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## com.trolltech.qt.core
Class QWaitCondition

    java.lang.Object
      com.trolltech.qt.internal.QSignalEmitterInternal
          com.trolltech.qt.QSignalEmitter
              com.trolltech.qt.QtJambiObject
                  com.trolltech.qt.core.QWaitCondition

All Implemented Interfaces:
  QtJambiInterface

    public class QWaitCondition

extends QtJambiObject

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()` or `wakeAll()`. Use `wakeOne()` to wake one randomly selected condition or `wakeAll()` to wake them all.

For example, let's suppose that we have three tasks that should be performed whenever the user presses a key. Each task could be split into a thread, each of which would have a run() body like this:

            while (true) {
                mutex.lock();
                keyPressed.wait(mutex);
                do_something();
                mutex.unlock();
            }

Here, the `keyPressed` variable is a global variable of type QWaitCondition.

A fourth thread would read key presses and wake the other three threads up every time it receives one, like this:

            while (true) {
                getchar();
                keyPressed.wakeAll();
            }

The order in which the three threads are woken up is undefined. Also, if some of the threads are still in `do_something()` when the key is pressed, they won't be woken up (since they're not waiting on the condition variable) and so the task will not be performed for that key press. This issue can be solved using a counter and a `QMutex` to guard it. For example, here's the new code for the worker threads:

            while (true) {
                mutex.lock();
                keyPressed.wait(mutex);
                ++count;
                mutex.unlock();

                do_something();

                mutex.lock();
                --count;
                mutex.unlock();
            }

Here's the code for the fourth thread:

            while (true) {
                getchar();

                mutex.lock();
                // Sleep until there are no busy worker threads
                while (count > 0) {
                    mutex.unlock();
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {
                        // handle exception
                    }
                    mutex.lock();
                }
                keyPressed.wakeAll();
                mutex.unlock();
            }

The mutex is necessary because the results of two threads attempting to change the value of the same variable simultaneously are unpredictable.

Wait conditions are a powerful thread synchronization primitive. The Wait Conditions example shows how to use QWaitCondition as an alternative to `QSemaphore` for controlling access to a circular buffer shared by a producer thread and a consumer thread.

See also:
  `QMutex`, `QSemaphore`, QThread, and Wait Conditions Example.

Nested Class Summary

Nested classes/interfaces inherited from class com.trolltech.qt.QSignalEmitter
`QSignalEmitter.AbstractSignal, QSignalEmitter.Signal0, QSignalEmitter.Signal1, QSignalEmitter.Signal2, QSignalEmitter.Signal3, QSignalEmitter.Signal4, QSignalEmitter.Signal5, QSignalEmitter.Signal6, QSignalEmitter.Signal7, QSignalEmitter.Signal8, QSignalEmitter.Signal9`

Nested classes/interfaces inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
`com.trolltech.qt.internal.QSignalEmitterInternal.AbstractSignalInternal`

Field Summary

Fields inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
`currentSender`

Constructor Summary
`QWaitCondition()` Constructs a new wait condition object.

Method Summary
`boolean` | `wait(QMutex mutex)` Releases the locked `mutex` and waits on the wait condition.
`boolean` | `wait(QMutex mutex, int time)` Releases the locked `mutex` and waits on the wait condition.
`boolean` | `wait(QReadWriteLock readWriteLock)` Releases the locked `readWriteLock` and waits on the wait condition.
`boolean` | `wait(QReadWriteLock readWriteLock, int time)` Releases the locked `readWriteLock` and waits on the wait condition.
`void` | `wakeAll()` Wakes all threads waiting on the wait condition.
`void` | `wakeOne()` Wakes one thread waiting on the wait condition.

Methods inherited from class com.trolltech.qt.QtJambiObject
`dispose, disposed, equals, finalize, reassignNativeResources, tr, tr, tr`

Methods inherited from class com.trolltech.qt.QSignalEmitter
`blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread`

Methods inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
`__qt_signalInitialization`

Methods inherited from class java.lang.Object
`clone, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Methods inherited from interface com.trolltech.qt.QtJambiInterface
`disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership`

Constructor Detail

### QWaitCondition

    public QWaitCondition()

  Constructs a new wait condition object.

Method Detail

### wait

    public final boolean wait(QMutex mutex)

  Releases the locked `mutex` and waits on the wait condition. The `mutex` must be initially locked by the calling thread. If `mutex` is not in a locked state, this function returns immediately. If `mutex` is a recursive mutex, this function returns immediately. The `mutex` will be unlocked, and the calling thread will block until either of these conditions is met:
  - Another thread signals it using `wakeOne()` or `wakeAll()`. This function will return true in this case.
  - `time` milliseconds has elapsed. If `time` is `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 mutex 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.

See also:
  `wakeOne()`, and `wakeAll()`.

### wait

    public final boolean wait(QMutex mutex,
                              int time)

  Releases the locked `mutex` and waits on the wait condition. The `mutex` must be initially locked by the calling thread. If `mutex` is not in a locked state, this function returns immediately. If `mutex` is a recursive mutex, this function returns immediately. The `mutex` will be unlocked, and the calling thread will block until either of these conditions is met:
  - Another thread signals it using `wakeOne()` or `wakeAll()`. This function will return true in this case.
  - `time` milliseconds has elapsed. If `time` is `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 mutex 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.

See also:
  `wakeOne()`, and `wakeAll()`.

### wait

    public final boolean wait(QReadWriteLock readWriteLock)

  Releases the locked `readWriteLock` and waits on the wait condition. The `readWriteLock` must be initially locked by the calling thread. If `readWriteLock` is not in a locked state, this function returns immediately. The `readWriteLock` must not be locked recursively, otherwise this function will not release the lock properly. The `readWriteLock` will be unlocked, and the calling thread will block until either of these conditions is met:
  - Another thread signals it using `wakeOne()` or `wakeAll()`. This function will return true in this case.
  - `time` milliseconds has elapsed. If `time` is `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 `readWriteLock` 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.

See also:
  `wakeOne()`, and `wakeAll()`.

### wait

    public final boolean wait(QReadWriteLock readWriteLock,
                              int time)

  Releases the locked `readWriteLock` and waits on the wait condition. The `readWriteLock` must be initially locked by the calling thread. If `readWriteLock` is not in a locked state, this function returns immediately. The `readWriteLock` must not be locked recursively, otherwise this function will not release the lock properly. The `readWriteLock` will be unlocked, and the calling thread will block until either of these conditions is met:
  - Another thread signals it using `wakeOne()` or `wakeAll()`. This function will return true in this case.
  - `time` milliseconds has elapsed. If `time` is `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 `readWriteLock` 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.

See also:
  `wakeOne()`, and `wakeAll()`.

### wakeAll

    public final void wakeAll()

  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.

See also:
  `wakeOne()`.

### wakeOne

    public final void 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.

See also:
  `wakeAll()`.

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