Characteristics and usage of QT signal and slot

1. Concept ：

 The signal （ Signal ） It's an event launched in a particular situation  ,  for example  PushButton  The most common signal is the mouse 
 Fired on Impact  clicked（）  The signal .
 Slot （ Slot ） Is a function of the response to the signal . Slot is a function , With general  C＋＋ The function is the same , Can be defined in any part of the class （ public 、 private  or  protected ）, Can have any parameter , It can also be called directly .
 The difference between slot function and general function is  ： Slot functions can be associated with a signal , When the signal is transmitted , The associated slot function is automatically executed  .

Qt Meta object compiler C Meta-0 all ect Compiler, MOC ） It's a preprocessor , Put these before the source program is compiled Qt Feature program is converted to standard C＋＋ Compatible form , Then by the standard C＋＋ The compiler compiles . That's why in classes that use the signal and slot mechanism , You must add a Q_OBJECT The reason for the macro , Only by adding this macro , moc To preprocess the signal and slot code in the class .

About the use of signals and slots , There are some rules to note .
( 1） One signal can be connected to multiple slots ：

connect (spinNum, SIGNAL(valueChanged(int)), this , SLOT(addFun(int)) ;
connect (spinNum, SIGNAL(valueChanged (int)), this , SLOT(updateStatus (int));

When the signal and slot function have parameters , stay connect（） In the function , Specify the type of parameter , But you can not write the parameter name .

( 2 ） Multiple signals can To connect with A slot ：

connect (ui->rBtnBlue , SIGNAL (clicked()) , this, SLOT (setTextFontColor ()));
connect (ui->rBtnRed , SIGNAL (clicked()) , this , SLOT (setTextFontColor ())) ;
connect (ui->rBtnBlack , SIGNAL (clicked()), this , SLOT (setTextFontColor ()));

(3 ） A letter No To connect another Another signal ：

connect (spinNum, SIGNAL(valueChanged （int)), this , SIGNAL (refreshinfo(int)) ;

( 4 ） Strictly speaking , The number and type of parameters of signal and slot shall be consistent , At least the parameters of the signal shall not be less than the slot Of ginseng
Count . If it doesn't match , There will be compilation errors or running errors .

( 5 ） In classes that use signals and slots , You must add macros to the class definition QOBJECT.

( 6 ） When a signal is transmitted when , It's related to Slot letter Numbers are usually set The perform , Like a normal call Like a function .
Only when all slot functions associated with the signal are executed , Will execute the code behind the transmitting signal .

class QMyClass : public QObject
{
    
	QOBJECT// Reference signal and slot time , Must quote 
public：
	// Constructors 
    Widget(QWidget *parent = nullptr);
    // Destructor 
    ~Widget();
private：
	//
protected:
	//
private slots:	
	//
}

2. connect（） Different parameter forms of functions ：

(1) A function prototype in the form of parameters is ：

connect(sender , SIGNAL(signal()), , receiver, SLOT (slot())) ;

characteristic ： Macros are used here SIGNAL（） and l SLOT（） Specify signal and slot functions , And if the signal and slot function have parameters , The parameter type shall also be indicated

connect (sender , SIGNAL(valueChanged （int)), this , SLOT(updateStatus (int) );

（2） Another prototype of a function in the form of parameters is  ：

Applicable to ： For signals and slots with default parameters （ That is, the signal name is unique , There are no two signals with different parameters and the same name ） , You can use this form of function pointer for Association , Such as ：

connect (lineEdit,&QlineEdit::textChanged ,this, &widget::on_textChanged);

(3) It's easy to get wrong ：
 For signals with the same name with different parameters, the function pointer cannot be used to correlate the signal with the slot , for example QSpinBox()  There are two  valueChanged() The signal  :
 Insert picture description here