Qt4.7中，線程，信號，事件的一點理解

首先，寫個線程類，繼承自QThread，該線程做的事情很簡單：每兩秒打印一次自己的線程id，由於我對Qt的console打印函數不太瞭解，這裏還是使用c++的cout！

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <iostream>
using namespace std;

class Mythread : public QThread
{
    Q_OBJECT
public:
    explicit Mythread(QObject *parent = 0);

signals:

public slots:

protected:
	void run();

};

#endif // MYTHREAD_H

#include "mythread.h"

Mythread::Mythread(QObject *parent) :
    QThread(parent)
{
}

void Mythread::run()
{
	while(1)
	{
		cout << "thread id: " << QThread::currentThreadId() << endl;
		sleep(2);
	}
}

#include <QtCore/QCoreApplication>
#include "mythread.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    cout << "main thread id:" << QThread::currentThreadId() << endl;

    Mythread thread;
    thread.start();

    return a.exec();
}

這是一個Qt console進程，好了，運行一下，每隔兩秒打印一次子線程id。

接下來，我們給線程設置定時器，來替換sleep

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <QTimer>
#include <iostream>
using namespace std;

class Mythread : public QThread
{
    Q_OBJECT
public:
    explicit Mythread(QObject *parent = 0);

signals:

public slots:
	void mytimedout();

protected:
	void run();

	QTimer _timer;

};

#endif // MYTHREAD_H

#include "mythread.h"

Mythread::Mythread(QObject *parent) :
    QThread(parent)
{
}

void Mythread::run()
{
	connect(&_timer, SIGNAL(timeout()), this, SLOT(mytimedout()));
	cout << "child thread id: " << QThread::currentThreadId() << endl;
	_timer.start(2000);

	exec();

}

void Mythread::mytimedout()
{
	cout << "thread id: " << QThread::currentThreadId() << endl;
}

main與以前相同

發現並不能正常隔兩秒打印一次。根據我查的資料，大概原因是：QTimer是在主線程裏創建的，它發的信號只能與主線程相關的QObject來處理。詳細信息還需要細看文檔。

修改代碼，我們在子線程裏創建一個QTimer，並用它來發超時信號，子線程會阻塞在exec()這裏，所以不用擔心mytimer的生存期

將run改爲如下：

void Mythread::run()
{
	QTimer mytimer;
	connect(&mytimer, SIGNAL(timeout()), this, SLOT(mytimedout()));
	cout << "child thread id: " << QThread::currentThreadId() << endl;
	mytimer.start(2000);

	exec();

}

此時可以打印了，不過，我們發現打印的時候，顯示的是主線程id，也就是說，mytimedout這個函數是在主線程裏執行的。根據所查資料，這裏大概是因爲：工作線程在創建的時候，線程的事件處理，是依附在創建者身上的，我們需要在創建之後，調用QObject::moveToThread()來改變依附性，

main修改如下：

#include <QtCore/QCoreApplication>
#include "mythread.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

	cout << "main thread id:" << QThread::currentThreadId() << endl;

	Mythread thread;
	thread.start();
	thread.moveToThread(&thread);

    return a.exec();
}

這時候，便由子線程自己來打印了

接下來看下事件，QEvent！我們讓子線程往主線程發送QEvent，主線程接受到後，打印信息表示接收到了。

要讓主線程可以處理自定義事件，需要從QCoreApplication繼承一個類，並改寫虛函數event(QEvent *eve)

#ifndef MYAPP_H
#define MYAPP_H

#include <QCoreApplication>
#include <mythread.h>

class Myapp : public QCoreApplication
{
    Q_OBJECT
public:
	explicit Myapp(int argc, char* argv[], QObject *parent = 0);

	bool event( QEvent * e );

signals:

public slots:

};

#endif // MYAPP_H

#include "myapp.h"

Myapp::Myapp(int argc, char* argv[], QObject *parent) :
	QCoreApplication(argc, argv)
{
}

bool Myapp::event( QEvent * e )
{
	if(e->type() == (mytype1)){
		cout << "thread id:" << QThread::currentThreadId() << "recv a event, type is:" << e->type() << endl;
	}
	return QCoreApplication::event(e);
}

子線程裏，發送事件

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <QTimer>
#include <QEvent>
#include <iostream>
using namespace std;

enum Type{ mytype1 = QEvent::User+1, mytype2 }; // 這是我自定義的事件類型。這裏有個問題，可能是qt的bug，如果我編譯成功了，然後改mytype1=QEvent::User，再編譯運行，就會收不到事件


class Mythread : public QThread
{
    Q_OBJECT
public:
    explicit Mythread(QObject *parent = 0);

signals:

public slots:
	void mytimedout();

protected:
	void run();

	QTimer _timer;

};

#endif // MYTHREAD_H

#include <QCoreApplication>
#include "mythread.h"


Mythread::Mythread(QObject *parent) :
    QThread(parent)
{
}

void Mythread::run()
{
	QTimer mytimer;
	connect(&mytimer, SIGNAL(timeout()), this, SLOT(mytimedout()));
	cout << "child thread id: " << QThread::currentThreadId() << endl;

	mytimer.start(2000);

	exec();

