float webViewHeight = [[webView stringByEvaluatingJavaScriptFromString:
                        @"document.body.scrollHeight"] floatValue];

WKWebView的高度計算方法:

[webView evaluateJavaScript:@"document.body.scrollHeight" completionHandler:^(id _Nullable result, NSError * _Nullable error) {
         //獲取頁面高度
         double webViewHeight = [result doubleValue];
    }];