iOS pdf file in webview to load a black screen problem

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Project development process, after the completion of the process of loading a pdf file, often black screen appears;
because webview call -webViewDidFinishLoad:after responsible for rendering UIWebPDFView pdf file has not finished loading, all want to get rid of the black background may be in -(void)viewDidLayoutSubviewsor

- (void)webView:(WKWebView *)webView 
didFinishNavigation:(WKNavigation *)navigation

Re-set the background color is white UIWebPDFView

- (void)webView:(WKWebView *)webView 
didFinishNavigation:(WKNavigation *)navigation 
{
   UIView *v = webView;
    while (v) {
        v.backgroundColor = [UIColor whiteColor];
        v = [v.subviews firstObject];
    }
}

Reference Links: https://stackoverflow.com/questions/45626151/ios-wkwebview-load-local-pdf-files-some-pages-become-black