Returns whether there are any handlers connected to instance for the
given signal id and detail.
One example of when you might use this is when the arguments to the
signal are difficult to compute. A class implementor may opt to not
emit the signal if no one is attached anyway, thus saving the cost
of building the arguments.
Returns whether there are any handlers connected to instance for the given signal id and detail. One example of when you might use this is when the arguments to the signal are difficult to compute. A class implementor may opt to not emit the signal if no one is attached anyway, thus saving the cost of building the arguments.