Clientside PreventUpdate and no_update

[jjaraalm]

It would be helpful for clientside callbacks to be able to conditionally update components. This is not supported right now (as far as I can tell). This should be pretty simple, my idea would be to define

window.dash_clientside.no_update = {};

then in updateClientsideOutput just conditionally block updating by adding

if(window.dash_clientside.no_update && outputValue === window.dash_clientside.no_update)
    return;

Similarly we could define window.dash_clientside.PreventUpdate and allow the use of

throw window.dash_clientside.PreventUpdate;

If this seems reasonable I could put together a PR.

[alexcjohnson]

Good idea! Yes, I think everything you're proposing makes sense, feel free to make a PR. One thing I'd add then is that no_update and PreventUpdate need to be forbidden as namespaces in dash_clientside.

[jjaraalm]

Yes, this is what I wanted to get feedback on first. I'm not sure that dash_clientside is actually the best place to put these as we expect it to be modified by users. The only way I see to do this reliably is to insert no_update and PreventUpdate as non-writable attributes after all user callbacks are loaded (practically, just insert them before invoking any callbacks), but then what do we do if a user has already defined them? I guess just gracefully proceed as if no_update and PreventUpdate didn't exist, otherwise it could be a breaking change.

Is there a better namespace to put these in?

[alexcjohnson]

I think that’s the right place. Yes, you could argue that forbidding those two names is a breaking change, but it seems to me extremely unlikely anyone has used those as a namespace so far. I’d put a check on the Python side that errors if those namespaces are referenced, and add them to dash_clientside during callback invocation.

[jjaraalm]

Sounds like a plan. I'll put one together shortly.