为了对网页中的图片做缓存加快运行速度，百度到这么一个抽象类
NSURLProtocol可以监听所有请求。

上代码，新建一个NSURLProtocol的分类，

#import <Foundation/Foundation.h>

@interface NSURLProtocol (VCWebView)
+ (void)wk_registerScheme:(NSString*)scheme;
+ (void)wk_unregisterScheme:(NSString*)scheme;
@end

#import "NSURLProtocol+VCWebView.h"
#import <WebKit/WebKit.h>

//FOUNDATION_STATIC_INLINE 属于属于runtime范畴，你的.m文件需要频繁调用一个函数,可以用static inline来声明。从SDWebImage从get到的。
FOUNDATION_STATIC_INLINE Class ContextControllerClass() {
    static Class cls;
    if (!cls) {
        cls = [[[WKWebView new] valueForKey:@"browsingContextController"] class];
    }
    return cls;
}

FOUNDATION_STATIC_INLINE SEL RegisterSchemeSelector() {
    return NSSelectorFromString(@"registerSchemeForCustomProtocol:");
}

FOUNDATION_STATIC_INLINE SEL UnregisterSchemeSelector() {
    return NSSelectorFromString(@"unregisterSchemeForCustomProtocol:");
}

@implementation NSURLProtocol (VCWebView)

+ (void)wk_registerScheme:(NSString *)scheme {
    Class cls = ContextControllerClass();
    SEL sel = RegisterSchemeSelector();
    if ([(id)cls respondsToSelector:sel]) {
        // 放弃编辑器警告
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        [(id)cls performSelector:sel withObject:scheme];
#pragma clang diagnostic pop
    }
}

+ (void)wk_unregisterScheme:(NSString *)scheme {
    Class cls = ContextControllerClass();
    SEL sel = UnregisterSchemeSelector();
    if ([(id)cls respondsToSelector:sel]) {
        // 放弃编辑器警告
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
        [(id)cls performSelector:sel withObject:scheme];
#pragma clang diagnostic pop
    }
}
@end

然后创建一个子类，继承NSURLProtocol

#import "VCCustomURLProtocol.h"
#import "SDImageCache.h"
#import "NSData+ImageContentType.h"
#import "UIImage+MultiFormat.h"

static NSString * const hasInitKey = @"VCCustomURLProtocolKey";
@interface VCCustomURLProtocol ()

@property (nonatomic, strong)NSMutableData *responseData;
@property (nonatomic, strong)NSURLConnection *connection;

@end


@implementation VCCustomURLProtocol

+ (BOOL)canInitWithRequest:(NSURLRequest *)request{
    if ([request.URL.scheme isEqualToString:@"http"]) {
        NSString *str = request.URL.absoluteString;
        NSString *str1 = request.URL.path;
        NSLog(@"=================================%@",str);
        NSLog(@"=================================%@",str1);
        //只处理http请求的图片
        if (([str hasSuffix:@".png"] || [str hasSuffix:@".jpg"] || [str hasSuffix:@".gif"])
            && ![NSURLProtocol propertyForKey:hasInitKey inRequest:request]) {
            //yes 表示子类能处理该请求
            NSLog(@"===============yes  yes   yes  yes ");
            return YES;
        }
    }
    return NO;
}

+ (NSURLRequest *)canonicalRequestForRequest:(NSURLRequest *)request {
    
    NSMutableURLRequest *mutableReqeust = [request mutableCopy];
    //这边可用干你想干的事情。。更改地址，提取里面的请求内容，或者设置里面的请求头。。
    return mutableReqeust;
}

+ (BOOL)requestIsCacheEquivalent:(NSURLRequest *)a toRequest:(NSURLRequest *)b{
    
    return YES;
}

- (void)startLoading
{
    NSMutableURLRequest *mutableReqeust = [[self request] mutableCopy];
    //做下标记，防止递归调用
    [NSURLProtocol setProperty:@YES forKey:hasInitKey inRequest:mutableReqeust];
    
    //查看本地是否已经缓存了图片
    NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
    
    NSData *data = [[SDImageCache sharedImageCache] diskImageDataBySearchingAllPathsForKey:key];
    
    if (data) {
        NSURLResponse *response = [[NSURLResponse alloc] initWithURL:mutableReqeust.URL
                                                            MIMEType:[NSData sd_contentTypeForImageData:data]
                                               expectedContentLength:data.length
                                                    textEncodingName:nil];
        [self.client URLProtocol:self
              didReceiveResponse:response
              cacheStoragePolicy:NSURLCacheStorageNotAllowed];
        
        [self.client URLProtocol:self didLoadData:data];
        [self.client URLProtocolDidFinishLoading:self];
    }
    else {
        self.connection = [NSURLConnection connectionWithRequest:mutableReqeust delegate:self];
    }
}

- (void)stopLoading
{
    [self.connection cancel];
}

#pragma mark- NSURLConnectionDelegate

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    
    [self.client URLProtocol:self didFailWithError:error];
}

#pragma mark - NSURLConnectionDataDelegate
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    self.responseData = [[NSMutableData alloc] init];
    [self.client URLProtocol:self didReceiveResponse:response cacheStoragePolicy:NSURLCacheStorageNotAllowed];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [self.responseData appendData:data];
    [self.client URLProtocol:self didLoadData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    UIImage *cacheImage = [UIImage sd_imageWithData:self.responseData];
    //利用SDWebImage提供的缓存进行保存图片
    [[SDImageCache sharedImageCache] storeImage:cacheImage
                           recalculateFromImage:NO
                                      imageData:self.responseData
                                         forKey:[[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL]
                                         toDisk:YES];
    
    [self.client URLProtocolDidFinishLoading:self];
}

@end

这样只要在你加载网页的时候启动或者关闭监听就好了。

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    //注册这个方法，用于缓存网页的图片
    [NSURLProtocol registerClass:[VCCustomURLProtocol class]];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];
    [NSURLProtocol unregisterClass:[VCCustomURLProtocol class]];
}

这样就可以实现了对网页图片的缓存，大家可以尽情的试试了。