Thursday, October 16, 2008

PenWag and GWT update

So, I mentioned that I'm using GWT to develop my new web site PenWag. I ran into an interesting problem with GWT, so let's talk about that in case you run into the same problem. When you do GWT development, most of the time spent viewing your app is in "hosted mode," that is, using their custom browser. Your app runs there as a java program. Once you've made some progress, you might deploy it to say, Tomcat, and hit it with a full-fledged browser such as Firefox.

So, I'm tooling along throwing this app together, viewing it in hosted mode, and all looks good. It's looking the way I want it, so I deploy it to Tomcat and view it in Firefox. Hmm, no love. I can see most of the content, but there's a blank space where there's supposed to be a grid. I'll cut to the chase here - after a fair amount of digging, I found the problem in this method in a class used to populate the grid:

public List<T> getList() {
return hasNextPage() ? results.subList(0, pageSize) : results;

To my eye, it looks pretty innocuous. But no. Here's where the problem is. Apparently, not sure why, "results.sublist(...)" works fine in hosted mode as Java, but fails when it's compiled to Javascript and viewed with Firefox. Ah, well. The fix was to move this logic to the server side, and just store the desired value that we will return with getList(). So now, the method looks like:

public List<T> getList() {
return results;

and all is well.

This was something of a difficult problem to solve. There was no error message to point to the problem, so I used a divide-and-conquer approach to solving the problem. Took out all the code, and the other surrounding components began to show up. Started to introduce pieces until it broke, then narrowed the problem down to sublist. There ya go, hope this helps.

No comments: