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每次加載WebView內容，如果圖片可以緩存的話，速度就會非常快。默認情況下，WebView自己來加載圖片，緩存的策略也是自己定的。如想要自己緩存的話，那么可以使用URLSessionDataDelegate來截獲HTTP訪問，如果訪問的是圖片的話，就自己去發起請求，緩存，如果緩存了的話，就提取緩存，自己構建一個HTTP響應對象返回。

如下案例，使用了Kingfisher庫做實際的圖片緩存，從而隔離開和文件操作相關的細節。代碼來自: github.com/Finb/V2ex-S… 。可以使用，但是代碼有些破碎，晚點有時間才調整。

import UIKit
import Kingfisher
class Page: UIViewController,UIGestureRecognizerDelegate{var c : UIWebView!var tapGesture : UITapGestureRecognizer!override func viewDidLoad() {super.viewDidLoad()c = UIWebView()c.frame = super.view.frameview.addSubview(c)c.frame.origin.y += 100c.frame.size.height = 100c.frame.size.width = 100let button = UIButton()button.setTitle("reload", for: .normal)button.addTarget(self, action: #selector(tap), for: .touchDown)button.frame = CGRect(x: 0, y: 70, width: 100, height: 20)view.addSubview(button)}func tap(){loadurl()}func loadurl(){let url = URL(string:"https://httpbin.org/image/png")//must be a https url ,otherwise iOS will fobidden itlet ro = URLRequest(url:url!)c.loadRequest(ro)}
}
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {var window: UIWindow?func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {URLProtocol.registerClass(WebViewImageProtocol.self)self.window = UIWindow(frame: UIScreen.main.bounds)self.window!.rootViewController = Page()self.window?.makeKeyAndVisible()return true}
}
fileprivate let WebviewImageProtocolHandledKey = "WebviewImageProtocolHandledKey"class WebViewImageProtocol: URLProtocol ,URLSessionDataDelegate {var session: URLSession?var dataTask: URLSessionDataTask?var imageData: Data?override class func canInit(with request: URLRequest) -> Bool{let b = request.url?.absoluteURL.absoluteString.contains("png")if b! {if let tag = self.property(forKey: WebviewImageProtocolHandledKey, in: request) as? Bool , tag == true {return false}return true}return false}override class func canonicalRequest(for request: URLRequest) -> URLRequest{return request}override class func requestIsCacheEquivalent(_ a: URLRequest, to b: URLRequest) -> Bool {return super.requestIsCacheEquivalent(a, to: b)}override func startLoading() {let resource = ImageResource(downloadURL: self.request.url!)let data = try? Data(contentsOf:URL(fileURLWithPath: KingfisherManager.shared.cache.cachePath(forKey: resource.cacheKey)))if let data = data {//在磁盤上找到Kingfisher的緩存，則直接使用緩存print("hitted")var mimeType = data.contentTypeForImageData()mimeType.append(";charset=UTF-8")let header = ["Content-Type": mimeType,"Content-Length": String(data.count)]let response = HTTPURLResponse(url: self.request.url!, statusCode: 200, httpVersion: "1.1", headerFields: header)!self.client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .allowed)self.client?.urlProtocol(self, didLoad: data)self.client?.urlProtocolDidFinishLoading(self)}else{//沒找到圖片則下載print("caching")guard let newRequest = (self.request as NSURLRequest).mutableCopy() as? NSMutableURLRequest else {return}WebViewImageProtocol.setProperty(true, forKey: WebviewImageProtocolHandledKey, in: newRequest)self.session = URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: nil)self.dataTask = self.session?.dataTask(with:newRequest as URLRequest)self.dataTask?.resume()}}override func stopLoading() {self.dataTask?.cancel()self.dataTask = nilself.imageData = nil}func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {self.client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .allowed)completionHandler(.allow)}func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {self.client?.urlProtocol(self, didLoad: data)if self.imageData == nil {self.imageData = data}else{self.imageData!.append(data)}}func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {if let error = error {self.client?.urlProtocol(self, didFailWithError: error)}else{self.client?.urlProtocolDidFinishLoading(self)let resource = ImageResource(downloadURL: self.request.url!)guard let imageData = self.imageData else { return }//保存圖片到Kingfisherguard  let image = DefaultCacheSerializer.default.image(with: imageData, options: nil) else { return }KingfisherManager.shared.cache.store(image, original: imageData, forKey: resource.cacheKey,  toDisk: true){print("cached")}}}
}
fileprivate extension Data {func contentTypeForImageData() -> String {var c:UInt8 = 0self.copyBytes(to: &c, count: MemoryLayout<UInt8>.size * 1)switch c {case 0xFF:return "image/jpeg";case 0x89:return "image/png";case 0x47:return "image/gif";default:return ""}}
}復制代碼

運行后點擊按鈕reload兩次輸出為：
caching
cached
hitted