[Fixed] ListBase TypeError #1010

Today I spent hours tracking down a bug related to inserting and/or deleting data from a Tree control. In certain circumstances, an update to the underlying dataProvider would cause the following to be reported:

After a bit of Googling, I found that I am not alone. Other folks have the same issue with Tree. Even more common, it looks like people are getting the same error when manipulating data in an AdvancedDataGrid, in which case it is reported as:

After drilling down into the Flex Framework source and exploring the makeRowsAndColumnsWithExtraRows function, I added some watch expressions in the Flash Builder Debug pane. (The debug features are really, really useful for this kind of thing.) The culprit? The value for verticalScrollPosition was being reported as -1, causing a line of code to look for an array element at index -1. This array element was in turn undefined, so the error message suddenly makes perfect sense.

The real puzzler here is that verticalScrollPosition could ever be less than zero. This is the logical equivalent of scrolling to the top of the list, then scrolling up one more spot, which is of course impossible. In any case, I have decided to assume that -1 is invalid at all times, and that -1 really means 0. I hope that is a safe assumption!

So, the solution is simple. In my case I had already subclassed the Tree control. If you're using the stock control from the framework, you'll need to subclass it and simply add the following:

All it does is intercept any request for verticalScrollPosition and guarantee that it is greater than or equal to zero. In brief testing, the error has disappeared and I have seen no side effects.

Like anything else on this blog, use this fix at your own risk, since I can't guarantee it won't have any negative impact.

Happy coding!