All Gmail users should have already noticed that file upload progress bar has been updated recently.
I'm wondering such effect is possible to implement with GWT. I'm fairly new with GWT, so if any GWT source code that can help me test out the function would be very helpful.
Update
I ended up going with SWFUpload. However, other suggestions under this question are all valid. Just try different options and choose the one you like!
Check out GWTC Upload, which has an implementation of exactly what you're looking for.
It's trivial to write your own if you have a java back end, you just start a file upload and then poll the server on a timer to see where it's up to (say every second or two). The java file upload binaries (the apache commons ones) support telling you the current progress so it's trivial to do.
Use SWFUpload via swfupload-gwt
The main advantage over the other methods is this does not require any special server code. You could even upload to another domain (if there is a crossdomain.xml which allows it).
You can use GwtSwfExt which is wrapper on top of SWFUpload (Its same as Swfupload-gwt lib ) you can download example and source code from http://code.google.com/p/gwtswfext.
When creating your own file upload progress, instead of pulling it form server at a small set time, you can have the client to display a indeterminate bar for 2 seconds and have the server calculate the estimated finish time the change back to determinate and pull new estimates every 5, 10 seconds instead. that should have little to no effect on the traffic.
Take a look to this library: http://code.google.com/p/gwtupload/. It is really easy to to use and works fine in all browsers and OS I've checked. It uses ajax requests to calculate progress. BTW Swfupload doesn't do well in linux and Mac.