It’s all doable.
What do you want the custom button to be able to do?
How do you get the javascript Quill object and use the import?
import anvil_extras.Quill # this will add Quill to window
from anvil.js.window import Quill
icons = Quill["import"]("ui/icons")
# js proxy objects can use subscript notation
# which gets around using .import
# (we could also have used getattr(Quill, "import")
icons['foo'] = '<i class="fa fa-eye"></i>'
# taken from a quill github issue https://github.com/quilljs/quill/issues/1099
How to use a custom button in Quill?
It’s probably not the right approach since it’s not documented in Quill.
But, that placeholder element for the button foo adds a class ql-foo to the button
We can access that button using some anvil.js
quill_dom = anvil.js.get_dom_node(self.quill_1)
foo_btn = quill_dom.querySelector('.ql-foo')
foo_btn.addEventListener('click', self.foo)
We can’t use the getModule('toolbar') approach since quill doesn’t know anything about a foo button control. Whereas it knows about a bold button control.
Note:
The suggested approach to custom buttons in the quill docs is to create your own toolbar container
This would work too but it’s (even) more of a faff.