详解 Qt 线程间共享数据

使用共享内存。即使用一个两个线程都能够共享的变量（如全局变量），这样两个线程都能够访问和修改该变量，从而达到共享数据的目的。

Qt 线程间共享数据是本文介绍的内容，多的不说，先来啃内容。Qt线程间共享数据主要有两种方式：

使用共享内存。即使用一个两个线程都能够共享的变量（如全局变量），这样两个线程都能够访问和修改该变量，从而达到共享数据的目的；

使用singal/slot机制，把数据从一个线程传递到另外一个线程。

第一种办法在各个编程语言都使用普遍，而第二种方式倒是QT的特有方式，下面主要学习一下这种方式：

在线程之间传递signal与在一个线程内传递signal是不一样的。在一个线程内传递signal时，emit语句会直接调用所有连接的slot并等待到所有slot被处理完；在线程之间传递signal时，slot会被放到队列中(queue)，而emit这个signal后会马上返回；默认情况，线程之间使用queue机制，而线程内使用direct机制，但在connect中可以改变这些默认的机制。

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//TextDevice.h
#ifndef TEXTDEVICE_H
#define TEXTDEVICE_H
#include <QThread>
#include <QString>
#include <QMutex>
class TextDevice : public QThread {
Q_OBJECT
public:
TextDevice();
void run();
void stop();
public slots:
void write(const QString& text);
private:
int m_count;
QMutex m_mutex;
};
#endif // TEXTDEVICE_H

//TextDevice.cpp
#include <QMutexLocker>
#include <QDebug>
#include <QString>
#include "TextDevice.h"
TextDevice::TextDevice() {
m_count = 0;
}
void TextDevice::run() {
exec();
}
void TextDevice::stop() {
quit();
}
void TextDevice::write(const QString& text) {
QMutexLocker locker(&m_mutex);
qDebug() << QString("Call %1: %2").arg(m_count++).arg(text);
}

//TextThread.h
#ifndef TEXTTHREAD_H
#define TEXTTHREAD_H
#include <QThread>
#include <QString>
class TextThread : public QThread {
Q_OBJECT
public:
TextThread(const QString& text);
void run();
void stop();
signals:
void writeText(const QString&);
private:
QString m_text;
bool m_stop;
};
#endif // TEXTTHREAD_H

//TextThread.cpp
#include "TextThread.h"
TextThread::TextThread(const QString& text) : QThread() {
m_text = text;
m_stop = false;
}
void TextThread::stop() {
m_stop = true;
}
void TextThread::run() {
while(!m_stop) {
emit writeText(m_text);
sleep(1);
}
}

//main.cpp
#include <QApplication>
#include <QMessageBox>
#include "TextDevice.h"
#include "TextThread.h"

int main(int argc, char** argv) {
QApplication app(argc, argv);
//启动线程
TextDevice device;
TextThread foo("foo"), bar("bar");
//把两个线程使用signal/slot连接起来
QObject::connect(&foo, SIGNAL(writeText(const QString&)), &device, SLOT(write(const QString&)));
QObject::connect(&bar, SIGNAL(writeText(const QString&)), &device, SLOT(write(const QString&)));
//启动线程
foo.start();
bar.start();
device.start();
QMessageBox::information(0, "Threading", "Close me to stop.");
//停止线程
foo.stop();
bar.stop();
device.stop();
//等待线程结束
device.wait();
foo.wait();
bar.wait();
return 0;
}
//TextDevice.h
#ifndef TEXTDEVICE_H
#define TEXTDEVICE_H
#include <QThread>
#include <QString>
#include <QMutex>
class TextDevice : public QThread {
Q_OBJECT
public:
TextDevice();
void run();
void stop();
public slots:
void write(const QString& text);
private:
int m_count;
QMutex m_mutex;
};
#endif // TEXTDEVICE_H

//TextDevice.cpp
#include <QMutexLocker>
#include <QDebug>
#include <QString>
#include "TextDevice.h"
TextDevice::TextDevice() {
m_count = 0;
}
void TextDevice::run() {
exec();
}
void TextDevice::stop() {
quit();
}
void TextDevice::write(const QString& text) {
QMutexLocker locker(&m_mutex);
qDebug() << QString("Call %1: %2").arg(m_count++).arg(text);
}

//TextThread.h
#ifndef TEXTTHREAD_H
#define TEXTTHREAD_H
#include <QThread>
#include <QString>
class TextThread : public QThread {
Q_OBJECT
public:
TextThread(const QString& text);
void run();
void stop();
signals:
void writeText(const QString&);
private:
QString m_text;
bool m_stop;
};
#endif // TEXTTHREAD_H

//TextThread.cpp
#include "TextThread.h"
TextThread::TextThread(const QString& text) : QThread() {
m_text = text;
m_stop = false;
}
void TextThread::stop() {
m_stop = true;
}
void TextThread::run() {
while(!m_stop) {
emit writeText(m_text);
sleep(1);
}
}

