'SDWebImage', '~> 4.0'
运行 pod 安装
现在导入 SDWebImage
import SDWebImage
现在从 url 设置图像
imageView.sd_setImage(with: URL(string: "http://www.domain/path/to/image.jpg"), placeholderImage: UIImage(named: "placeholder.png"))
它会显示占位符图像，但是当下载图像时，它会显示来自 url 的图像。您的应用永远不会崩溃
这是 SDWebImage 的主要特点
为 UIImageView、UIButton、MKAnnotationView 添加网页图片和缓存管理的类别
异步图片下载器
具有自动缓存过期处理的异步内存 + 磁盘映像缓存
背景图片解压
保证不会多次下载同一个网址
保证不会一次又一次地重试虚假网址
保证主线程永远不会被阻塞 表演！
使用 GCD 和 ARC
【讨论】：
使用 UIImageView 的扩展来加载 URL 图片。
let imageCache = NSCache<NSString, UIImage>()
extension UIImageView {
func imageURLLoad(url: URL) {
DispatchQueue.global().async { [weak self] in
func setImage(image:UIImage?) {
DispatchQueue.main.async {
self?.image = image
}
}
let urlToString = url.absoluteString as NSString
if let cachedImage = imageCache.object(forKey: urlToString) {
setImage(image: cachedImage)
} else if let data = try? Data(contentsOf: url), let image = UIImage(data: data) {
DispatchQueue.main.async {
imageCache.setObject(image, forKey: urlToString)
setImage(image: image)
}
}else {
setImage(image: nil)
}
}
}
}
【讨论】：
let url = ("https://firebasestorage.googleapis.com/v0/b/qualityaudit-678a4.appspot.com/o/profile_images%2FBFA28EDD-9E15-4CC3-9AF8-496B91E74A11.png?alt=media&token=b4518b07-2147-48e5-93fb-3de2b768412d")
self.myactivityindecator.startAnimating()
let urlString = url
guard let url = URL(string: urlString) else { return }
URLSession.shared.dataTask(with: url)
{
(data, response, error) in
if error != nil {
print("Failed fetching image:", error!)
return
}
guard let response = response as? HTTPURLResponse, response.statusCode == 200 else {
print("error")
return
}
DispatchQueue.main.async {
let image = UIImage(data: data!)
let myimageview = UIImageView(image: image)
print(myimageview)
self.imgdata.image = myimageview.image
self.myactivityindecator.stopanimating()
}
}.resume()
【讨论】：
我使用 AlamofireImage，它适用于 ImageView 中的 Loading url，它也有 Placeholder 选项.
func setImage (){
let image = “https : //i.imgur.com/w5rkSIj.jpg”
if let url = URL (string: image)
{
//Placeholder Image which was in your Local(Assets)
let image = UIImage (named: “PlacehoderImageName”)
imageViewName.af_setImage (withURL: url, placeholderImage: image)
}
}
注意：- 不要忘记在你的 Pod 文件和 Import Statment 中添加 AlamofireImage
举例，
pod 'AlamofireImage' 在您的 PodFile 和 ViewController 中 import AlamofireImage
【讨论】：