为何需要多线程？

1、进行耗时操作时，可以处理用户的其他输入输出。比如，如果在UI线程里面进行耗时操作，界面会不响应用户操作。

2、提升程序性能。现在的电脑一般都是多核CPU，多线程并行处理事务，可以大大提升程序的性能。

针对第一点，为我们定位界面不响应问题指明了一个方向；针对第二点，为我们提升软件处理效率指明了一个方向。

那么，基于Qt开发的应用程序，如何实现多线程呢？

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1、继承QThread，重载run函数。

这种方法比较适用于处理耗时很长的业务。示例代码如下：

 class WorkerThread : public QThread
  {
      Q_OBJECT
      void run() override {
          QString result;
          /* ... here is the expensive or blocking operation ... */
          emit resultReady(result);
      }
  signals:
      void resultReady(const QString &s);
  };
 
  void MyObject::startWorkInAThread()
  {
      WorkerThread *workerThread = new WorkerThread(this);
      connect(workerThread, &WorkerThread::resultReady, this, &MyObject::handleResults);
      connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
      workerThread->start();
  }

需要注意以下几点：

1）run函数在新线程中执行，run函数执行结束，线程结束。

2）WorkerThread实例化的对象属于创建他的线程，而不是run函数所在线程。

3）WorkerThread没有事件循环，除非在run()函数中调用exec();

4）队列连接到WorkerThread的slot函数，slot函数在创建WorkerThread对象的线程中执行。

5）直接调用WorkerThread的方法，该方法的执行线程为调用处的线程。

2、继承QObject，调用void QObject::moveToThread(QThread *targetThread)。

这种方法适用于在一个类中处理多个耗时任务，且这个些任务不会并行执行的情况。示例代码如下：

  class Worker : public QObject
  {
      Q_OBJECT
 
  public slots:
      void doWork(const QString &parameter) {
          QString result;
          /* ... here is the expensive or blocking operation ... */
          emit resultReady(result);
      }
  signals:
      void resultReady(const QString &result);
  };
 
  class Controller : public QObject
  {
      Q_OBJECT
      QThread workerThread;
  public:
      Controller() {
          Worker *worker = new Worker;
          worker->moveToThread(&workerThread);
          connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
          connect(this, &Controller::operate, worker, &Worker::doWork);
          connect(worker, &Worker::resultReady, this, &Controller::handleResults);
          workerThread.start();
      }
      ~Controller() {
          workerThread.quit();
          workerThread.wait();
      }
  public slots:
      void handleResults(const QString &);
  signals:
      void operate(const QString &);
  };

需要注意以下几点：

1）调用moveToThread函数的对象不能设置父对象。

2）Worker类中的槽函数可以跟任意线程的任意信号建立连接，队列连接时，在新线程中执行。

3）直接调用Worker类中的函数，在调用线程内执行。

4）同时发送多个与Worker类中槽函数连接的信号，槽函数依次执行。

3、QThreadPool and QRunnabl。

我们都知道频繁创建和销毁线程会带来较大的性能开销，影响程序执行效率。Qt的线程池技术，给了我们一个解决这个问题的有效方法。示例代码如下：

  class HelloWorldTask : public QRunnable
  {
      void run() override
      {
          qDebug() << "Hello world from thread" << QThread::currentThread();
      }
  };
 
  HelloWorldTask *hello = new HelloWorldTask();
  // QThreadPool takes ownership and deletes 'hello' automatically
  QThreadPool::globalInstance()->start(hello);

注意：

1）默认情况下，run函数执行完，hello对象会被线程池自动删除。可以使用setAutoDelete函数设置。

2）QThreadPool::start()多次启动设置为autoDelete的QRunnable对象，可能导致崩溃。

4、Qt Concurrent。

QtConcurrent提供了高级api，使编写多线程程序时，不需要使用诸如互斥锁、读写锁、等待条件或信号量等低级线程安全类。具体用法，我们下回分解。传送门：Qt Concurrent 线程使用详解

Concurrent Run

 extern void aFunction();
 QFuture<void> future = QtConcurrent::run(aFunction);

