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Progress Implementation Note

Chartlets progress components are normal components, like all the others. CircularProgress and LinearProgress render when hidden is false and do not render when hidden is true.

The extra progress logic (chartlets.js/packages/lib/src/actions/helpers/pendingProgress.ts) exists only to cover a timing problem: a backend callback can return component state changes only after it has finished. Without some frontend help, a Python callback computes for several seconds cannot show a spinner during that same callback.

How Pending Progress Works

A panel configures a progress component in its layout:

CircularProgress(id="loading_progress", hidden=True)

If a callback's output includes the progress component's hidden property, chartlets treats it as a pending-progress target:

Output("loading_progress", "hidden")

When the callback is invoked, the frontend does the following before sending the backend request:

  1. Find callback outputs that target hidden on a progress component.
  2. Set those targets to hidden: false locally.
  3. Send the backend callback request.

When the backend response arrives, normal callback outputs are applied. The Python callback should return the final progress state, usually True for hidden so the spinner disappears.

return True, result

Why The Output Is Required

The output declaration is the only signal that connects the callback to the progress component. If a panel removes this output:

# Output("loading_progress", "hidden")

then the frontend does not know that this callback should show that spinner. The spinner remains in its layout state.

Failure Handling

If the backend callback fails, no backend output will arrive to hide the spinner. In that case, the frontend hides completed pending-progress targets by setting hidden: true.

Overlapping Callbacks

Multiple callbacks can target the same progress component. chartlets keeps a small pending count per progress target so one finished callback does not hide a spinner that is still needed by another pending callback.

Alternative

The alternative would be to let every panel manage the state of long-running callback requests. This would keep the logic out of the frontend, but it would make each panel more complicated to set-up and maintain for the contributors.