#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

signals:    //添加自定义的信号，信号强制为公有类型，所以没有前缀
    void SendMsg(QString str);  //信号只需要声明，不要给信号写实体代码

public slots:   //接收按钮信号的槽函数
    void ButtonClicked();

private:
    Ui::Widget *ui;
};

#endif // WIDGET_H

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);

    //关联
    connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(ButtonClicked()));
}

Widget::~Widget()
{
    delete ui;
}
//槽函数
void Widget::ButtonClicked()
{
    //用 emit 发信号
    emit SendMsg( tr("This is the message!") );
}

#ifndef SHOWMSG_H
#define SHOWMSG_H

#include <QObject>

class ShowMsg : public QObject
{
    Q_OBJECT
public:
    explicit ShowMsg(QObject *parent = 0);
    ~ShowMsg();

signals:

public slots:
    //接收 SendMsg 信号的槽函数
    void RecvMsg(QString str);
};

#endif // SHOWMSG_H

#include "showmsg.h"
#include <QMessageBox>

ShowMsg::ShowMsg(QObject *parent) : QObject(parent)
{

}

ShowMsg::~ShowMsg()
{

}

//str 就是从信号里发过来的字符串
void ShowMsg::RecvMsg(QString str)
{
    QMessageBox::information(NULL, tr("Show"), str);
}

#include "widget.h"
#include <QApplication>
#include "showmsg.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;   //①主窗体对象，内部会发送 SendMsg 信号
    ShowMsg s;  //②接收端对象，有槽函数 RecvMsg
    //③关联，信号里的字符串参数会自动传递给槽函数
    QObject::connect(&w, SIGNAL(SendMsg(QString)), &s, SLOT(RecvMsg(QString)));

    //显示主界面
    w.show();

    return a.exec();
}