Concurrent Map and Map-Reduce

  QList<QImage> images = ...;
  // Each call blocks until the entire operation is finished.
  QList<QImage> future = QtConcurrent::blockingMapped(images, scaled);
  QtConcurrent::blockingMap(images, scale);
  QImage collage = QtConcurrent::blockingMappedReduced(images, scaled, addToCollage);

Concurrent Filter and Filter-Reduce

 QStringList strings = ...;
  // each call blocks until the entire operation is finished
  QStringList lowerCaseStrings = QtConcurrent::blockingFiltered(strings, allLowerCase);
  QtConcurrent::blockingFilter(strings, allLowerCase);
  QSet<QString> dictionary = QtConcurrent::blockingFilteredReduced(strings, allLowerCase, addToDictionary);

5、测试代码

main.cpp

#include "qtthreaddemo.h"
#include <QtWidgets/QApplication>
#include <QDebug>
#include <QThread>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qDebug() << "Main Thread Id:" << QThread::currentThreadId();
    QtThreadDemo w;
    w.show();
    return a.exec();
}

qtthreaddemo.h

#pragma once
 
#include <QtWidgets/QWidget>
#include "ui_qtthreaddemo.h"
 
class WorkerThread1;
class WorkerThread2;
 
class QtThreadDemo : public QWidget
{
    Q_OBJECT
 
public:
    QtThreadDemo(QWidget *parent = Q_NULLPTR);
private slots:
    void on_btnCreateThread1_clicked();
    void on_btnSlotFun_clicked();
    void on_btnCreateThread2_clicked();
    void on_btnTestOrders_clicked();
    void on_btnRunnableThread_clicked();
    void workerThreadEnd();
signals:
    void run_slot();
private:
    Ui::QtThreadDemoClass ui;
    WorkerThread1 *m_pThread1 = nullptr;
    WorkerThread2 *m_pThread2 = nullptr;
};

qtthreaddemo.cpp

#include "qtthreaddemo.h"
#include "workerthread1.h"
#include "workerthread2.h"
#include "runnablethread.h"
#include <QDebug>
#include <QThreadPool>
 
QtThreadDemo::QtThreadDemo(QWidget *parent)
    : QWidget(parent)
{
    ui.setupUi(this);
}
 
void QtThreadDemo::on_btnCreateThread1_clicked()
{
    qDebug() << "btnCreateThread clicked Thread Id:" << QThread::currentThreadId();
    m_pThread1 = new WorkerThread1(this);
    connect(this, SIGNAL(run_slot()), m_pThread1, SLOT(doJob()), Qt::QueuedConnection);
    connect(m_pThread1, SIGNAL(finished()), this, SLOT(workerThreadEnd()));
    m_pThread1->start();
}
 
void QtThreadDemo::on_btnSlotFun_clicked()
{
    qDebug() << "btnSlotFun clicked Thread Id:" << QThread::currentThreadId();
    emit run_slot();
}
 
void QtThreadDemo::on_btnCreateThread2_clicked()
{
    qDebug() << "btnCreateThread2 clicked";
    if (m_pThread2 == nullptr)
    {
        m_pThread2 = new WorkerThread2(nullptr);
    }
    m_pThread2->doJob();
}
 
void QtThreadDemo::on_btnTestOrders_clicked()
{
    qDebug() << "btnTestOrders clicked";
    if (m_pThread2 == nullptr)
    {
        m_pThread2 = new WorkerThread2(nullptr);
    }
    m_pThread2->testOrder();
}
 
void QtThreadDemo::on_btnRunnableThread_clicked()
{
    qDebug() << "btnRunnableThread clicked";
    RunnableThread *pThread = new RunnableThread;
    QThreadPool::globalInstance()->tryStart(pThread);
    QThread::msleep(2000);
    //QThreadPool::globalInstance()->tryStart(pThread);   //会发生异常
}
 
void QtThreadDemo::workerThreadEnd()
{
    qDebug() << "Worker Thread Ended!";
}

workthread1.h

#pragma once
 
#include <QThread>
 
class WorkerThread1 : public QThread
{
    Q_OBJECT
 
public:
    WorkerThread1(QObject *parent);
    ~WorkerThread1();
    static void publicFun();
protected:
    void run();
 
private slots:
    void doJob();
};

workerthread1.cpp

#include "workerthread1.h"
#include <QDebug>
 
WorkerThread1::WorkerThread1(QObject *parent)
    : QThread(parent)
{
    qDebug() << "WorkerThread1 Object Thread Id:" << QThread::currentThreadId();
}
 
