iOS开发学习笔记之swift3.0几种加载网络图片的方法

iOS加载本地图片很简单，只需要将图片当做资源加入到工程，imageView.image = UIImage(named: "这里加入导入的资源名字")就可以了，然而对于看图app，不可能将海量的图片都加入到app当中，而是通过网络获取图片，然后再用来显示，这样做就能实时从网络上获取最新的图片。废话不多说，现在讲讲再swfit3.0中用到的几种加载网络图片的方法。

// storyboard拉一个imageView，关联输出口
@IBOutlet weak var loadImageFromURL: UIImageView!

方法一：同步加载网络图片

// 方法一: 同步加载网络图片
let url = URL(string: "http://i.imgur.com/w5rkSIj.jpg")
// 从url上获取内容
// 获取内容结束才进行下一步
let data = try? Data(contentsOf: url!)

if let imageData = data {
let image = UIImage(data: data!)
loadImageFromURL.image = image
}

方法二：异步加载网络图片

// 方法二： 异步加载网络图片
let urlString = "http://i.imgur.com/w5rkSIj.jpg"
let catPictureURL = URL(string: urlString)!

// url带中文的，urlString必须经过处理，加addingPercentEncoding，不然catPictureURL为nil,不带中文就不需要
// 这个是url带中文的处理方法
//        let urlString = "http://localhost:8080/AlbumImage/杀破狼.jpg"
//        let unsafeP = urlString.addingPercentEncoding(withAllowedCharacters: NSCharacterSet(charactersIn:"`#%^{}\"[]|\\<> ").inverted)!
//        let catPictureURL = URL(string: unsafeP)!

// 用默认配置创建一个session对象
let session = URLSession(configuration: .default)

// 定义一个下载task，下载task会把url的内容读到data中，然后你可以用data来操作，如加载图片，缓存图片等
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.
DispatchQueue.main.async {
self.loadImageFromURL.image = image
}

} else {
print("Couldn't get image: Image is nil")
}
} else {
print("Couldn't get response code for some reason")
}
}
}

// 这一句必须要加，不然task不会开始
downloadPicTask.resume()

方法三：异步加载网络图片

//方法三： 异步加载网络图片
//创建URL对象
let url = URL(string:"http://i.imgur.com/w5rkSIj.jpg")!
//创建请求对象
let request = URLRequest(url: url)

let session = URLSession.shared
let dataTask = session.dataTask(with: request, completionHandler: {
(data, response, error) -> Void in
if error != nil{
print(error.debugDescription)
}else{
//将图片数据赋予UIImage
let img = UIImage(data:data!)

// 这里需要改UI，需要回到主线程
//                DispatchQueue.main.async {
self.loadImageFromURL.image = img
//                }

}
}) as URLSessionTask

//使用resume方法启动任务
dataTask.resume()

以上三个方法在swift3.0可以使用，这里用到http，所以在运行之前先设置一下Info.plist，在Info.plist中添加“App Transport Security Settings”，然后在“App Transport Security Settings”中将Allow Arbitrary Loads设置为YES（如图1），然后在运行，要不然运行时看不到任何效果的。



图1

最后将加载网络图片做个简单封装，扩展UIImageView：

extension UIImageView {
func downloadedFrom(url: URL, contentMode mode: UIViewContentMode = .scaleToFill) {
contentMode = mode
URLSession.shared.dataTask(with: url) { (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.
DispatchQueue.main.async() { () -> Void in
self.image = image
}

} else {
print("Couldn't get image: Image is nil")
}
} else {
print("Couldn't get response code for some reason")
}
}

}.resume()

}
func downloadedFrom(link: String, contentMode mode: UIViewContentMode = .scaleAspectFit) {
guard let url = URL(string: link) else { return }
downloadedFrom(url: url, contentMode: mode)
}
}

调用：

let urlString = "http://i.imgur.com/w5rkSIj.jpg"
if let url = URL(string: urlString) {
loadImageFromURL.downloadedFrom(url: url)
}