Locking and unlocking a
QMutexin complex functions and statements or in exception handling code is error-prone and difficult to debug.
QMutexLockercan be used in such situations to ensure that the state of the mutex is always well-defined.
QMutexLockershould be created within a function where a
QMutexneeds to be locked. The mutex is locked when
QMutexLockeris created. You can unlock and relock the mutex with
relock(). If locked, the mutex will be unlocked when the
For example, this complex function locks a
QMutexupon entering the function and unlocks the mutex at all the exit points:def complexFunction(flag): mutex.lock() retVal = 0 if flag == 0 or flag == 1: mutex.unlock() return moreComplexFunction(flag) elif flag == 2: status = anotherFunction() if status < 0: mutex.unlock() return -2 retVal = status + flag else: if flag > 10: mutex.unlock() return -1 mutex.unlock() return retVal
This example function will get more complicated as it is developed, which increases the likelihood that errors will occur.
QMutexLockergreatly simplifies the code, and makes it more readable:def complexFunction(flag): locker = QMutexLocker(mutex) retVal = 0 if flag == 0 or flag == 1: return moreComplexFunction(flag) elif flag == 2: status = anotherFunction() if status < 0: return -2 retVal = status + flag else: if flag > 10: return -1 return retVal
Now, the mutex will always be unlocked when the
QMutexLockerobject is destroyed (when the function returns since
lockeris an auto variable).
The same principle applies to code that throws and catches exceptions. An exception that is not caught in the function that has locked the mutex has no way of unlocking the mutex before the exception is passed up the stack to the calling function.
QMutexLockeralso provides a
mutex()member function that returns the mutex on which the
QMutexLockeris operating. This is useful for code that needs access to the mutex, such as
wait(). For example:class SignalWaiter: def __init__(mutex): self.locker = mutex def waitForSignal(): # ... while not signalled: waitCondition.wait(self.locker.mutex()) # ...
- param m
__exit__(arg__1, arg__2, arg__3)¶
arg__1 – object
arg__2 – object
arg__3 – object
- Return type
Returns the mutex on which the
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