I have a project where I am communicating with a USB camera.
I have a slider that controls stuff related to the camera which needs careful checking of the status before sending the control actions to it.
I got this to work reasonably well, except in one situation:
When the window is out of focus (e.g. when MacOSX Finder is active and the app window is grey in the background) and I click on the slider to activate the window, the app crashes, the USB is stalling.
If I click somewhere else on the window to get it in focus (or is it called "activated"?) and THEN touch the slider, it works.

How can I catch his focus/activate event?
I probably need to disable slider action in this case?