[saket0187]

Fixes #7652

• Fixes RuntimeError: Cannot add watch ... - it is already scheduled on macOS by replacing per-instance Observer objects with a single class-level _shared_observer across MapsetWatchdog instances.
• Adds weakref.WeakSet instance tracking (_instances) so ScheduleWatchCurrentMapset() can schedule event handlers for all active GUI components.

I have used Claude for this.

[petrasovaa]

Have you figured out why this is even necessary? It would be good to understand the source of the problem. Could you please confirm you tested it?

[saket0187]

Could you please confirm you tested it?

Yes, I tested it before creating the PR, and it does fixes the issue.

Have you figured out why this is even necessary? It would be good to understand the source of the problem. Could you please confirm you tested it?

From what I was able to understand, Originally MapsetWatchdog only monitored the Layer Manager / Main Window (GMFrame), which is why the docstring mentions "Schedules 1 watches (raster)" I think.

self._mapset_watchdog = MapsetWatchdog(
            elements_dirs=(("raster", "cell"),),

When the Data Catalog (DataCatalogTree) was added alongside GMFrame, it initialized its own MapsetWatchdog. When interacting with layers/mapsets (_loadItemsDone() / _updateAfterMapsetChanged()), It calls ScheduleWatchCurrentMapset(), creating a separate watchdog.Observer() monitoring the exact same directories that GMFrame's observer is already watching.

On Windows/Linux, It allows two separate observers in the same process to watch the same directory independently without error.
On macOS (FSEvents), watchdog uses a process-global C library that forbids duplicate watches. When the second observer tries to watch a directory that the first observer is already monitoring, it throws RuntimeError: ... already scheduled and stops receiving file updates.