Discussion on QT learning 10 message processing in QT

Qt Study 10 On Qt Message processing in

Qt Message model

• Qt encapsulation Of the specific operating system Message mechanism
• Qt follow classical GUI Message driven time model

reflection

• Qt in How to represent user messages
• Qt in How to map user messages to message handling functions ？
• Qt in What rules do message mappings need to follow ？

Signal and slot

• Qt Concepts related to system messages are defined in

  The signal ( Signal )

  • Messages generated by the operating system

  Slot ( Slot )

  • Message handling functions in programs

  Connect ( Connect )

  • Bind system messages to message handling functions

• Qt Message handling mechanism in

The signal to slot connection must occur in two Qt Between class objects

• Qt Of The core - QObject::connect function

bool connect(const QObject* sender,		//  Send to 
             const char* signal,		//  Message name 
             const QObject* receiver,	//  The recipient 
             const char* method,		//  Accept member functions of objects 
             Qt::ConnectionType type=Qt::AutoConnection)

remarks ： stay Qt in , For message character string Describe ;connect Function in Message name and Processing function Between Building mapping

• Qt Medium “ new ” keyword

  SIGNAL

  • Used to specify the message name

  SLOT

  • Used to specify the name of the message handling function 1

  Q_OBJECT

  • All custom slot classes You must add... At the beginning of the class declaration Q_OBJECT

  slot

  • Used in classes Declare the message handling function

Custom slot

• Only OBject Subclass of can Custom slot
• The class that defines the operation must be in At the beginning of the declaration, use Q_OBJECT
• Class Declare slot When needed slots keyword
• Slot And the The signal stay Function signatures must be consistent
• SIGNAL and SLOT In the specified name ：
  • Can contain parameter types
  • Cannot contain specific parameter names
• Slot function through sender() function You can get the object of the signal

Calculator program plus signal slot

The following code contains only the signals and slots added , The previously repeated part is no longer shown

/* QCalculatorUI.h */
//  The header file included in the previous code 
class QCalculatorUI : public QWidget
{
    
    Q_OBJECT	//  Use the configuration that must be added to the signal slot 
private:
    //  Content in previous code 
private slots:	//  Custom slot function 
    void onButtonClicked();
public:
    //  Content in previous code 
};

/* QCalculatorUI.cpp */

//  Content in previous code 
bool QCalculatorUI::construct()
{
    
    //  Content in previous code 
    for(int i=0; (i<4) && ret; i++) {
    
        for(int j=0; (j<5) && ret; j++) {
    
            m_buttons[i*5 + j] = new QPushButton(this);
            if( m_buttons[i*5 + j] != NULL ) {
    
                //  Content in previous code 
                connect(m_buttons[i*5 + j], SIGNAL(clicked()), this, SLOT(onButtonClicked()));	//  Add a slot function to the click event of each button 
            }
            else {
    
                ret = false;
            }
        }
    }

    return ret;
}
//  Button click event custom slot function 
void QCalculatorUI::onButtonClicked()
{
    
    QPushButton* btn = (QPushButton*)sender();	//  adopt sender() The function knows which button was pressed 

    qDebug() << "onButtonClicked()";
    qDebug() << btn->text();
}
//  Content in previous code 

Tips:

• Solve classic problems ：Object::connect: No such slot …

  1. Inspection class Whether to inherit from QObject

  2. Check the beginning of the class declaration Whether to add Q_OBJECT

  3. Check Whether to use slots Keyword for slot declaration

  4. Check Whether the slot name is wrong

  5. Re execution qmake

Summary

• Signal and slot yes Qt The core mechanism of the system
• Different Qt Objects can communicate through signals and slots
• Only QObject Subclass of can Custom signals and slots
• Signals and slots must be used in At the beginning of the declaration, use Q_OBJECT
• The signal With the handler in Function signatures must be consistent