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Worker caveats #158
Description
I just realized, out of a one-off example in Discord, that if a worker has listeners added after some reference is created or populated with values, the synchronous dance might mislead because that reference on the main might have been already bootstrapped or triggered/resolved.
The explicit issue I am talking about is this one:
image = window.Image.new()
# if this is before, we're all good
image.onload = lambda e: document.body.append(image)
# if that is after though, setting an inline source of the image
# will resolve it before the next `.onload = ...` or even
# `.addEventListener` is attached
image.src = f'data:image/svg+xml;utf8,{svg}'The demo in case is this one https://pyscript.com/@agiammarchi/svg-image/latest?files=main.py,index.html
I had to double-check why, once added the worker attribute, that would not work.
Due inability to hook into listeners happening on the main, I think it's worth it to document the best way to be sure a listener is triggered, either on main or worker thread, is to define listeners related to such bootstrap AOT (Ahead of Time) and not later.
This is an important detail I could see myself failing at remembering on occasions, so I think it should be part, at least, of our F.A.Q.s section.
Activity
Neon22 commentedon Mar 26, 2025
Interestingly was trying to use this on svgs and all styling was ignored. I.e. image is always in B&W.
Had to change from utf-8 to base64 like so: (using the ltk here)
(the
dwgis an svg text description)The ltk has the nice side effect of not having to do the
on.loadand then trigger that by setting theimage.src. It all happens in one step. Maybe that's the jQuery interior... I don't know for sure.examples in main.py here: - https://pyscript.com/@neon22/satin-generator/latest?files=main.py
WebReflection commentedon Mar 26, 2025
to clarify, ltk on main uses MicroPython (AFAIK) so on main this issue does not exist, it exists on workers if you set the listener after, which is never the case for libraries so this is something to document as it cannot be fixed because we don't get to decide when listeners on the main are triggered and if we add those listeners after there's, by specs, no listener triggered.