Qt多线程编程中的对象线程与函数执行线程

近来用Qt编写一段多线程的TcpSocket通信程序，被其中Qt中报的几个warning搞晕了，一会儿是说“Cannot create children for a parent that is in a different thread”，有时候又是“QSocketNotifier: socket notifiers cannot be enabled from another thread”，还经常又Assert failure：Cannot send events toobjects owned by a different thread，从而导致程序崩溃。

    为彻底搞清原因并解决问题，在查阅大量资料和Qt文档之后，理清了其中的机制，也对多线程编程中的QObject对象创建以及connect执行有更清楚的认识：

    1. 一个对象的线程就是创建该对象时的线程，而不论该对象的定义是保存在那个线程中；

    2. QObject的connect函数有几种连接方式，

      a) DirectConnection，信号发送后槽函数立即执行，由sender的所在线程执行；

      b) QueuedConnection，信号发送后返回，相关槽函数由receiver所在的线程在返回到事件循环后执行；

      c) 默认使用的是Qt::AutoConnection，当sender和receiver在同一个线程内时，采用DirectConnection的方式，当sender和receiver在不同的线程时，采用QueuedConnection的方式。

    为了更清楚的理解这些问题，在此特编了个小例子说明一下。首先定义一个从QObject继承的类SomeObject，包含一个信号someSignal和一个成员函数callEmitSignal，此函数用于发送前面的someSignal信号。定义如下：

<span style="font-family: SimSun; font-size: 16px;">// define Object class</span>

// define Object class

<span style="font-family: SimSun; font-size: 16px;">class SomeObject : public QObject
{
    Q_OBJECT
public:
    SomeObject(QObject* parent=0) : QObject(parent) {}
    void callEmitSignal()  // 用于发送信号的函数
    {
        emit someSignal();
    }
signals:
    void someSignal();
};
</span>

class SomeObject : public QObject
{
    Q_OBJECT
public:
    SomeObject(QObject* parent=0) : QObject(parent) {}
    void callEmitSignal()  // 用于发送信号的函数
    {
        emit someSignal();
    }
signals:
    void someSignal();
};

然后再定义一个从QThread继承的线程类SubThread，它包含一个SomeObject的对象指针obj，另外有一个slot函数someSolt，定义如下：

<span style="font-family: SimSun; font-size: 16px;">class SubThread : public QThread
{
    Q_OBJECT
public:
    SubThread(QObject* parent=0) : QThread(parent){}
    virtual ~SubThread()
    {
        if (obj!=NULL) delete obj;
    }
public slots:
    // slot function connected to obj's someSignal
    void someSlot();
public:
    SomeObject * obj;
};
// slot function connected to obj's someSignal
void SubThread::someSlot()
{
    QString msg;
    msg.append(this->metaObject()->className());
    msg.append("::obj's thread is ");
    if (obj->thread() == qApp->thread())
    {
        msg.append("MAIN thread;");
    }
    else if (obj->thread() == this)
    {
        msg.append("SUB thread;");
    }
    else
    {
        msg.append("OTHER thread;");
    }
    msg.append(" someSlot executed in ");
    if (QThread::currentThread() == qApp->thread())
    {
        msg.append("MAIN thread;");
    }
    else if (QThread::currentThread() == this)
    {
        msg.append("SUB thread;");
    }
    else
    {
        msg.append("OTHER thread;");
    }
    qDebug() << msg;
    quit();
}</span>

class SubThread : public QThread
{
    Q_OBJECT
public:
    SubThread(QObject* parent=0) : QThread(parent){}
    virtual ~SubThread()
    {
        if (obj!=NULL) delete obj;
    }
public slots:
    // slot function connected to obj's someSignal
    void someSlot();
public:
    SomeObject * obj;
};
// slot function connected to obj's someSignal
void SubThread::someSlot()
{
    QString msg;
    msg.append(this->metaObject()->className());
    msg.append("::obj's thread is ");
    if (obj->thread() == qApp->thread())
    {
        msg.append("MAIN thread;");
    }
    else if (obj->thread() == this)
    {
        msg.append("SUB thread;");
    }
    else
    {
        msg.append("OTHER thread;");
    }
    msg.append(" someSlot executed in ");
    if (QThread::currentThread() == qApp->thread())
    {
        msg.append("MAIN thread;");
    }
    else if (QThread::currentThread() == this)
    {
        msg.append("SUB thread;");
    }
    else
    {
        msg.append("OTHER thread;");
    }
    qDebug() << msg;
    quit();
}

这里someSlot函数主要输出了obj所在的线程和slot函数执行线程。

    接着从SubThread又继承了3个线程类，分别是SubThread1, SubThread2, SubThread3.分别实现线程的run函数。定义如下：

<span style="font-family: SimSun; font-size: 16px;">// define sub thread class 1
class SubThread1 : public SubThread
{
    Q_OBJECT
public:
    SubThread1(QObject* parent=0);
    // reimplement run
    void run();
};
class SubThread2 : public SubThread
{
    Q_OBJECT
public:
    SubThread2(QObject* parent=0);
    // reimplement run
    void run();
};
class SubThread3 : public SubThread
{
    Q_OBJECT
public:
    SubThread3(QObject* parent=0);
    // reimplement run
    void run();
};</span>

