Flex Data Binding Performance Pitfall

A friend of mine recently asked me to help him troubleshoot some performance problems with his Flex application. In his scenario he had a large list of data and wanted to filter the data such that each time the search string grew by a character the complex filter would only be run on the results of the previous filter. A very simple approach to this is just to keep a record in each item indicating if the item matched the filter for each search string. Even though the filter function will still run for each item, the complex part of the filter function could easily be isolated and only run for the subset of data which matched the previous filter. This may not be the best way to do this (I’m open to other suggestions) but it was simple. You can see the results of my first attempt here:

Demo 1 (Enter a few numbers in the search field and see how many items are being filtered and how long it takes.)

Source 1

Notice that in order to keep track of how many items are reaching the main part of the filter I increment a Bindable variable inside the filter function:

[Bindable] private var ffCount:Number = 0;
private function ff(item:Object):Boolean

Now since I’ve been programming in Flex for over four years you’d think I’d know better. Every time a Bindable object is modified Flex does a bunch of event dispatching and other plumbing. In this case the Label which is bound to the ffCount variable is also doing a bunch of work each time ffCount changes. As you can see, the results are not great. It takes almost a full second (on my machine) to do the filter on 20,000 items. Obviously this isn’t right.

Lesson learned: A Flex application should never, ever, ever, update Bindable objects in filter functions or other places where the user won’t ever be able to distinguish something changing that rapidly.

A better approach is to update a non-Bindable variable in the loop or filter function, then when the loop or filter is done, copy the non-Bindable variable to a Bindable one. You can see the updated demo using this approach here:

Demo 2

Source 2

This approach is about 10x faster! And only a little bit more work. In this case it’s very easy to be notified when the filter has completed and then update the Bindable variable:

if (event.kind == CollectionEventKind.REFRESH)
    ffCount = _ffCount;

In retrospect this seems like a silly mistake but I would not be surprised if this sort of thing is causing performance problems in many Flex applications. Data Binding is a very powerful part of Flex but it can also cause performance problems when used incorrectly. I hope this helps some of you to avoid this pitfall. Let me know what you think.