QT——信号槽

结合教程，写出如下关于信号槽的代码，将教程中信号槽两种方式写入同一个界面中。

#include "mainwindow.h"
#include <QApplication>
#include <QPushButton>
#include <QDebug>
#include <QHBoxLayout>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    //MainWindow w;
    //w.show();

    QWidget *myWidget = new QWidget;
    myWidget->setWindowTitle("Hello World");

    //信号槽
    QPushButton *button0 = new QPushButton;
    button0->setText("Quit");
    QObject::connect(button0, &QPushButton::clicked, &QApplication::quit);
    // Lambda表达式标准输出
    QPushButton *button = new QPushButton;
    button->setText("std::out");
    QObject::connect(button, &QPushButton::clicked, [](bool) {
        qDebug() << "You clicked me!";
    });

    QHBoxLayout *layout = new QHBoxLayout;
    //layout->addItem(button0);
    layout->addWidget(button0);
    layout->addWidget(button);
    myWidget->setLayout(layout);

    myWidget->show();

    return a.exec();
}

Qt5中信号槽的定义：

QMetaObject::Connection connect(const QObject *, const char *,
                                const QObject *, const char *,
                                Qt::ConnectionType);

QMetaObject::Connection connect(const QObject *, const QMetaMethod &,
                                const QObject *, const QMetaMethod &,
                                Qt::ConnectionType);

QMetaObject::Connection connect(const QObject *, const char *,
                                const char *,
                                Qt::ConnectionType) const;

QMetaObject::Connection connect(const QObject *, PointerToMemberFunction,
                                const QObject *, PointerToMemberFunction,
                                Qt::ConnectionType)

QMetaObject::Connection connect(const QObject *, PointerToMemberFunction,
                                Functor);

connect()一般会使用前面四个参数，第一个是发出信号的对象，第二个是发送对象发出的信号，第三个是接收信号的对象，第四个是接收对象在接收到信号之后所需要调用的函数。也就是说，当 sender 发出了 signal 信号之后，会自动调用 receiver 的 slot 函数。

connect(sender,   signal,   receiver,   slot);

自定义信号槽：

//newspaper.h

#ifndef NEWSPAPER_H
#define NEWSPAPER_H

#include <QObject>

//使用信号槽必须继承QBject类
class Newspaper : public QObject
{
    //QBject类及其子类，都必须在第一行写Q_OBJECT
    //该宏应放于头文件中
    Q_OBJECT
public:
    Newspaper(const QString & name) :
        m_name(name)
    {
    }

    void send()
    {
        //emit 的含义是发出newPaper()的信号
        emit newPaper(m_name);
    }

    //信号函数，返回值为void，参数是类要让外界获取的数据
signals:
    void newPaper(const QString &name);

private:
    QString m_name;
};
#endif // NEWSPAPER_H

//reader.h

#ifndef READER_H
#define READER_H

#include <QObject>
#include <QDebug>

class Reader : public QObject
{
    Q_OBJECT
public:
    Reader() {}

    //槽函数
    void receiveNewspaper(const QString & name)
    {
        qDebug() << "Receives Newspaper: " << name;
    }
};

#endif // READER_H

#include "mainwindow.h"
#include "newspaper.h"
#include "reader.h"
#include <QApplication>
#include <QPushButton>
#include <QDebug>
#include <QHBoxLayout>

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

    QCoreApplication app(argc, argv);
    //MainWindow w;
    //w.show();

    Newspaper newspaper("Newspaper A");
    Reader reader;
    //信号连接机制
    QObject::connect(&newspaper, &Newspaper::newPaper,
                     &reader,    &Reader::receiveNewspaper);
    newspaper.send();

    return app.exec();

}

编译文件，报错——LNK2001，无法链接解析的外部符号。

解决方法：将Project的同级编辑文件删除，重新编译既可。

输出：