QMutexLocker

The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes. More…

Synopsis

Functions

• def __enter__ ()

• def __exit__ (arg__1, arg__2, arg__3)

• def mutex ()

• def relock ()

• def unlock ()

Detailed Description

Locking and unlocking a QMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug. QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined.

QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created. You can unlock and relock the mutex with unlock() and relock() . If locked, the mutex will be unlocked when the QMutexLocker is destroyed.

For example, this complex function locks a QMutex upon 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.

Using QMutexLocker greatly 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 QMutexLocker object is destroyed (when the function returns since locker is 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.

QMutexLocker also provides a mutex() member function that returns the mutex on which the QMutexLocker is 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())
        # ...

See also

QReadLocker QWriteLocker QMutex

class PySide2.QtCore.QMutexLocker(m)

PySide2.QtCore.QMutexLocker(m)

param m:

PySide2.QtCore.QBasicMutex

Constructs a QMutexLocker and locks mutex . The mutex will be unlocked ( unlock() called) when the QMutexLocker is destroyed. If mutex is None , QMutexLocker does nothing.

See also

lock()

PySide2.QtCore.QMutexLocker.__enter__()

PySide2.QtCore.QMutexLocker.__exit__(arg__1, arg__2, arg__3)

Parameters:

• arg__1 – object

• arg__2 – object

• arg__3 – object

PySide2.QtCore.QMutexLocker.mutex()

Return type:

PySide2.QtCore.QMutex

Returns the mutex on which the QMutexLocker is operating.

PySide2.QtCore.QMutexLocker.relock()

Relocks an unlocked mutex locker.

See also

unlock()

PySide2.QtCore.QMutexLocker.unlock()

Unlocks this mutex locker. You can use relock() to lock it again. It does not need to be locked when destroyed.

See also

relock()