怎么在Qt中写两个类，它们可以相互发信号与接收信号

#ifndef A_H#define A_H#include <QObject>#include <QDebug>class A : public QObject{    Q_OBJECTpublic:    explicit A(QObject *parent = 0) :        QObject(parent)    {    }    void sendSignalToB()    {        emit toB();    }signals:    void toB();public slots:    void fromB()    {        qDebug() << "i am A ,Rcv from B.";    }};#endif // A_H