Qt学习：线程间共享数据

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。

//TextDevice.h#ifndef TEXTDEVICE_H#define TEXTDEVICE_H#include <QThread>#include <QString>#include <QMutex>class TextDevice : public QThread { Q_OBJECTpublic: 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_OBJECTpublic: 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&");

//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_OBJECTpublic: 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_OBJECTpublic: 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;}