Xamarin Android WebView 下载内联pdf内容

Question

Android experts ... I'm in a quandry. I have a WebView embedded in a Forms app which also has a custom renderer (for the WebView) implemented for it.

I'm wrapping a basic web app (SAP Fiori) and within that app, it has a salary statement section where a user can view their pay slip. On that page, there is a button to download the pay slip to a PDF.

In iOS within my app, this all works as standard, ie the PDF opens and is viewed in the browser. On both Android and iOS, any browser (like Chrome or Safari) all works fine as well.

In my Android app, clicking on the "Open as PDF" button produces no outcome, nothing on the front end ever happens but within the app itself, the implemented DownloadListener method is triggered ...

protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.WebView> e)
    {
        base.OnElementChanged(e);

        Control.Settings.AllowUniversalAccessFromFileURLs = true;

        Control.SetWebViewClient(new MySAPWebViewClient(Context));
        Control.SetDownloadListener(new MySAPDownloadListener());
    }

public class MySAPDownloadListener : Java.Lang.Object, IDownloadListener
{
    public new void Dispose() { }

    public void OnDownloadStart(string url, string userAgent, string contentDisposition, string mimetype, long contentLength)
    {
        System.Diagnostics.Debug.WriteLine("Url = " + url);
        System.Diagnostics.Debug.WriteLine("Content Disposition = " + contentDisposition);
        System.Diagnostics.Debug.WriteLine("Mime Type = " + mimetype);
    }
}

These are the main DownloadEventArgs values ...

url = "https://organisation.com/sap/opu/odata/sap/HCM_MY_PAYSTUBS_SRV/PDFPaystubs(SEQUENCENUMBER=189,PersonnelAssignment='00044411')/$value"
contentDisposition = "inline; filename=Paystub_01.31.2018.pdf"
mimetype = "application/pdf"

... now, behind that url, there is a piece of javascript that generates the dynamic PDF so it's not stored anywhere on the server and this is where I seem to be having an issue.

Everything I read online says to use the HttpClient to download the contents of the PDF to file and then open it. However, if I pass in that URL to the HttpClient, I don't get the PDF data, I get the HTML text.

I've tried GoogleDocs and numerous other links here on StackOverflow but no matter what I do, that URL returns the HTML text, not the contents of the PDF document.

I have very little control over what SAP is giving me as well so changing the server side is an unlikely option.

Any help would be much appreciated.

Answer 1

Unfortunately, for some reason Android WebView cannot display PDF files. So, if using WebView is crucial for you, where is only one option - use Google Docs Viewer. But it will works only with PDFs available via public URLs. It will looks something like that:

 WebView view=(WebView)findViewById(R.id.webView);
 view.setWebViewClient(new WebViewClient());
 String url=getIntent().toString("value1");
 String URL="https://docs.google.com/viewer?url="+url; 
 view.loadUrl(Url);

If using WebView is not so crucial, you can use:

1. Some third-party library to display PDF
2. Call an Intent to display PDF via another app
3. If you usin API level 21 and above - you can get your hands dirty with Android PdfRenderer

Hope it help

Answer 2

You can do this by two solutions :

Solution 1: Open URL in System Default Browser.

private async Task MWebview_Download(object sender, DownloadEventArgs e)
{
    await Launcher.OpenAsync(e.Url);               
}

Solution 2 Downloading using HttpClient.

    private async void MWebview_Download(object sender, DownloadEventArgs e)
    {
        string fileName = GetFileName(e);

        Device.BeginInvokeOnMainThread(() =>
        {
            Toast.MakeText(MainActivity.Instance, "Downloading File...", ToastLength.Long).Show();
        });


        await DownloadPdfFile(e, fileName);
    }

    private async Task DownloadPdfFile(DownloadEventArgs e, string fileName)
    {
        try
        {
            //Bypass the certificate 
            HttpClientHandler clientHandler = new HttpClientHandler();
            clientHandler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => { return true; };