}

void Mythread::mytimedout()
{
	cout << "thread id: " << QThread::currentThreadId() << endl;
	QEvent *eve = new QEvent((QEvent::Type)(mytype1));
	QCoreApplication::postEvent(QCoreApplication::instance(), eve);
}

#include <QtCore/QCoreApplication>
#include "mythread.h"
#include "myapp.h"

int main(int argc, char *argv[])
{
	Myapp a(argc, argv);

	cout << "reg value:" << QEvent::registerEventType(mytype1) << endl; // 這裏註冊一下，看該類型的事件是否被別人註冊過了，避免混淆
	cout << "reg value:" << QEvent::registerEventType(mytype2) << endl;

	cout << "main thread id:" << QThread::currentThreadId() << endl;

	Mythread thread;
	thread.start();
	thread.moveToThread(&thread);

    return a.exec();
}

我們在Mythread::mytimedout()裏new 了一個QEvnet對象，卻沒有去delete它，那麼它會內存泄漏嗎？其實不會，因爲這個事件被發出去後，處理該事件的對象在處理完該事件後，會delete它，比如這裏是

bool Myapp::event( QEvent * e )
{
	if(e->type() == (mytype1)){
		cout << "thread id:" << QThread::currentThreadId() << "recv a event, type is:" << e->type() << endl;
	}
	return QCoreApplication::event(e);
}

return QCoreApplication::event(e); 這裏是轉給父類去處理，估計最終會由QObject的event函數來delete掉。

我們可以自己寫個類繼承自QEvent，QEvent的析構函數爲虛。我們的類繼承它，別人delete這個類時，除了會執行自己的析構函數以外，也會調用QEvent的析構函數。

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <QTimer>
#include <QEvent>
#include <iostream>
using namespace std;

enum Type{ mytype1 = QEvent::User+1, mytype2 };

class Myevent:public QEvent
{
public:
	Myevent(QEvent::Type type):QEvent(type){}
	~Myevent(){
		cout << "thread id: " << QThread::currentThreadId() << "destroy event, type is:" << type() << endl;
	}
};


class Mythread : public QThread
{
    Q_OBJECT
public:
    explicit Mythread(QObject *parent = 0);

signals:

public slots:
	void mytimedout();

protected:
	void run();

	QTimer _timer;

};

#endif // MYTHREAD_H

void Mythread::mytimedout()
{
	cout << "thread id: " << QThread::currentThreadId() << endl;
	QEvent *eve = new Myevent((QEvent::Type)(mytype1));
	QCoreApplication::postEvent(QCoreApplication::instance(), eve);
}

可以看到Myapp的線程在處理完事件後，delete掉了事件，因爲調用了Myevent的析構函數。

最後，我們讓父類接收到mytype1事件後，往子線程發一個mytype2事件。

類Mythread需要重載event方法

bool Mythread::event( QEvent * e )
{
	switch(e->type())
	{
	case mytype2:
		cout << "child thread:" << QThread::currentThreadId() << "recv a event, type is:" << e->type() << endl;
		break;
	default:
		break;
	}
	return QThread::event(e);
}

Myapp的event變爲

bool Myapp::event( QEvent * e )
{
	if(e->type() == (mytype1)){
		cout << "thread id:" << QThread::currentThreadId() << "recv a event, type is:" << e->type() << endl;
		QEvent *eve = new Myevent((QEvent::Type)mytype2);
		postEvent(_recv, eve);
	}
	return QCoreApplication::event(e);
}

這裏_recv是子線程的QObject指針

void setReceive(QObject *obj){
		_recv = obj;
	}
protected:
	QObject *_recv;

main函數

#include <QtCore/QCoreApplication>
#include "mythread.h"
#include "myapp.h"

int main(int argc, char *argv[])
{
	Myapp a(argc, argv);

	cout << "reg value:" << QEvent::registerEventType(mytype1) << endl;
	cout << "reg value:" << QEvent::registerEventType(mytype2) << endl;

	cout << "main thread id:" << QThread::currentThreadId() << endl;

	Mythread thread;
	a.setReceive(&thread);

	thread.start();
	thread.moveToThread(&thread);

    return a.exec();
}

這樣，子線程每2秒處理一個超時信號，打印相關信息和往主線程發事件mytype1，主線程接收到該事件後，往子線程發一個mytype2的事件。子線程處理mytype2事件，打印信息

注意：

thread.moveToThread(&thread) // 這不是一個好的方法，這裏作爲例子這麼用可以更快速讓例子運行起來

參考：

http://www.qtcn.org/bbs/simple/?t32303.html

http://www.cnblogs.com/tankery/archive/2011/03/09/2004561.html

http://www.cnblogs.com/andreitang/archive/2011/08/03/2125815.html

Qt4.7中，線程，信號，事件的一點理解

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Qt4.7中，線程，信號，事件的一點理解

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