Qt 多线程学习

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最近的项目上用到了关于多线程的知识，自己也比较感兴趣，所以就拿了那本《C++ GUI Qt4 编程》来学习。

这本书的第14章是关于多线程的知识，使用的Qt版本是Qt4.x。在下用的是最新的Qt 5.2，所以代码上有一些不兼容，稍加修改就可以运行了。

Qt的多线程简单来说就是继承QThread类，重载run()函数，start()启动线程。首先来看下书上的第一个例子：（修改版的代码已上传，点击下载）

class Thread : public QThread
{
Q_OBJECT
public:
Thread(QString message = "", QObject *parent = NULL);
~Thread();
void setMessage(QString);
QString getMessage(); void stop();protected:
void run();private:
QString message;
volatile bool stopped;
};

Thread类继承了QThread类，并实现了run函数。stopped变量前面的volatile声明stopped为易失性变量，这样每次读取stopped时都是最新的值。

继续看Thread类的实现：

Thread::Thread(QString message, QObject *parent) :
stopped(false)
, QThread(parent)
, message(message)
{
}Thread::~Thread()
{
this->stop();
this->wait();
qDebug() << this;
}void Thread::setMessage(QString message)
{
this->message = message;
}QString Thread::getMessage()
{
return this->message;
}void Thread::stop()
{
stopped = true;
}void Thread::run()
{
while (!stopped)
std::cerr << qPrintable(message);
stopped = false;
std::cerr << std::endl;
}

初始化时将stopped设置为false，run函数中持续检查stopped的值，为true时才退出。

Dialog::Dialog(QWidget *parent)
: QDialog(parent)
{
QPushButton *buttonQuit = new QPushButton(QString::fromLocal8Bit("Quit"));
connect(buttonQuit, &QPushButton::clicked, this, &Dialog::close); QBoxLayout *layout = new QBoxLayout(QBoxLayout::LeftToRight, this); QStringList list = QString("ABCDEFGHIJKLMN").split("",QString::SkipEmptyParts); foreach (QString name, list)
{
Thread *thread = new Thread(name, this);
QPushButton *button = new QPushButton(QString("Start ")+name, this);
mappingTable.insert(button, thread);
connect(button, &QPushButton::clicked, this, &Dialog::startOrStopThread);
layout->addWidget(button);
} layout->addWidget(buttonQuit);
this->setLayout(layout);
}void Dialog::startOrStopThread()
{
QPushButton *buttonNow = dynamic_cast<QPushButton*>(sender());
Thread *threadNow = (Thread*)mappingTable[buttonNow]; if (threadNow == NULL) return; if(threadNow->isRunning())
{
threadNow->stop();
buttonNow->setText( buttonNow->text().replace(QString("Stop"),QString("Start")) );
}
else
{
threadNow->start();
buttonNow->setText( buttonNow->text().replace(QString("Start"),QString("Stop")) );
}
}

在Dialog界面类中，将button与thread实现一一对应的连接，在槽函数中就可以方便的找到对应的线程了。其中mappingTable是QMap<QObject*, QObject*>类型的。

这样就可以方便的实现多个线程的修改，如下图：

另外，第四个例子对我也很有启发：

TransactionThread::TransactionThread(QObject *parent) :
QThread(parent)
{
start();
}TransactionThread::~TransactionThread()
{
{
QMutexLocker locker(&mutex); while (!transactions.isEmpty())
delete transactions.dequeue(); transactionCondition.wakeOne();
} wait();
}void TransactionThread::addTransaction(Transaction *transaction)
{
QMutexLocker locker(&mutex);
transactions.enqueue(transaction);
transactionCondition.wakeOne();
}void TransactionThread::run()
{
Transaction *transaction = ;
QImage oldImage; forever
{
{
QMutexLocker locker(&mutex); if (transactions.isEmpty())
transactionCondition.wait(&mutex); if (transactions.isEmpty())
break; transaction = transactions.dequeue();
oldImage = currentImage;
} emit transactionStarted(transaction->message(), );
QImage newImage = transaction->apply(oldImage);
delete transaction; {
QMutexLocker locker(&mutex);
currentImage = newImage; if (transactions.isEmpty())
emit allTransactionsDone();
}
}
}void TransactionThread::setImage(const QImage& image)
{
QMutexLocker locker(&mutex);
currentImage = image;
}QImage TransactionThread::getImage()
{
QMutexLocker locker(&mutex);
return currentImage;
}

以上为线程实现的关键代码。在读取和写入从线程与主线程共享的变量时，都要使用mutex互斥变量。使用QMutexLocker locker(&mutex)也更方便，在构造是lock，析构时unlock，临时变量超过了作用域自然被析构，不得不说实现者的方法很巧妙啊。至于transactionCondition.wait(&mutex)则是等待条件。当事务队列为空时，等待事务加入，或者析构。加入事务时唤醒即可，即transactionCondition.wakeOne()。