【QT5 多线程示例】条件变量

条件变量

QT的条件变量类是QWaitCondition，有wakeOne() 和 wakeAll() 两个方法

• wakeOne()：仅唤醒一个等待的线程。
• wakeAll()：唤醒所有等待的线程。

使用 wakeOne()

https://github.com/BinaryAI-1024/QtStudy/tree/master/thread/conditionVariable

有 3 个工作线程 在 QWaitCondition 上等待，wakeOne() 只唤醒其中一个线程。

// main.cpp
#include <QCoreApplication>
#include <QThread>
#include <QTimer>
#include "myworker.h"

int main(int argc, char *argv[]) {
    QCoreApplication app(argc, argv);

    const int numThreads = 3;
    QThread* threads[numThreads];
    MyWorker* workers[numThreads];

    for (int i = 0; i < numThreads; ++i) {
        threads[i] = new QThread();
        workers[i] = new MyWorker();

        workers[i]->moveToThread(threads[i]);

        // 线程启动时，工作对象执行 doWork()
        QObject::connect(threads[i], &QThread::started, workers[i], &MyWorker::doWork);

        // 任务完成后退出线程的事件循环
        QObject::connect(workers[i], &MyWorker::finished, threads[i], &QThread::quit);

        // 任务完成后删除工作对象
        QObject::connect(workers[i], &MyWorker::finished, workers[i], &QObject::deleteLater);

        // 线程退出后删除线程对象
        QObject::connect(threads[i], &QThread::finished, threads[i], &QObject::deleteLater);

        threads[i]->start();
    }

    // 2 秒后唤醒所有线程
    QTimer::singleShot(2000, [=]() {
        workers[0]->startWork();  // 让所有 worker 线程醒来
    });

    return app.exec();
}

// myworker.h
#ifndef MYWORKER_H
#define MYWORKER_H

#include <QObject>
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
#include <QDebug>

class MyWorker : public QObject {
    Q_OBJECT
public:
    explicit MyWorker(QObject *parent = nullptr);
    ~MyWorker();
signals:
    void finished();
public slots:
    void doWork();
    void startWork();

private:
    QMutex mutex;              // 每个实例都有一个互斥锁
    QWaitCondition condition;  // 每个实例都有一个条件变量
    bool ready = false;        // 每个实例都有一个标志变量
};

#endif // MYWORKER_H

// myworker.cpp
#include "myworker.h"

MyWorker::MyWorker(QObject *parent) : QObject(parent) {}

MyWorker::~MyWorker() {
    qDebug() << QThread::currentThread() << "Worker destroyed";
}

void MyWorker::doWork() {
    qDebug() << QThread::currentThread() << "Worker waiting...";

    {
        QMutexLocker locker(&mutex);  // 加锁
        while (!ready) {  // 等待被唤醒
            condition.wait(&mutex);
        }
    }


    qDebug() << QThread::currentThread() << "Worker processing...";
    QThread::msleep(2000);
    qDebug() << QThread::currentThread() << "Worker finished!";
    emit finished();
}


void MyWorker::startWork() {
    QMutexLocker locker(&mutex);
    ready = true;
    condition.wakeOne();  // 仅唤醒一个线程
}

运行结果

QThread(0x1481b00) Worker waiting...
QThread(0x1487b10) Worker waiting...
QThread(0x1487d58) Worker waiting...
QThread(0x1481b00) Worker processing...
QThread(0x1481b00) Worker finished!
QThread(0x1481b00) Worker destroyed

使用 wakeAll()

https://github.com/BinaryAI-1024/QtStudy/tree/master/thread/conditionVariableAll
有 3 个工作线程在 QWaitCondition 上等待，wakeAll() 同时唤醒所有线程。这需要将QMutex、QWaitCondition 和 ready 声明为静态变量，在多个 MyWorker实例间共享。

// main.cpp
#include <QCoreApplication>
#include <QThread>
#include <QTimer>
#include "myworker.h"

int main(int argc, char *argv[]) {
    QCoreApplication app(argc, argv);

    const int numThreads = 3;
    QThread* threads[numThreads];
    MyWorker* workers[numThreads];

    for (int i = 0; i < numThreads; ++i) {
        threads[i] = new QThread();
        workers[i] = new MyWorker();

        workers[i]->moveToThread(threads[i]);

        // 线程启动时，工作对象执行 doWork()
        QObject::connect(threads[i], &QThread::started, workers[i], &MyWorker::doWork);

        // 任务完成后退出线程的事件循环
        QObject::connect(workers[i], &MyWorker::finished, threads[i], &QThread::quit);

        // 任务完成后删除工作对象
        QObject::connect(workers[i], &MyWorker::finished, workers[i], &QObject::deleteLater);

        // 线程退出后删除线程对象
        QObject::connect(threads[i], &QThread::finished, threads[i], &QObject::deleteLater);

        threads[i]->start();
    }

    // 2 秒后唤醒所有线程
    QTimer::singleShot(2000, [=]() {
        workers[0]->startWork();  // 让所有 worker 线程醒来
    });

    return app.exec();
}

// myworker.h
#ifndef MYWORKER_H
#define MYWORKER_H

#include <QObject>
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
#include <QDebug>

class MyWorker : public QObject {
    Q_OBJECT
public:
    explicit MyWorker(QObject *parent = nullptr);
    ~MyWorker();
signals:
    void finished();
public slots:
    void doWork();
    void startWork();

private:
    static QMutex mutex;  // 互斥锁（多个 MyWorker 共享）
    static QWaitCondition condition;  // 等待条件（多个 MyWorker 共享）
    static bool ready;  // 共享的标志变量
};

#endif // MYWORKER_H

// myworker.cpp
#include "myworker.h"
#include <QMutexLocker>
// 共享变量初始化
QMutex MyWorker::mutex;
QWaitCondition MyWorker::condition;
bool MyWorker::ready = false;

MyWorker::MyWorker(QObject *parent) : QObject(parent) {}

MyWorker::~MyWorker() {
    qDebug() << QThread::currentThread() << "Worker destroyed";
}

void MyWorker::doWork() {
    qDebug() << QThread::currentThread() << "Worker waiting...";

    {
        QMutexLocker locker(&mutex);  // 加锁
        while (!ready) {  // 等待被唤醒
            condition.wait(&mutex);
        }
    }


    qDebug() << QThread::currentThread() << "Worker processing...";
    QThread::msleep(2000);
    qDebug() << QThread::currentThread() << "Worker finished!";
    emit finished();
}


void MyWorker::startWork() {
    QMutexLocker locker(&mutex);
    ready = true;
    condition.wakeAll();  // 唤醒所有等待的线程
}

运行结果

QThread(0xa9f1b00) Worker waiting...
QThread(0xa9f7fd8) Worker waiting...
QThread(0xa9f80d0) Worker waiting...
QThread(0xa9f1b00) Worker processing...
QThread(0xa9f80d0) Worker processing...
QThread(0xa9f7fd8) Worker processing...
QThread(0xa9f1b00) Worker finished!
QThread(0xa9f80d0) Worker finished!
QThread(0xa9f7fd8) Worker finished!
QThread(0xa9f1b00) Worker destroyed
QThread(0xa9f7fd8) Worker destroyed
QThread(0xa9f80d0) Worker destroyed