On 2012-12-12 20:11, Wanderer wrote:
I have a program that has a main GUI and a camera. In the main GUI, you can
manipulate the images taken by the camera. You can also use the menu to check
the camera's settings. Images are taken by the camera in a separate thread, so
the long exposures don't block the GUI. I block conflicts between the camera
snapshot thread and the main thread by setting a flag called self.cameraActive.
I check to see if the cameraActive flag is false and set the cameraActive to
True just before starting the thread. I generate an event on exiting the thread
which sets the cameraActive flag to False. I also check and set and reset the
flag in all the menu commands that access the camera. Like this.
def onProperties(self, event):
""" Display a message window with the camera properties
event -- The camera properties menu event
"""
# Update the temperature
if not self.cameraActive:
self.cameraActive = True
self.camera.getTemperature()
camDict = self.camera.getPropertyDict()
self.cameraActive = False
else:
camDict = {'Error': 'Camera Busy'}
dictMessage(camDict, 'Camera Properties')
This works but my question is, is there a better way using semaphores, locks or
something else to prevent collisions between threads?
That suffers from a race condition in that self.cameraActive might be
False when it's checked in the 'if' condition but set to True just
afterwards by the other thread.
You could try a non-blocking semaphore:
def __init__(self):
self.cameraActive = Semaphore()
def onProperties(self, event):
""" Display a message window with the camera properties
event -- The camera properties menu event
"""
# Update the temperature
if self.cameraActive.acquire(False): # Non-blocking
# Successfully acquired the semaphore, so the camera wasn't active
self.camera.getTemperature()
camDict = self.camera.getPropertyDict()
self.cameraActive.release()
else:
camDict = {'Error': 'Camera Busy'}
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