How does the atomic module work

[s-cork]

from the forum @hugetim:

https://anvil.works/forum/t/state-management-with-atomic-module/10689/4

Puzzled by the way atoms.todo.update_db is defined, I was trying to understand what the writeback function call in Todo.ItemTemplate does. (It seemed to me that self.item in that writeback call was neither an atom nor a selector, but then I realized it is a DictAtom.)

So I guess my question is: when that DictAtom's __setitem__ is called, could/should the update requested propagate upward to any parent atoms, such as the ListAtom that contains it, in this case? (If that doesn’t make sense, I can try to explain more fully.)

edit: Answering my own question: No, probably not. But maybe it could work the other way. When update_db is registered as depending on the todos attribute, maybe it would make sense for it to be recursively registered as depending on any “child” atoms of todos: i.e., the “todo” DictAtom's here?

[s-cork]

Yes - so when you have lists or dicts as attributes of an atom, they get converted to ListAtoms and DictAtoms. These are subclasses of list and dict and just means that they know how to request re-renders when something about them change.

Re whether __setitem__ should propogate. I went with no. Because - typically a list of dicts is used as the items for a repeating panel. If it propagated - any time the DictAtom changed the RepeatingPanel would re-render - and that's not usually what you want.

But - if the list is extended, or an item is removed - then you would want the RepeatingPanel to re-render.
(At some point we could look to make this cleverer and cache some of the RepeatingPanel components)

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The atoms.todo.update_db function:

is_first_run = True

@render
def update_db():
    # running this creates a dependency on todo_atom.todos and each todo
    todos = [dict(todo) for todo in todos_atom.todos]
    if not is_first_run:
        indexed_db["todos"] = todos

update_db()
is_first_run = False

I wanted to depend on both the list atom and each key of each DictAtom within the list.
By calling [dict(todo) for todo in todos_atom.todos] I access the list of todos and each key of each DictAtom.
This implicitly subscribes this render method to any change i'm interested in.
So if you tick completed on one of the todos - Then this method will be called - which then updates the indexed_db.