WorkerThread1::~WorkerThread1()
{
}
 
void WorkerThread1::publicFun()
{
    qDebug() << "WorkerThread1::publicFun Thread Id:" << QThread::currentThreadId();
}
 
void WorkerThread1::run()
{
    qDebug() << "WorkerThread1 Create Thread Id:" << QThread::currentThreadId();
}
 
void WorkerThread1::doJob()
{
    qDebug() << "WorkerThread1 Slot-doJob Thread Id:" << QThread::currentThreadId();
}

workerthread2.h

#pragma once
 
#include <QObject>
class QThread;
 
class WorkerThread2 : public QObject
{
    Q_OBJECT
 
public:
    WorkerThread2(QObject *parent);
    ~WorkerThread2();
    void doJob();
    void testOrder();
 
private slots:
    void onDoJob();
    void onOrder1();
    void onOrder2();
    void onOrder3();
 
signals:
    void sig_do_job();
    void sig_order1();
    void sig_order2();
    void sig_order3();
private:
    QThread *m_pThread;
};

workerthread2.cpp

#include "workerthread2.h"
#include <QThread>
#include <QDebug>
#include "workerthread1.h"
 
WorkerThread2::WorkerThread2(QObject *parent)
    : QObject(parent)
{
    m_pThread = new QThread;
    moveToThread(m_pThread);
    m_pThread->start();
 
    connect(this, SIGNAL(sig_do_job()), this, SLOT(onDoJob()), Qt::QueuedConnection);
    connect(this, SIGNAL(sig_order1()), this, SLOT(onOrder1()), Qt::QueuedConnection);
    connect(this, SIGNAL(sig_order2()), this, SLOT(onOrder2()), Qt::QueuedConnection);
    connect(this, SIGNAL(sig_order3()), this, SLOT(onOrder3()), Qt::QueuedConnection);
}
 
WorkerThread2::~WorkerThread2()
{
}
 
void WorkerThread2::doJob()
{
    qDebug() << "WorkerThread2::doJob thread id:" << QThread::currentThreadId();
    emit sig_do_job();
}
 
void WorkerThread2::testOrder()
{
    emit sig_order1();
    emit sig_order2();
    emit sig_order3();
}
 
void WorkerThread2::onDoJob()
{
    qDebug() << "WorkerThread2::onDoJob thread id:" << QThread::currentThreadId();
    WorkerThread1::publicFun();
}
 
void WorkerThread2::onOrder1()
{
    qDebug() << "WorkerThread2::onOrder1 begin";
    QThread::msleep(1000);
    qDebug() << "WorkerThread2::onOrder1 end";
}
 
void WorkerThread2::onOrder2()
{
    qDebug() << "WorkerThread2::onOrder2 begin";
    QThread::msleep(1000);
    qDebug() << "WorkerThread2::onOrder2 end";
}
 
void WorkerThread2::onOrder3()
{
    qDebug() << "WorkerThread2::onOrder3 begin";
    QThread::msleep(1000);
    qDebug() << "WorkerThread2::onOrder3 end";
}

runnablethread.h

#pragma once
 
#include <QRunnable>
 
class RunnableThread : public QRunnable
{
public:
    RunnableThread();
    ~RunnableThread();
    void run();
};

runnablethread.cpp

#include "runnablethread.h"
#include <QDebug>
#include <QThread>
 
RunnableThread::RunnableThread()
{
}
 
RunnableThread::~RunnableThread()
{
}
 
void RunnableThread::run()
{
    qDebug() << "RunnableThread::run Thread Id:" << QThread::currentThreadId();
}

运行结果：