思路如下：

1. 在webview加载完之后添加一个contentSize的KVO
2. 根据contentSize改变webView 高度约束

// 坚听高度变化
    self.KVOController = [FBKVOController controllerWithObserver:self];
    [self.KVOController observe:self.webView.scrollView keyPath:@"contentSize" options:NSKeyValueObservingOptionNew block:^(id  _Nullable observer, id  _Nonnull object, NSDictionary<NSKeyValueChangeKey,id> * _Nonnull change) {
        float height = selfWeak.webView.scrollView.contentSize.height;
        NSLog(@"geight = %f",height);
        [selfWeak changeLayoutByWebViewHeight:height];
    }];
// 改变高度约束
- (void)changeLayoutByWebViewHeight:(float)height{
    [self.webView mas_updateConstraints:^(MASConstraintMaker *make) {
        make.height.mas_equalTo(height);
    }];
}

遇到了一个问题～webView死循环， 高度越来越大直至crash

原因是:WKWebView 自动缩放导致webview.scrollerView.contentSize.height 一直大于webview.height

解决方案:禁用WKWebView的缩放功能

- (WKWebView *)webView{
    if (!_webView) {
        // 高度自适应js
        NSString *sizejScript = @"var meta = document.createElement('meta'); meta.setAttribute('name', 'viewport'); meta.setAttribute('content', 'width=device-width'); document.getElementsByTagName('head')[0].appendChild(meta);";
        WKUserScript *sizeUScript = [[WKUserScript alloc] initWithSource:sizejScript injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:YES];
        // 禁止缩放
        NSString *zoomingjScript = @" $('meta[name=description]').remove(); $('head').append( '<meta name=\"viewport\" content=\"width=device-width, initial-scale=1,user-scalable=no\">' );";
        WKUserScript *zoomingUScript = [[WKUserScript alloc] initWithSource:zoomingjScript injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:NO];
        WKUserContentController *wkUController = [[WKUserContentController alloc] init];
        [wkUController addUserScript:sizeUScript];
        [wkUController addUserScript:zoomingUScript];
        WKWebViewConfiguration *wkWebConfig = [[WKWebViewConfiguration alloc] init];
        wkWebConfig.userContentController = wkUController;
        _webView =  [[WKWebView alloc] initWithFrame:CGRectZero configuration:wkWebConfig];
        _webView.backgroundColor = [UIColor redColor];//PCH_BANK_GRAY_ededed;//
        _webView.scrollView.scrollEnabled = NO;
        _webView.scrollView.layer.masksToBounds = NO;
        _webView.scrollView.bounces = NO;
    }
    return _webView;
}