网页重定向时，返回卡顿延迟严重！代码执行如下，作者可以测一下
//网页重定向

• (void)urlChange:(NSString*)urlStr
  {
  NSURLRequest * request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:urlStr]];
  NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
  NSURLSession *session = [NSURLSession sessionWithConfiguration:config
  delegate:self
  delegateQueue:[NSOperationQueue mainQueue]];
  NSURLSessionDataTask * task = [session dataTaskWithRequest:request];
  [task resume];
  }