QT槽函数获取信号发送对象

Posted 2020-12-11 arvinhuang

Qt 在槽函数中获取信号发送对象

Qt中提供了一个函数 qobject_cast(QObject *object),可以通过这个函数判断信号发出对象

Qt 帮助文档的解释：
Returns the given object cast to type T if the object is of type T (or of a subclass); otherwise returns nullptr. If object is nullptr then it will also return nullptr.
The class T must inherit (directly or indirectly) QObject and be declared with the Q_OBJECT macro.
A class is considered to inherit itself.
The qobject_cast() function behaves similarly to the standard C++ dynamic_cast(), with the advantages that it doesn‘t require RTTI support and it works across dynamic library boundaries.

• 简单来说 就是使用这个函数会得到你转型的的一个实例，但是这个类必须继承自QObject 或者其子类并且声明Q_OBJECT 这个宏
• QT 帮助文档中一个Example

  QObject *obj = new QTimer; 
  QTimer *timer = qobject_cast<QTimer *>(obj);
  

这里顺便记录一下RTTI 与RAII，RAII 通常应用于对象资源管理，RTTI 可以动态判断对象类型，但使用RTTI会增加程序运行时间，这里简单记录区分一下 。

1. RTTI : Run-time type information

   #include <iostream>
   #include <typeinfo>
   
   class Base {
   public:
       virtual ~Base() = default;
   };
   
   class Derived : public Base {};
   
   int main() {
       Base base;
       Derived derived;
       Base* ptr = &derived;
       Base& ref = derived;
       std::cout << typeid(base).name()<< std::endl;  // class Base 
       std::cout << typeid(derived).name()<< std::endl;  // class Derived
       std::cout << typeid(ptr).name()<< std::endl;  // class Base *
       std::cout << typeid(*ptr).name() << std::endl;  //class Derived           
       std::cout << typeid(ref).name() << std::endl;  //class Derived
   
   }
   

2. RAII : Resource Acquisition Is Initialization

   代码来源 https://en.cppreference.com/w/cpp/language/raii
   std::mutex m;
   
   void bad() 
   {
       m.lock();                    // acquire the mutex
       f();                         // if f() throws an exception, the mutex is never released
       if(!everything_ok()) return; // early return, the mutex is never released
       m.unlock();                  // if bad() reaches this statement, the mutex is released
   }
   
   void good()
   {
       std::lock_guard<std::mutex> lk(m); // RAII class: mutex acquisition is initialization
       f();                               // if f() throws an exception, the mutex is released
       if(!everything_ok()) return;       // early return, the mutex is released
   }
   

下面是QT通过qobject_cast获取信号发送对象的一个Demo,通过Qt Desinger 绘制两个按钮和一个文本框，将两个按钮的点击事件连接到同一个槽函数，在槽函数里面判断信号的发送者并作出不同的响应
主要的代码如下：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

public slots:
    void onButtonClicked();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H


#include "mainwindow.h"
#include "ui_mainwindow.h"
#include<QDebug>
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(ui->leftButton,&QPushButton::clicked,this,&MainWindow::onButtonClicked);
    connect(ui->rightButton,&QPushButton::clicked,this,&MainWindow::onButtonClicked);

}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onButtonClicked()
{
    QPushButton *button =  qobject_cast<QPushButton *>(sender());

    ui->textLabel->setText(button->text());

    if(ui->leftButton == button)
    {
        qDebug()<<"left Button clicked";
        ui->textLabel->setStyleSheet("background-color:yellow");
        button->setStyleSheet("background-color:yellow");

    }
    else
    {
        ui->textLabel->setStyleSheet("background-color:green");
        button->setStyleSheet("background-color:green");
    }

}

完整的代码已上传Github