//main.cpp
#include <QApplication>
#include <QMessageBox>
#include "TextDevice.h"
#include "TextThread.h"
int main(int argc, char** argv) {
QApplication app(argc, argv);
//启动线程
TextDevice device;
TextThread foo("foo"), bar("bar");
//把两个线程使用signal/slot连接起来
QObject::connect(&foo, SIGNAL(writeText(const QString&)), &device, SLOT(write(const QString&)));
QObject::connect(&bar, SIGNAL(writeText(const QString&)), &device, SLOT(write(const QString&)));
//启动线程
foo.start();
bar.start();
device.start();
QMessageBox::information(0, "Threading", "Close me to stop.");
//停止线程
foo.stop();
bar.stop();
device.stop();
//等待线程结束
device.wait();
foo.wait();
bar.wait();
return 0;
}

　　上面例子代码可以看出两个线程之间传送了类型为QString的信息。像QString等这些QT本身定义的类型，直接传送即可。但如果是自己定义的类型如果想使用signal/slot来传递的话，则没有这么简单。直接使用的话，会产生下面这种错误：

QObject::connect: Cannot queue arguments of type 'TextAndNumber' (Make sure 'TextAndNumber' is registed using qRegisterMetaType().)

　　

原因：当一个signal被放到队列中（queued）时，它的参数(arguments)也会被一起一起放到队列中（queued起来），这就意味着参数在被传送到slot之前需要被拷贝、存储在队列中（queue）中；为了能够在队列中存储这些参数(argument)，Qt需要去construct、destruct、copy这些对象，而为了让Qt知道怎样去作这些事情，参数的类型需要使用qRegisterMetaType来注册（如错误提示中的说明）

步骤：(以自定义TextAndNumber类型为例）

自定一种类型，在这个类型的顶部包含：#include <QMetaType>

在类型定义完成后，加入声明：Q_DECLARE_METATYPE(TextAndNumber);

在main()函数中注册这种类型：qRegisterMetaType<TextAndNumber>("TextAndNumber");

如果还希望使用这种类型的引用，可同样要注册：qRegisterMetaType<TextAndNumber>("TextAndNumber&");

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//TextAndNumber.h
#ifndef TEXTANDNUMBER_H
#define TEXTANDNUMBER_H
#include <QMetaType>
//必须包含QMetaType，否则会出现下面错误：
//error: expected constructor, destructor, or type conversion before ‘;’ token
#include <QString>
class TextAndNumber {
public:
TextAndNumber();
TextAndNumber(int, QString);
int count();
QString text();
private:
int m_count;
QString m_text;
};
Q_DECLARE_METATYPE(TextAndNumber);
#endif // TEXTANDNUMBER_H

//TextAndNumber.cpp
#include "TextAndNumber.h"
TextAndNumber::TextAndNumber() {
}
TextAndNumber::TextAndNumber(int count, QString text) {
m_count = count;
m_text = text;
}
int TextAndNumber::count() {
return m_count;
}
QString TextAndNumber::text() {
return m_text;
}

//TextDevice.h
#ifndef TEXTDEVICE_H
#define TEXTDEVICE_H
#include <QThread>
#include <QDebug>
#include <QString>
#include "TextAndNumber.h"
class TextDevice : public QThread {
Q_OBJECT
public:
TextDevice();
void run();
void stop();
public slots:
void write(TextAndNumber& tran);
private:
int m_count;
};
#endif // TEXTDEVICE_H

//TextDevice.cpp
#include "TextDevice.h"
TextDevice::TextDevice() : QThread() {
m_count = 0;
}
void TextDevice::run() {
exec();
}
void TextDevice::stop() {
quit();
}
void TextDevice::write(TextAndNumber& tran) {
qDebug() << QString("Call %1 (%3): %2").arg(m_count++).arg(tran.text()).arg(tran.count());
}

//TextThread.h
#ifndef TEXTTHREAD_H
#define TEXTTHREAD_H
#include <QThread>
#include <QString>
#include "TextAndNumber.h"
class TextThread : public QThread {
Q_OBJECT
public:
TextThread(const QString& text);
void run();
void stop();
signals:
void writeText(TextAndNumber& tran);
private:
QString m_text;
int m_count;
bool m_stop;
};

#endif // TEXTTHREAD_H

//TextThread.cpp
#include "TextThread.h"
TextThread::TextThread(const QString& text) : QThread() {
m_text = text;
m_stop = false;
m_count = 0;
}
void TextThread::run() {
while(!m_stop) {
TextAndNumber tn(m_count++, m_text);
emit writeText(tn);
sleep(1);
}
}
void TextThread::stop() {
m_stop = true;
}

//main.cpp
#include <QApplication>
#include <QMessageBox>
#include "TextThread.h"
#include "TextDevice.h"
#include "TextAndNumber.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
qRegisterMetaType<TextAndNumber>("TextAndNumber");
qRegisterMetaType<TextAndNumber>("TextAndNumber&");
TextDevice device;
TextThread foo("foo"), bar("bar");
QObject::connect(&foo, SIGNAL(writeText(TextAndNumber&)), &device, SLOT(write(TextAndNumber&)));
QObject::connect(&bar, SIGNAL(writeText(TextAndNumber&)), &device, SLOT(write(TextAndNumber&)));
device.start();
foo.start();
bar.start();
QMessageBox::information(0, "Threading", "Click me to close");
foo.stop();
bar.stop();
device.stop();
foo.wait();
bar.wait();
device.wait();
qDebug() << "Application end.";
return 0;
}