# Trying out the Chickadee custom vat **URL:** \ **Category:** Spritely Goblins\ **Tags:** debugging, ui\ **Created:** [April 2, 2023, 4:54pm UTC](https://community.spritely.institute/t/trying-out-the-chickadee-custom-vat/231 "2023-04-02T16:54:49Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![jfred](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.spritely.institute/jfred/32/18_2.png) [@jfred](https://community.spritely.institute/u/jfred)\ **Post date:** [April 2, 2023, 4:54pm UTC](https://community.spritely.institute/t/trying-out-the-chickadee-custom-vat/231/1 "2023-04-02T16:54:49Z")
Hi! I was attempting to play around with the Chickadee example vat in the community-garden repo. I’m having a bit of trouble, though, and I’m probably just misunderstanding something 😅 Here’s my attempt at the moment: ```scheme (define-module (community-garden test-chickadee-vat) #:use-module (community-garden chickadee-vat) #:use-module (goblins) #:use-module (goblins vat) #:use-module (oop goops) #:use-module (chickadee) #:use-module (chickadee scripting) #:use-module (chickadee graphics text) #:use-module (chickadee math vector)) (define (^artist bcom) (lambda () ;;(display "Drawing...") (draw-text "Hello world!" (vec2 260.0 240.0)))) (define-class () (vat #:accessor vat) (started #:accessor started? #:init-value #f) (artist #:accessor artist)) (define-method (start (self )) (define (draw alpha) (if (started? self) (script (with-vat (vat self) ($ (artist self)))))) (define (update dt) (update-agenda 1)) (define (load) (script (slot-set! self 'vat (make-chickadee-vat)) (vat-start! (vat self)) (slot-set! self 'artist (with-vat (vat self) (spawn ^artist))) (slot-set! self 'started #t))) (run-game #:draw draw #:update update #:load load)) (define tester (make )) (start tester) ``` I’m trying to have actors that are able to draw on the screen when requested, as I described in [a previous forum post](https://community.spritely.institute/t/asynchronous-visual-applications-and-the-unum-pattern/200). The game window opens, and while I’m able to see the “Drawing…” message that my `artist` actor prints if I uncomment that, the text is never drawn. I wasn’t sure if I needed to make the method call to my `artist` object in a `script` (the “Hello world” program on the Chickadee site does not) but if I don’t I get an error as follows: ```auto Backtrace: In ice-9/boot-9.scm: 1752:10 12 (with-exception-handler _ _ #:unwind? _ # _) In unknown file: 11 (apply-smob/0 #) In ice-9/boot-9.scm: 724:2 10 (call-with-prompt _ _ #) In ice-9/eval.scm: 619:8 9 (_ #(#(#))) In ice-9/boot-9.scm: 2836:4 8 (save-module-excursion _) 4388:12 7 (_) In chickadee.scm: 414:11 6 (run-game #:window-title _ #:window-width _ # _ # _ # _ …) In chickadee/game-loop.scm: 96:26 5 (run-game* #:init _ #:update _ #:render _ #:time _ # _ # …) 43:8 4 (call-with-error-handling _ _) 97:28 3 (_) In chickadee.scm: 388:10 2 (render-sdl-opengl 0.002672420290764435) In goblins/vat.scm: 213:2 1 (call-with-vat # _) In unknown file: 0 (_ (script) #) ERROR: In procedure abort: Abort to unknown prompt ``` But I’m wondering if the use of `script` is what’s resulting in nothing being displayed here, because the return value of a `script` call is a handle to the script. So if drawing relies on the return value of `draw-text`, that could be the culprit. ---
**Author:** ![dthompson](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.spritely.institute/dthompson/32/95_2.png) [@dthompson](https://community.spritely.institute/u/dthompson)\ **Post date:** [April 3, 2023, 1:30pm UTC](https://community.spritely.institute/t/trying-out-the-chickadee-custom-vat/231/2 "2023-04-03T13:30:45Z")
Hi @jfred! I’ll start with a brief disclaimer that the community garden demo is built on top of a git snapshot of chickadee _and_ an as-of-yet-unreleased game engine built on top of it so it’s a bit of a moving target. That said, I’m happy to help you understand what’s going on. The main issue with your code is that you are attempting to render using an actor. When you call `(with-vat (vat self) ($ (artist self)))` in your `draw` procedure, it is actually scheduling work to be performed later during the game loop’s update step by pushing a message onto the vat’s queue. This means that the `draw-text` call doesn’t happen within the context of the `draw` procedure, and the result is that you don’t see any text because the screen will have been cleared before the next frame is rendered. The fix is to move all rendering code outside of actor behaviors. Your `^artist` could tell you what text it currently represents, but leave it up to the UI layer to choose how to render it. The simplest way to do this is to periodically poll the actor for its current state and update some rendering state accordingly. That is actually what the Community Garden demo does because I didn’t have enough time to implement push-based updates, which would be more efficient. The Community Garden demo works by using actors to model the behavior and essential state of the application, but notably the actors know _nothing_ about rendering. This decouples the actors from any particular UI. This decoupling allowed me to start with simple terminal rendering and move on to a graphical view later on. It also avoids the issues you are running into where graphical rendering needs to be run in the correct context for it to be visible. You might be wondering “well, what’s the point of the Chickadee vat, then?” The Chickadee vat integrates with Chickadee’s single-threaded game loop by using it’s agenda scheduling system for async tasks. This allows us to modify game state from the proper thread and avoid multiple thread writing issues. For your specific example code, I would add more slots to `` to hold the artist’s current rendering state (the string and position vector, perhaps) and use that information in `draw`. Remove the `script` form in `draw`, too, as it’s unnecessary and using any scripting features would just defer the rendering into the update step, which as mentioned above won’t be visible. I hope this helps! Let me know if you have additional questions! ---
**Author:** ![jfred](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.spritely.institute/jfred/32/18_2.png) [@jfred](https://community.spritely.institute/u/jfred)\ **Post date:** [April 11, 2023, 2:29am UTC](https://community.spritely.institute/t/trying-out-the-chickadee-custom-vat/231/3 "2023-04-11T02:29:53Z")
Ah! Right, that makes sense. I was missing the detail that `script` forms result in things running during the update step. (Is calling `update-agenda` the thing that actually runs those?) My example code was a lot simpler than what I’m actually trying to do (I want the actors to have a decent amount of control over what gets displayed) but I’ll have to reconsider my design a bit given this. Thank you for the explanation! That cleared things up a lot. ---
**Author:** ![dthompson](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.spritely.institute/dthompson/32/95_2.png) [@dthompson](https://community.spritely.institute/u/dthompson)\ **Post date:** [April 11, 2023, 2:25pm UTC](https://community.spritely.institute/t/trying-out-the-chickadee-custom-vat/231/4 "2023-04-11T14:25:40Z")
Yup, calling `update-agenda` is what moves the scheduler forward in time and runs scheduled tasks.