获取网页整个图片失败

需要捕获网页的全屏图片，但是显示出来是空白。

应该怎么实现？

我的失败代码：

CGSize overallSize = [WebPageImage sizeThatFits:CGSizeZero];
UIGraphicsBeginImageContext(WebPageImage.scrollView.contentSize);
WebPageImage.bounds = CGRectMake(0, 0, overallSize.width, overallSize.height);
while (WebPageImage.loading) {
    [NSThread sleepForTimeInterval:0.1];
}
UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,(unsigned long)NULL), ^(void) {
    UIImageWriteToSavedPhotosAlbum(viewImage, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
});