I can't add a comment, so I will add an answer saying this code should be triggered by a call to the updateDisplayList method (during the appropriate phase of the flex component life-cycle). I would suggest reading and understanding this update cycle before doing any further Flex development (it will save you a lot of time in the long-run).
More info can be found here: http://livedocs.adobe.com/flex/3/html/help.html?content=ascomponents_advanced_2.html
UPDATE
Since you mentioned Canvas, I updated the link to point to the 3.x version of the article