The QWaitCondition class provides a condition variable for synchronizing threads. 更多...
| 头： | #include <QWaitCondition> |
| qmake: | QT += core |
注意： 此类的所有函数 thread-safe .
| QWaitCondition () | |
| ~QWaitCondition () | |
| void | notify_all () |
| void | notify_one () |
| bool | wait (QMutex * lockedMutex , unsigned long time = ULONG_MAX) |
| bool | wait (QMutex * lockedMutex , QDeadlineTimer deadline ) |
| bool | wait (QReadWriteLock * lockedReadWriteLock , unsigned long time = ULONG_MAX) |
| bool | wait (QReadWriteLock * lockedReadWriteLock , QDeadlineTimer deadline ) |
| void | wakeAll () |
| void | wakeOne () |
The QWaitCondition class provides a condition variable for synchronizing threads.
QWaitCondition allows a thread to tell other threads that some sort of condition has been met. One or many threads can block waiting for a QWaitCondition to set a condition with wakeOne () 或 wakeAll ()。使用 wakeOne () 去唤醒某一随机选中线程或 wakeAll () 去唤醒它们所有。
例如，假设有 3 个任务应该履行每当用户按下键时。每个任务可以被分割成线程，每个线程都有 run() 本体像这样：
forever {
mutex.lock();
keyPressed.wait(&mutex);
do_something();
mutex.unlock();
}
在这里，
keyPressed
variable is a global variable of type
QWaitCondition
.
第 4 个线程将读取键按下，并在每次收到一个时唤醒其它 3 个线程，像这样：
forever {
getchar();
keyPressed.wakeAll();
}
3 个线程被唤醒的次序不确定。另外，若某些线程仍在
do_something()
当键被按下时，它们不会被唤醒 (因为它们没有等待条件变量)，所以任务不会因键按下而被履行。此问题可以被解决使用计数器和
QMutex
去守卫它。例如，这里是工作者线程的新代码：
forever {
mutex.lock();
keyPressed.wait(&mutex);
++count;
mutex.unlock();
do_something();
mutex.lock();
--count;
mutex.unlock();
}
这里是第 4 个线程的代码：
forever {
getchar();
mutex.lock();
// Sleep until there are no busy worker threads
while (count > 0) {
mutex.unlock();
sleep(1);
mutex.lock();
}
keyPressed.wakeAll();
mutex.unlock();
}
互斥是必要的，因为试图同时改变同一变量值的 2 个线程的结果是不可预测的。
等待条件是强大的线程同步原语。 等待条件范例 范例展示如何使用 QWaitCondition as an alternative to QSemaphore 为控制由生产者线程和消费者线程共享的循环缓冲的访问。
另请参阅 QMutex , QSemaphore , QThread ，和 等待条件范例 .
构造新等待条件对象。
销毁等待条件对象。
此函数为兼容 STL (标准模板库) 提供。它相当于 wakeAll ().
该函数在 Qt 5.8 引入。
此函数为兼容 STL (标准模板库) 提供。它相当于 wakeOne ().
该函数在 Qt 5.8 引入。
释放 lockedMutex 并等待等待条件。 lockedMutex must be initially locked by the calling thread. If lockedMutex is not in a locked state, the behavior is undefined. If lockedMutex is a recursive mutex, this function returns immediately. The lockedMutex will be unlocked, and the calling thread will block until either of these conditions is met:
ULONG_MAX
(the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out.
The lockedMutex will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state.
释放 lockedReadWriteLock 并等待等待条件。 lockedReadWriteLock must be initially locked by the calling thread. If lockedReadWriteLock is not in a locked state, this function returns immediately. The lockedReadWriteLock must not be locked recursively, otherwise this function will not release the lock properly. The lockedReadWriteLock will be unlocked, and the calling thread will block until either of these conditions is met:
ULONG_MAX
(the default), then the wait will never timeout (the event must be signalled). This function will return false if the wait timed out.
The lockedReadWriteLock will be returned to the same locked state. This function is provided to allow the atomic transition from the locked state to the wait state.
该函数在 Qt 4.4 引入。
Wakes all threads waiting on the wait condition. The order in which the threads are woken up depends on the operating system's scheduling policies and cannot be controlled or predicted.
另请参阅 wakeOne ().
Wakes one thread waiting on the wait condition. The thread that is woken up depends on the operating system's scheduling policies, and cannot be controlled or predicted.
If you want to wake up a specific thread, the solution is typically to use different wait conditions and have different threads wait on different conditions.
另请参阅 wakeAll ().