Question

Okay, so I have this small procedural SVG editor in Clojure.

It has a code pane where the user creates code that generates a SVG document, and a preview pane. The preview pane is updated whenever the code changes.

Right now, on a text change event, the code gets recompiled on the UI thread (Ewwwww!) and the preview pane updated. The compilation step should instead happen asynchronously, and agents seem a good answer to that problem: ask an agent to recompile the code on an update, and pass the result to the image pane.

I have not yet used agents, and I do not know whether they work with an implicit queue, but I suspect so. In my case, I have zero interest in computing “intermediate” steps (think about fast keystrokes: if a keystroke happens before a recompilation has been started, I simply want to discard the recompilation) — ie I want a send to overwrite any pending agent computation.

How do I make that happen? Any hints? Or even a code sample? Is my rambling even making sense?

Thanks!

Answer 1

You describe a problem that has more to deal with execution flow control rather than shared state management. Hence, you might want to leave STM apart for a moment and look into futures: they’re still executed in a thread pool as agents, but instead of agents they can be stopped by calling future-cancel, and inspecting their status with future-cancelled?.

There are no strong guarantees that the thread the future is executing can be effectively stopped. Still, your code will be able to try to cancel the future, and move on to schedule the next recompilation.