iOS WKWebView 图片适配屏幕，最终解决方案（适配iOS 10 11 12）

        WKUserContentController *userContentController = [[WKUserContentController alloc] init];
        
        NSString * webWidth = @"";
        
        if (@available(iOS 11, *)) {
            webWidth = @"device-width";
        }else{
            webWidth = [NSString stringWithFormat:@"%f",[UIScreen mainScreen].scale *  [[UIScreen mainScreen] bounds].size.width];
        }
        NSString *jSString = jSString = [NSString stringWithFormat:@"var meta = document.createElement('meta'); meta.setAttribute('name', 'viewport'); meta.setAttribute('content', 'width=%@'); document.getElementsByTagName('head')[0].appendChild(meta);",webWidth];
        
        WKUserScript *wkUserScript = [[WKUserScript alloc] initWithSource:jSString injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:YES];
        [userContentController addUserScript:wkUserScript];
        WKWebViewConfiguration *wkWebConfig = [[WKWebViewConfiguration alloc] init];
        wkWebConfig.userContentController = userContentController;