Optimized List: A pure AS3 List with renderer recycling

One of the things that the Flex List component does pretty well is handle large data collections (Arrays, ArrayCollections, etc). The List uses renderer recycling when displaying data, ie, it creates an itemRenderer only for the items that are displayed and not for each item in the dataProvider. As items scroll offscreen, the renderers associated with those data items are reused to render the new data that appeared on screen.

For the last few projects I have worked on Flex has not been an option because of file size concerns besides others which prompted me to start working on OpenPyro, a light weight Flex-esque framework and I have been pretty happy with it so far. However its List component is a far cry from Flex’s, implemented more like a Repeater, creating as many renderers as the length of the dataProvider, and as my new projects seem to get more data intensive, I started feeling a real need for a List that does the efficient renderer recycling that Flex does. Not finding any on the web, I decided to write one for myself.

The algorithm is as follows: The List creates an instance of a class called ObjectPool, which manages used and unused renderers. Whenever the List needs a new renderer, it asks the pool for a new one and when it does not need a renderer, it returns it to the pool. The pool tries to find unused renderers to satisfy the request for a new renderer and if it cannot, will create a new object and return that.

The core part of the logic resides in the onVerticalScroll function. The list always maintains a map of all visible renderers and as the scrollbar scrolls, the map is recalculated and renderers managed if the map is changed. The source and embedded example are the first stab at the implementation. The example renders an array of 200 values but ends up creating only the 8 or 9 renderers it needs to display.

The code is not complete but I am blogging it here so that if someone feels I am on a wrong track or should investigate a different approach, you can let me know.


It should be noted that this technique only works right now for fixed rowHeight lists. I haven’t thought of the variable height renderers yet, so suggestions on that front are welcome.