参考文档：
iOS【终极方案】精准获取webView内容高度，自适应高度
iOS精准获取webView内容高度，自适应高度（cell篇）

#pragma mark-懒加载webView
-(UIWebView *)webView{
    if (!_webView) {
//        _webView = [[UIWebView alloc] init];
        // 1随便多少,不能为0
        _webView = [[UIWebView alloc]initWithFrame:CGRectMake(0,0,self.view.frame.size.width,1)];
        _webView.delegate = self;
        _webView.scrollView.scrollEnabled = NO;
        //这个属性不加,webview会显示很大.
        _webView.scalesPageToFit = YES;
        _webView.opaque =NO;
        //预先加载url
        [_webView loadRequest:[[NSURLRequest alloc] initWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"%@/api/product/piclist?item_id=%@",SERVER,self.model.itemid]]]];
        //给webView的scrollView的contentSize属性添加监听，每当内容发生变化，contentSize一定会跟着变，捕获这个变动
//        [self.webView.scrollView addObserver:self forKeyPath:@"contentSize" options:(NSKeyValueObservingOptionNew) context:nil];
        [self.webView.scrollView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew context:nil];
    }
    return _webView;
}
- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view addSubview:self.webView];
}
#pragma mark - UIWebView Delegate Methods
-(void)webViewDidFinishLoad:(UIWebView *)webView
{
     [[MBProgressHUD HUDForView:self.view] removeFromSuperview];
//    //获取到webview的高度
////    CGFloat height =  [[webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.scrollHeight"]floatValue];
//    self.webViewHeight = [[self.webView stringByEvaluatingJavaScriptFromString:@"document.body.scrollHeight"] floatValue];
//    CGSize webSize = [webView sizeThatFits:CGSizeZero];
//    [webView mas_updateConstraints:^(MASConstraintMaker *make) {
//        make.height.equalTo(@(webSize.height));
//    }];
//    cellhight = self.webViewHeight;
 //   [self.tableView reloadData];

}
- (void)webViewDidStartLoad:(UIWebView *)webView
{
    NSLog(@"webViewDidStartLoad");
//    [MBProgressHUD showHUDAddedTo:self.view animated:YES];
}
- (void)webView:(UIWebView *)webView didFailLoadWithError:(nonnull NSError *)error
{
    NSLog(@"didFailLoadWithError===%@", error);
    isOpen = !isOpen;
     [[MBProgressHUD HUDForView:self.view] removeFromSuperview];
       [AMShowMessage showMessage:@"加载失败" withDuration:2];
    [self.tableView reloadData];
}
#pragma mark-监听的处理1、监听tableview偏移（控制导航的透明度）2、监听webView.scrollView的contentSize的变化
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
   
    if ([keyPath isEqualToString:@"contentOffset"])
    {
      
          UIButton *  bottom = [self.view viewWithTag:1000];
        CGPoint offset = [change[NSKeyValueChangeNewKey] CGPointValue];
        CGFloat alpha = offset.y/NAVIGATION_BAR_HEIGHT >1.0f ? 1:offset.y/NAVIGATION_BAR_HEIGHT;
        //设置一个颜色并转化为图片
        UIImage *image = [self imageWithColor:[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:alpha]];
        
        self.navigationView.image = image;
        self.navigationController.navigationBar.alpha = alpha;
        self.topView.backgroundColor = [UIColor colorWithPatternImage:image];
        self.topView.alpha = alpha;

        if (offset.y > 0) {
        self.navigationView.isAlphZero = NO;
            bottom.hidden = NO;
            
        }else{
            self.navigationView.isAlphZero = YES;
             bottom.hidden = YES;
        }
        
    }


    if ([keyPath isEqualToString:@"contentSize"]) {
         //通过JS代码获取webview的内容高度
        //self.webViewHeight = [[self.webView stringByEvaluatingJavaScriptFromString:@"document.body.scrollHeight"] floatValue];
       //通过webview的contentSize获取内容高度
       //self.webViewHeight = [self.showWebView.scrollView contentSize].height;
        CGRect frame = self.webView.frame;
        //通过webview的contentSize获取内容高度
        frame.size.height = self.webView.scrollView.contentSize.height;
        
        //这里获取webView的高度
        _webViewHeight = frame.size.height;
        NSLog(@"%f",frame.size.height);

        cellhight =_webViewHeight;
        CGSize webSize = [self.webView sizeThatFits:CGSizeZero];
        [self.webView mas_updateConstraints:^(MASConstraintMaker *make) {
            make.height.equalTo(@(webSize.height));
        }];

        [self.tableView reloadData];
    }
}

反思：
1、在webViewDidFinishLoad方法中获取webView的内容高度，会导致获取的内容高度不精确，因为webViewDidFinishLoad代理方法被调用时，页面并不一定完全展现完成，可能有图片还未加载出来，导致此时获取的高度是并不是最终高度，过会儿图片加载出来后，浏览器会重新排版，而我们在这之前给了一个错误高度，导致显示异常。所以需要在监听到webView.scrollView的contentSize变化时，获取到的webView的内容高度才时精确的。

#pragma mark - UIWebView Delegate Methods
-(void)webViewDidFinishLoad:(UIWebView *)webView
{
     [[MBProgressHUD HUDForView:self.view] removeFromSuperview];
//    //获取到webview的高度
////    CGFloat height =  [[webView stringByEvaluatingJavaScriptFromString:@"document.documentElement.scrollHeight"]floatValue];
//    self.webViewHeight = [[self.webView stringByEvaluatingJavaScriptFromString:@"document.body.scrollHeight"] floatValue];
//    CGSize webSize = [webView sizeThatFits:CGSizeZero];
//    [webView mas_updateConstraints:^(MASConstraintMaker *make) {
//        make.height.equalTo(@(webSize.height));
//    }];
//    cellhight = self.webViewHeight;
    [self.tableView reloadData];

}

2、实例化创建webView的时候，必须设置frame值，否值不显示

// 高度随便多少,可以为1，但不能为0，为0 则webView不显示
_webView = [[UIWebView alloc]initWithFrame:CGRectMake(0,0,self.view.frame.size.width,1)];