            // Getting Cookie
            String cookies = CookieManager.Instance.GetCookie(e.Url);

            var client = new HttpClient(clientHandler);
            var httpRequestMessage = new HttpRequestMessage
            {
                Method = HttpMethod.Get,
                RequestUri = new Uri(e.Url),
                Headers = {
                { HttpRequestHeader.TransferEncoding.ToString(), "binary" },
                { HttpRequestHeader.ContentLength.ToString(), e.ContentLength+"" },
                { HttpRequestHeader.ContentType.ToString(), "application/pdf" },
                {HttpRequestHeader.Cookie.ToString(),cookies },
            },
            };

            // Api Call
            var response = await client.SendAsync(httpRequestMessage);

            // Read content as Byte[]
            var bytes = response.Content.ReadAsByteArrayAsync().Result;

            // Save to Local DB
            var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.MyDocuments);
            var filePath = Path.Combine(documentsPath, fileName);

            // convert byte to stream
            MemoryStream stream = new MemoryStream(bytes);

            // Write file
            SaveAndView(fileName, "application/pdf", stream);

        }
        catch (Exception ee)
        {
            Toast.MakeText(MainActivity.Instance, "Failed !!", ToastLength.Long).Show();
        }
    }

    public void SaveAndView(string fileName, String contentType, MemoryStream stream)
    {
        string exception = string.Empty;
        string root = null;

        if (ContextCompat.CheckSelfPermission(MainActivity.Instance, Manifest.Permission.WriteExternalStorage) != Permission.Granted)
        {
            ActivityCompat.RequestPermissions((Activity)MainActivity.Instance, new String[] { Manifest.Permission.WriteExternalStorage }, 1);
        }

        if (Android.OS.Environment.IsExternalStorageEmulated)
        {
            root = Android.OS.Environment.ExternalStorageDirectory.ToString();
        }
        else
            root = System.Environment.GetFolderPath(System.Environment.SpecialFolder.MyDocuments);

        Java.IO.File myDir = new Java.IO.File(root + "/PDFFiles");
        myDir.Mkdir();

        Java.IO.File file = new Java.IO.File(myDir, fileName);

        if (file.Exists()) file.Delete();

        try
        {
            FileOutputStream outs = new FileOutputStream(file);
            outs.Write(stream.ToArray());

            outs.Flush();
            outs.Close();
        }
        catch (Exception e)
        {
            exception = e.ToString();
        }

        if (file.Exists() && contentType != "application/html")
        {
            string extension = MimeTypeMap.GetFileExtensionFromUrl(Android.Net.Uri.FromFile(file).ToString());
            string mimeType = MimeTypeMap.Singleton.GetMimeTypeFromExtension(extension);
            Intent intent = new Intent(Intent.ActionView);
            intent.SetFlags(ActivityFlags.ClearTop | ActivityFlags.NewTask);
            Android.Net.Uri path = FileProvider.GetUriForFile(MainActivity.Instance, Android.App.Application.Context.PackageName + ".provider", file);
            intent.SetDataAndType(path, mimeType);
            intent.AddFlags(ActivityFlags.GrantReadUriPermission);

            MainActivity.Instance.StartActivity(Intent.CreateChooser(intent, "Choose App"));
        }

    }

Note : Make sure to handle FileProvider :

 **Changes in Manifest file:** 

   - Add android:requestLegacyExternalStorage="true" in application tag
   
   - Add between application tag

    <provider
        android:name="android.support.v4.content.FileProvider"
        android:authorities="${applicationId}.provider"
        android:exported="false" android:grantUriPermissions="true">
          <meta-data
             android:name="android.support.FILE_PROVIDER_PATHS"
             android:resource="@xml/file_paths" />
    </provider>

Add file_paths.xml in xml folder inside Resources folder in Xamarin.Android Project:

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
   <external-files-path name="my_images" path="Pictures" />
   <external-files-path name="my_movies" path="Movies" />
   <external-path name="external_files" path="."/>
</paths>