计算wkwebview实际高度

- (void)viewDidLoad
{
    [self.webView.scrollView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];
}

- (void)dealloc {
    
    [self.webView.scrollView removeObserver:self forKeyPath:@"contentSize" context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (object == self.webView.scrollView && [keyPath isEqual:@"contentSize"]) {
        // we are here because the contentSize of the WebView's scrollview changed.
        
        UIScrollView *scrollView = self.webView.scrollView;
        NSLog(@"New contentSize: %f x %f", scrollView.contentSize.width, scrollView.contentSize.height);
    }
}