Xamarin.Forms 5.0.0.2196版WebView加载Google Viewer预览PDF链接失效如何解决？

该问题是Xamarin.Forms 5.0.0.2196版本对WebView的默认配置做了调整，同时谷歌文档查看器的内部链接都基于iframe内跳转，默认不会被外层WebView的导航事件捕获导致的。

需要分平台自定义WebView渲染器拦截内部跳转请求，手动触发导航事件：

Android端 #

• 自定义WebView渲染器，开启谷歌文档必备的JavaScript、DomStorage权限，重写ShouldOverrideUrlLoading方法拦截所有跳转
  示例代码：

[assembly: ExportRenderer(typeof(WebView), typeof(CustomWebViewRenderer))]
namespace YourAppNamespace.Droid.Renderers
{
    public class CustomWebViewRenderer : WebViewRenderer
    {
        public CustomWebViewRenderer(Context context) : base(context) { }

        protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.WebView> e)
        {
            base.OnElementChanged(e);
            if (Control != null)
            {
                Control.Settings.JavaScriptEnabled = true;
                Control.Settings.DomStorageEnabled = true;
                Control.SetWebViewClient(new CustomWebViewClient(Element));
            }
        }
    }

    internal class CustomWebViewClient : WebViewClient
    {
        private readonly Xamarin.Forms.WebView _formsWebView;

        public CustomWebViewClient(Xamarin.Forms.WebView webView)
        {
            _formsWebView = webView;
        }

        public override bool ShouldOverrideUrlLoading(Android.Webkit.WebView view, IWebResourceRequest request)
        {
            var navArgs = new WebNavigatingEventArgs(WebNavigationEvent.NewPage, 
                new UrlWebViewSource { Url = request.Url.ToString() }, request.Url.ToString());
            _formsWebView.SendNavigating(navArgs);
            if (navArgs.Cancel) return true;
            view.LoadUrl(request.Url.ToString());
            return true;
        }
    }
}

iOS端 #

• 自定义WebView渲染器，重写DecidePolicy方法拦截iframe跳转，手动触发Navigating事件
  示例代码：

[assembly: ExportRenderer(typeof(WebView), typeof(CustomWebViewRenderer))]
namespace YourAppNamespace.iOS.Renderers
{
    public class CustomWebViewRenderer : WkWebViewRenderer
    {
        protected override void OnElementChanged(VisualElementChangedEventArgs e)
        {
            base.OnElementChanged(e);
            if (e.NewElement != null)
            {
                NavigationDelegate = new CustomNavigationDelegate(Element as Xamarin.Forms.WebView);
            }
        }
    }

    internal class CustomNavigationDelegate : WKNavigationDelegate
    {
        private readonly Xamarin.Forms.WebView _formsWebView;

        public CustomNavigationDelegate(Xamarin.Forms.WebView webView)
        {
            _formsWebView = webView;
        }

        public override void DecidePolicy(WKWebView webView, WKNavigationAction navigationAction, Action<WKNavigationActionPolicy> decisionHandler)
        {
            var navArgs = new WebNavigatingEventArgs(WebNavigationEvent.NewPage, 
                new UrlWebViewSource { Url = navigationAction.Request.Url.ToString() }, navigationAction.Request.Url.ToString());
            _formsWebView.SendNavigating(navArgs);
            decisionHandler(navArgs.Cancel ? WKNavigationActionPolicy.Cancel : WKNavigationActionPolicy.Allow);
        }
    }
}

• 如果需要处理新窗口打开的链接，iOS端可额外配置UIDelegate拦截新窗口请求，在当前WebView加载或者跳转外部浏览器
• 不要在Forms层重复给WebView的Source赋值，避免重复加载导致事件丢失
• 拦截到下载类链接时可直接调用原生下载接口处理，无需走WebView加载

内容的提问来源于stack exchange，提问作者tuha