Signal and slot mechanism of QT learning

1. Concept
   The signal and slot are Qt Define a communication mechanism in , Realize the interaction between objects ; When the state of the object changes , Will send a signal , The signal can be received by other objects , After receiving the signal, it will execute a specified operation function ( Slot ).

2. Definition

   class XX:public QObject{
         
   	Q_OBJECT //moc Metaobject compiler 
   signals:
   	void sig_func(...);// The signal 
   public slots:
   	void slot_func(...){
         ...}// Slot  
   };
   

   notes ： Signal functions need only be declared , Cannot write definitions
   notes ： Slot function can be connected to a signal , The execution of slot functions can be triggered by signals ; In addition, the slot function can also be used as a general member function , Call directly .

3. Signal and slot connection

   QObject::connect(
   	const QObject* sender,// Signal sending object  
   	const char* signal, // Signal function 
   	const QObject* receiver,// Receiving object of signal  
   	const char* method)// Slot function 
   

   SIGNAL( Signal function ( Parameter table ))// Convert the signal function to const char*
   SLOT( Slot function ( Parameter table ))// Convert the slot function to const char*

   Case study ： establish Qt Applications , It contains a label (QLabel) And a button (QPushButton), Click the button to close the label function .

   #include <QApplication>
   #include <QLabel>
   #include <QPushButton>
   #include <QFont>
   #include <QDialog>
   
   int main(int argc,char** argv){
         
       QApplication app(argc,argv);
       QDialog parent;// The parent window 
       parent.resize(480,320);
       // Create button and label controls 
       QPushButton button(" Turn off the tag ",&parent);
       QLabel label(" I'm the label ",&parent);
       // Resize and position 
       button.move(50,200);
       label.move(50,50);
       button.resize(150,50);
       label.resize(150,50);
       // Set font size 
       QFont font;
       font.setPointSize(20);
       parent.setFont(font);
       
       // Dynamically create button objects 
       QPushButton* button2 = 
           new QPushButton(" sign out ",&parent);
       button2->move(300,200);
       button2->resize(150,50);
   
       // Show parent window 
       parent.show();
       
       // Click the button to close the tab 
       QObject::connect(
           &button,SIGNAL(clicked(void)),
           &label,SLOT(close(void)));
       // practice : Use new Operate to dynamically create a button object , Wen benxian 
       // in " sign out ", Click this button to exit the application .
       QObject::connect(
           button2,SIGNAL(clicked(void)),
           //&app,SLOT(closeAllWindows(void)));
           &app,SLOT(quit(void)));
           //&parent,SLOT(close(void)));
   
       return app.exec();
   }
   

4. Syntax requirements for signal and slot connections
   1） Signal and slot parameters shall be consistent

   QObject::connect(A,SIGNAL(sigfun(int)), B,SLOT(slotfun(int)));//ok
   QObject::connect(A,SIGNAL(sigfun(void)), B,SLOT(slotfun(int)));//error
   

   2） There can be default parameters

   class B:public QObject{
         
   	Q_OBJECT
   public slots:
   	void slotfun(int = 0){
         }
   };
   QObject::connect(A,SIGNAL(sigfun(void)), B,SLOT(slotfun(void)));//ok
   

   3） The parameters of the signal can be more than slots , Redundant parameters will be ignored
   QObject::connect(A,SIGNAL(sigfun(int)),B,SLOT(slotfun(void)));//ok
   4） A signal can be connected to multiple slots at the same time （ One to many ）
   QObject::connect(A,SIGNAL(sigfun(int)),B1,SLOT(slotfun1(int)));
QObject::connect(A,SIGNAL(sigfun(int)),B2,SLOT(slotfun2(int)));
   notes ： When A Sending signal ,B1 and B2 All slot functions of will be executed , But the execution sequence is uncertain under multithreading .
   5） Multiple signals can be connected to the same slot at the same time ( How to 1)
   QObject::connect(A1,SIGNAL(sigfun1(int)), B,SLOT(slotfun(int)));
   QObject::connect(A2,SIGNAL(sigfun2(int)), B,SLOT(slotfun(int)));
   notes ： No matter what A1 still A2 Sending signal ,B All slot functions are executed
   6） The two signals can be directly connected ( The signals are connected in series )
   QObject::connect(A1,SIGNAL(sigfun1(int)), A2,SIGNAL(sigfun2(int)));
   A1–A2–>B
   notes ： If A1 Sending signal ,A2 The signal will also be sent

   Case study ： Event synchronization , Implement slider (QSlider) And the value selection box (QSpinBox) Synchronous operation
   1） slider ：QSlider
   QSlider( vertical / level , Parent window pointer );// Constructors
   void setRange(int min,int max);// Set the sliding range
   void valueChanged(int value);// The signal sent when the slider slides
   void setValue(int value);// Set the current slider position slot function
   2） Value selection box ：QSpinBox
   QSpinBox( Parent window pointer );// Constructors
   void setRange(int min,int max);// Set the value range of the value selection box
   void valueChanged(int value);// The signal sent when the value changes
   void setValue(int value);// Set the current value slot function

   #include <QApplication>
   #include <QSlider>// slider 
   #include <QSpinBox>// Value selection box 
   #include <QDialog>
   #include <QFont>
   int main(int argc,char** argv){
         
       QApplication app(argc,argv);
       // The parent window 
       QDialog parent;
       parent.resize(480,320);
       QFont font;
       font.setPointSize(20);
       parent.setFont(font);
       // slider 
       QSlider slider(Qt::Horizontal,&parent);
       slider.setRange(1,100);
       slider.move(100,50);
       // Value selection box 
       QSpinBox spin(&parent);
       spin.setRange(1,100);
       spin.move(100,200);
       // Show parent window 
       parent.show();
       // Slide the slider , The value of the selection box changes 
       QObject::connect(
           &slider,SIGNAL(valueChanged(int)),
           &spin,SLOT(setValue(int)));
       // The value of the selection box changes , The slider slides with it 
       QObject::connect(
           &spin,SIGNAL(valueChanged(int)),
           &slider,SLOT(setValue(int)));
   
       return app.exec();
   }
   

Note that the above code should be compiled according to qmake -project qmeke make …