// define sub thread class 1
class SubThread1 : public SubThread
{
    Q_OBJECT
public:
    SubThread1(QObject* parent=0);
    // reimplement run
    void run();
};
class SubThread2 : public SubThread
{
    Q_OBJECT
public:
    SubThread2(QObject* parent=0);
    // reimplement run
    void run();
};
class SubThread3 : public SubThread
{
    Q_OBJECT
public:
    SubThread3(QObject* parent=0);
    // reimplement run
    void run();
};

    在主程序中分别创建3个不同的线程并运行，查看运行结果。

<span style="font-family: SimSun; font-size: 16px;">int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    SubThread1* t1 = new SubThread1(&a); //由主线程创建
    t1->start();
    SubThread2* t2 = new SubThread2(&a); //由主线程创建
    t2->start();
    SubThread3* t3 = new SubThread3(&a); //由主线程创建
    t3->start();
    return a.exec();
}</span>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    SubThread1* t1 = new SubThread1(&a); //由主线程创建
    t1->start();
    SubThread2* t2 = new SubThread2(&a); //由主线程创建
    t2->start();
    SubThread3* t3 = new SubThread3(&a); //由主线程创建
    t3->start();
    return a.exec();
}

    下面我们来分析不同写法的程序，其obj对象所在的线程空间和someSlot函数执行的线程空间分别是怎样的。

    首先看SubThread1的实现：

<span style="font-family: SimSun; font-size: 16px;">////////////////////////////////////////////////////////
// class SubThread1
////////////////////////////////////////////////////////
SubThread1::SubThread1(QObject* parent)
    : SubThread(parent)
{
    obj = new SomeObject();//由主线程创建
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()));
}
// reimplement run
void SubThread1::run()
{
    obj->callEmitSignal();
    exec();
}</span>

////////////////////////////////////////////////////////
// class SubThread1
////////////////////////////////////////////////////////
SubThread1::SubThread1(QObject* parent)
    : SubThread(parent)
{
    obj = new SomeObject();//由主线程创建
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()));
}
// reimplement run
void SubThread1::run()
{
    obj->callEmitSignal();
    exec();
}

可以看到，obj是在构造函数中被创建的，那么创建obj对象的线程也就是创建SubThread1的线程，一般是主线程，而不是SubThread1所代表的线程。同时由于obj和this（即t1）都位于主线程，所以someSlot函数也是由主线程来执行的。

    而在线程SubThread2中，我们把obj对象的创建放到子线程的run函数中，那么obj对象的线程就应该SubThread2代表的线程，即t2，就不再是主线程了。

<span style="font-family: SimSun; font-size: 16px;">////////////////////////////////////////////////////////
// class SubThread2
////////////////////////////////////////////////////////
SubThread2::SubThread2(QObject* parent)
    : SubThread(parent)
{
    obj=0;
}
// reimplement run
void SubThread2::run()
{
    obj = new SomeObject(); //由当前子线程创建
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()));
    obj->callEmitSignal();
    exec();
}</span>

////////////////////////////////////////////////////////
// class SubThread2
////////////////////////////////////////////////////////
SubThread2::SubThread2(QObject* parent)
    : SubThread(parent)
{
    obj=0;
}
// reimplement run
void SubThread2::run()
{
    obj = new SomeObject(); //由当前子线程创建
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()));
    obj->callEmitSignal();
    exec();
}

同时，在connect函数中由于obj和this（这里是t2）不是在同一个线程中，因此会采用QueuedConnection的方式，其slot函数由this对象所在的线程即主线程来执行。这里有一个特别容易误解的地方，就是这个slot函数虽然是子线程SubThread2的一个成员函数，connect操作也是在子线程内完成的，但是该函数的执行却不在子线程内，而是在主线程内。

    那么如果想让相应的slot函数在子线程内执行，该如何做呢？在子线程的run函数中创建obj对象的同时，在执行connect时指定连接方式为DirectConnection，这样就可以使slot函数在子线程中运行，因为DirectConnection的方式始终由sender对象的线程执行。如

<span style="font-family: SimSun; font-size: 16px;">////////////////////////////////////////////////////////
// class SubThread3
////////////////////////////////////////////////////////
SubThread3::SubThread3(QObject* parent)
    : SubThread(parent)
{
    obj=0;
}
// reimplement run
void SubThread3::run()
{
    obj = new SomeObject();
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()),
            Qt::DirectConnection);
    obj->callEmitSignal();
    exec();
}</span>

////////////////////////////////////////////////////////
// class SubThread3
////////////////////////////////////////////////////////
SubThread3::SubThread3(QObject* parent)
    : SubThread(parent)
{
    obj=0;
}
// reimplement run
void SubThread3::run()
{
    obj = new SomeObject();
    connect(obj, SIGNAL(someSignal()), this, SLOT(someSlot()),
            Qt::DirectConnection);
    obj->callEmitSignal();
    exec();
}

    最后，该程序的运行结果应该是：

<span style="font-family: SimSun; font-size: 16px;">"SubThread1::obj's thread is MAIN thread; someSlot executed in MAIN thread;"
"SubThread2::obj's thread is SUB thread; someSlot executed in MAIN thread;"
"SubThread3::obj's thread is SUB thread; someSlot executed in SUB thread;" </span>

"SubThread1::obj's thread is MAIN thread; someSlot executed in MAIN thread;" 
"SubThread2::obj's thread is SUB thread; someSlot executed in MAIN thread;" 
"SubThread3::obj's thread is SUB thread; someSlot executed in SUB thread;"