多线程

1. 线程概念的起源

• 单核CPU
  早期还没有线程的概念，如何保证2个进程同时进行呢？时间片轮转调度
  每次被CPU选中来执行当前进程所用的时间，时间一到，无论进程是否运行结束，操作系统都会强制将CPU这个资源转到另一个进程去执行。
• 多核CPU
  随着运行的进程越来越多，人们发现进程的创建、撤销与切换存在着较大的时空开销，因此业界急需一种轻型的进程技术来减少开销。于是上世纪80年代出现了一种叫SMP的对称多处理技术，就是我们所知的线程概念。
  线程切换的开销要小很多，这是因为每个进程都有属于自己的一个完整虚拟地址空间，而线程隶属于某一个进程，与进程内的其他线程一起共享这片地址空间，基本上就可以利用进程所拥有的资源而无需调用新的资源，故对它的调度所付出的开销就会小很多。

2. 三种方式创建线程

1. 线程入口函数

• 全局函数

#include <QThread>
void helloThread()
{for (int i = 0; i < 99999999; ++i) {qDebug() << "hello";}
}int main(int argc, char* argv[])
{QApplication a(argc, argv);QThread* th1 = QThread::create(helloThread);th1->start();return a.exec();
}

void helloThread(QString s)
{for (int i = 0; i < 99999999; ++i) {qDebug() << "hello" << s;}
}int main(int argc, char* argv[])
{QApplication a(argc, argv);QThread* th1 = QThread::create(helloThread, "thread1");th1->start();return a.exec();
}

• 静态成员函数

class A {
public:A() { }static void print(){for (int i = 0; i < 10; i++)qDebug() << __FUNCTION__;}
};int main(int argc, char* argv[])
{QApplication a(argc, argv);QThread* th1 = QThread::create(A::print);th1->start();return a.exec();
}

• 成员函数

class A {
public:A() { }void print(){for (int i = 0; i < 10; i++)qDebug() << __FUNCTION__;}
};int main(int argc, char* argv[])
{QApplication a(argc, argv);A aFunc;QThread* th1 = QThread::create(&A::print, &aFunc);th1->start();return a.exec();
}

• lamda表达式

    QThread* th2 = QThread::create([=] { qDebug() << "lamda"; });th2->start();

• 连接信号与槽

    QObject::connect(th2, &QThread::started, [=] { qDebug() << "th2线程启动"; });QObject::connect(th1, &QThread::finished, [=] { qDebug() << "th1线程执行完毕";th1->deleteLater(); });

2. 继承QThread重写run

class MyThread : public QThread {
public:void run() override{for (int i = 0; i < 10; i++) {qDebug() << "MyThread" << i;if (i == 5)quit(); // 只能在线程中调用,但是要等事件结束才结束}}
};
int main(int argc, char* argv[])
{QApplication a(argc, argv);MyThread th3;th3.start();th3.exit(); // 结束线程，也要等事件结束才结束    return a.exec();    
}

3. QObject::moveToThread

class Download : public QObject {Q_OBJECT
public:Download(QObject* parent = nullptr): QObject(parent){}
public slots:void downloadFile(){for (int i = 0; i < 100; i++)qDebug() << "下载进度=====" << i << "%";emit finished();}
signals:void finished();
};class APP : public QObject {Q_OBJECT
public:APP(QObject* parent = nullptr): QObject(parent){d = new Download;th = new QThread;connect(this, &APP::startDownload, d, &Download::downloadFile);connect(d, &Download::finished, [=] { qDebug() << "下载完成"; });d->moveToThread(th);connect(th, &QThread::finished, th, &QThread::deleteLater);th->start();}
signals:void startDownload();private:Download* d;QThread* th;
};
int main(int argc, char* argv[])
{QApplication a(argc, argv);APP ap;emit ap.startDownload();return a.exec();
}#include "main.moc"

3. 启动线程前的准备工作

    // 建议最多同时开多少线程数qDebug() << QThread::idealThreadCount();// 改变栈空间，如果线程运行所占空间很大，那就会崩溃，需要改变qDebug() << "原始栈空间" << th3.stackSize(); // 原始栈空间 0th3.setStackSize(1024 * 1024 * 3);qDebug() << "现在栈空间" << th3.stackSize();//现在栈空间 3145728

4. 启动线程/退出线程

• 启动线程

调用start()函数后，新线程会优先执行run()中的代码，再执行其他的
默认run()会调用exec()函数，即启动一个局部的不占CPU的事件循环

5. 操作运行中的线程

• 休眠函数

void helloThread(QString s)
{for (int i = 0; i < 9; ++i) {qDebug() << "hello" << s;QThread::sleep(1); // 睡眠1秒QThread::msleep(100); // 睡眠100毫秒QThread::usleep(1000); // 睡眠1000微秒}
}

• 中断标志

class MyThread : public QThread {
public:void run() override{for (int i = 0; i < 10; i++) {qDebug() << "MyThread" << i;// 中断标志if (this->isInterruptionRequested())break;// 请求中断if (i == 3)this->requestInterruption();}}
};

6. 为每个线程提供独立数据

关于全局变量，在2个线程里修改会相互影响

int g_num = 5;
void fun()
{g_num = 8;qInfo() << "g_num" << g_num;
}
void fun1()
{g_num = 9;qInfo() << "1_g_num" << g_num;
}
int main(int argc, char* argv[])
{QApplication a(argc, argv);QThread* th = QThread::create(fun);QThread* th1 = QThread::create(fun1);th->start();th1->start();return a.exec();
}

可以通过QThreadStorage类把全局变量设置成线程独立的变量

QThreadStorage<int> g_num;
void fun()
{g_num.setLocalData(8);qInfo() << "g_num" << g_num.localData();
}
void fun1()
{g_num.setLocalData(9);qInfo() << "1_g_num" << g_num.localData();
}
int main(int argc, char* argv[])
{QApplication a(argc, argv);QThread* th = QThread::create(fun);QThread* th1 = QThread::create(fun1);th->start();th1->start();qDebug() << "main_g_num" << g_num.localData();    return a.exec();
}

7.子线程不能操作ui

Gui框架一般只允许ui线程操作界面组件，Qt也是如此，否则会出现崩溃

解决方案

• 通过信号与槽
  参考之前的moveToThread

• 通过QMetaObject::invokeMethod

#include "Widget.h"
#include "./ui_Widget.h"
#include <QMetaObject>
#include <QThread>Widget::Widget(QWidget* parent): QWidget(parent), ui(new Ui::Widget)
{ui->setupUi(this);QThread* th = QThread::create(&Widget::fun, this);th->start();
}Widget::~Widget()
{delete ui;
}void Widget::fun()
{QThread::sleep(5);QMetaObject::invokeMethod(ui->label, "setText", Q_ARG(QString, "hello"));
}void Widget::on_pushButton_clicked()
{fun();
}

• 通过QApplication::postEvent

#include "Widget.h"
#include "./ui_Widget.h"
#include <QCoreApplication>
#include <QMetaObject>
#include <QThread>class MyEvent : public QEvent {
public:MyEvent(): QEvent(QEvent::Type(QEvent::User)){}
};Widget::Widget(QWidget* parent): QWidget(parent), ui(new Ui::Widget)
{ui->setupUi(this);
}Widget::~Widget()
{delete ui;
}void Widget::fun()
{QCoreApplication::postEvent(this, new MyEvent);
}void Widget::customEvent(QEvent* ev)
{if (ev->type() == QEvent::User)this->move(-10, 0);
}