GeckoFX file download

Question

I have some issues with GeckoFX file download.

I tried the LauncherDialog method described in a few locations such as How to handle downloading in GeckoFX 29, and a WebRequest / Webclient method.

Both method work, but they do two requests to the server. The first one will trigger the LauncherDialog.Download event, and the LauncherDialog will make a new request to get the actual file.

I use GeckoFX in a custom web client for a customer, and in this particular case, this download request requires several seconds of processing on server side and modifies the data state. The second request is delayed and the returned data isn't the same as for the first request.

Also, this particular application doesn't need a download progress window of any sorts.

Is there any way to get the data stream from the initial request? Modifying GeckoFx-Winforms is not a problem. I would prefer avoiding any modification to GeckoFX-Core, but I'll do it if required.

Answer 1

Well, I revisited my wrong assumptions about XPCOM programming, took a look at selected locations in Firefox/Gecko source code and found one solution. Which could be very obvious for someone with some XPCOM/XUL programming experience, but was not initially for me. So I guess sharing my solution could help a few people.

In my case, the LauncherDialog method is definitely not the way to go.

Instead, I implemented the nsIFactory, nsIExternalHelperAppService and nsIStreamListener interfaces.

nsiStreamListener

internal class MyStreamListener : nsIStreamListener
{
    public MyStreamListener(/*...*/) { }
    public void OnStartRequest(nsIRequest aRequest, nsISupports aContext)
    {
        // This will get called once, when the download "begins".
        // You can initialize your things here.
    }

    public void OnStopRequest(nsIRequest aRequest, nsISupports aContext, int aStatusCode)
    {
        // This will also get called once, when the download is 
        // complete or interrupted. You can perform the post-download 
        // actions here.

        if (aStatusCode != GeckoError.NS_OK) {
            // download interrupted
        }
        else {
            // download completed
        }
    }

    public void OnDataAvailable(nsIRequest aRequest, nsISupports aContext, nsIInputStream aInputStream, ulong aOffset, uint aCount)
    {
        // This gets called several times with small chunks of data. 
        // Do what you need with the stream. In my case, I read it 
        // in a small buffer, which then gets written to an output 
        // filestream (not shown).
        // The aOffset parameter is the sum of all previously received data.

        var lInput = InputStream.Create(aInputStream);
        byte[] lBuffer = new byte[aCount];
        lInput.Read(lBuffer, 0, (int)aCount);

    }
}

nsIExternalHelperAppService

public class MyExternalHelperAppService : nsIExternalHelperAppService
{
     public MyExternalHelperAppService(/* ... */)
     {
        /* ... */
     }

     public nsIStreamListener DoContent(nsACStringBase aMimeContentType, nsIRequest aRequest, nsIInterfaceRequestor aWindowContext, bool aForceSave)
     {
         var request = Request.CreateRequest(aRequest);
         var lChannel = request as HttpChannel;
         try {
             if (lChannel != null) {
                 var uri = lChannel.OriginalUri;
                 var contentType = lChannel.ContentType;
                 var contentLength = lChannel.ContentLength;
                 var dispositionFilename = lChannel.ContentDispositionFilename;

                 // Do your contenttype validation, keeping only what you need.
                 // Make sure you clean dispositionFilename before using it.

                 // If you don't want to do anything with that file, you can return null;

                 return new MyStreamListener(/* ... */);
            }
        }
        catch (COMException) {
            /* ... */
        }
        return null;
    }
}

nsIFactory (you can also overload GenericOneClassNsFactory<TFactory,TType>):

public IntPtr CreateInstance(nsISupports aOuter, ref Guid iid)
{
    // This is called when the content dispatcher gets a DISPOSITION_ATTACHMENT 
    // on the channel, or when it doesn't have any builtin handler 
    // for the content type. It needs an external helper to handle 
    // the content, so it creates one and calls DoContent on it.

    MyExternalHelperAppService _myExternalHelperAppService = new MyExternalHelperAppService(...);
    IntPtr result;
    IntPtr iUnknownForObject = Marshal.GetIUnknownForObject(_myExternalHelperAppService);
    Marshal.QueryInterface(iUnknownForObject, ref iid, out result);
    Marshal.Release(iUnknownForObject);
    return result;
}

public void LockFactory(bool @lock) {
    // do nothing here, it's not used, only kept for backwards compatibility.
}

Then, somewhere in my initialization code, I registered my nsIFactory with the proper contract:

Xpcom.RegisterFactory(typeof(MyExternalHelperAppService).GUID,
    "MyExternalHelperAppService", 
    "@mozilla.org/uriloader/external-helper-app-service;1", 
    new MyNsFactory());

And that's all.