QT槽函式獲取訊號傳送物件

平凡鍵客發表於2020-06-21
QT函式物件

Qt 在槽函式中獲取訊號傳送物件

Qt中提供了一個函式 qobject_cast(QObject *object),可以通過這個函式判斷訊號發出物件

Qt 幫助文件的解釋：
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.

  • 簡單來說 就是使用這個函式會得到你轉型的的一個例項，但是這個類必須繼承自QObject 或者其子類並且宣告Q_OBJECT 這個巨集
  • QT 幫助文件中一個Example
    QObject *obj = new QTimer; 
QTimer *timer = qobject_cast<QTimer *>(obj);

這裡順便記錄一下RTTI 與RAII，RAII 通常應用於物件資源管理，RTTI 可以動態判斷物件型別，但使用RTTI會增加程式執行時間，這裡簡單記錄區分一下 。

  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
    程式碼來源 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通過qobject_cast獲取訊號傳送物件的一個Demo,通過Qt Desinger 繪製兩個按鈕和一個文字框，將兩個按鈕的點選事件連線到同一個槽函式，在槽函式裡面判斷訊號的傳送者並作出不同的響應
主要的程式碼如下：

#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");
    }

}

完整的程式碼已上傳Github

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