截取webView里面的图片，并且保持

#pragma mark - ******************** 将发出通知时调用
- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType
{
    NSString *url = request.URL.absoluteString;
    NSRange range = [url rangeOfString:@"sx:src="];
    if (range.location != NSNotFound) {
        NSInteger begin = range.location + range.length;
        NSString *src = [url substringFromIndex:begin];
        [self savePictureToAlbum:src];
        return NO;
    }
    return YES;
}

#pragma mark - ******************** 保存到相册方法
- (void)savePictureToAlbum:(NSString *)src
{
    UIAlertController *alert = [UIAlertController alertControllerWithTitle:@"提示" message:@"确定要保存到相册吗？" preferredStyle:UIAlertControllerStyleActionSheet];
    
    [alert addAction:[UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleCancel handler:nil]];
    [alert addAction:[UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDestructive handler:^(UIAlertAction *action) {
        
        NSURLCache *cache =[NSURLCache sharedURLCache];
        NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:src]];
        NSData *imgData = [cache cachedResponseForRequest:request].data;
        UIImage *image = [UIImage imageWithData:imgData];
        UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil);
        
    }]];
    
    
    [self presentViewController:alert animated:YES completion:nil];
}