Custom locator with compute function?

Hi I’m writing a custom locator with Maya SDK. I’ve written nodes before but this is my first locator. I would like it to draw icons based on its velocity. I am having trouble visualizing the compute order for a locator. A typical node will check its dirty propagation and then update. What does a locator do? How does it handle incoming information and how does it pass information out?

more detail: I have accesss to the locators matrices very easily. How do I retain one of those matrices from one frame to another or from one refresh to another? And when does a locator redraw itself?