Signals Slots

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Signals & Slots



Signals & Slots

• Signals and slots is an object to objectcommunication technique in Qt

• Only QObjects can communicate viasignals and slots

• It can be used in both GUI and consoleapplications



To utilise signals & slots

A class should

• inherit from the QObject class orinherit from a class that inherits fromthe QObject class

• include the QO!"#$% macro



Signals and Slots: Definition and Implementation

Signals and slots

• are defined as void member functionsof a class

•

can have input parameter lists




A slot

• must be implemented in the class

• can be invoed as a member function

on a object of the class




A signal

• does not have an implementation

• cannot be invoed as a member

function



Signals and Slots: Example 1 (non-GUI)

Product definition Product implementation

#ifndef PRODUCT_H#define PRODUCT_H#include <QString>

class Product{ pulic!

Product"QString d int n$% &oid sell"int n$% pri&ate! QString description%

int nu'%(%#endif

Product!!Product"QString dint n$!description"d$ nu'"n${(

&oid Product!!sell"int n${ if"n <) nu'$ nu' ) nu' * n%(




Product definition Product implementation#ifndef PRODUCT_H#define PRODUCT_H#include <QString>#include <QObject>

class Product: public QObject{ Q_OBJECT pulic!

Product"QString d int n$% &oid sell"int n$% public signals:

void restoc!Q"tring d$ pri&ate! QString description% int nu'%(%#endif

Product!!Product"QString dint n$!description"d$ nu'"n${(

&oid Product!!sell"int n${ if"n <) nu'$ nu' ) nu' * n% i% !nu& '' ( e&it restoc!description$(




Order)ist definition Order)ist implementation

#ifndef ORD+R,-ST_H#define ORD+R,-ST_H#include <QString,ist>

class Order,ist{ pulic!

QString,ist get,ist"$const%

&oid add"QString d$%pri&ate! QString,ist descriptions%(%#endif

QString,ist Order,ist!!get,ist"$const{ return descriptions%(

&oid Order,ist!!add"QString d${ descriptions.append"d$%(




Order)ist definition Order)ist implementation#ifndef ORD+R,-ST_H#define ORD+R,-ST_H#include <QString,ist>#include <QObject>

class Order,ist ! publicQObject*Q_OBJECT pulic!

QString,ist get,ist"$const%

public slots: &oid add"QString d$%pri&ate! QString,ist descriptions%(%#endif

QString,ist Order,ist!!get,ist"$const{ return descriptions%

(

&oid Order,ist!!add"QString d${ descriptions.append"d$%(




One last step in this e'ample is toconnect the signal and slots(

&ain! function

Product c/eese""0reen 1ar'" 2$% 33 an instance of Product

Order,ist list% 33 an instance of Order,ist

QO4ect!!connect"5c/eese S-067,"restoc8"QString$$ 5listS,OT"add"QString$$$%

c/eese.sell"2$% 33 results in "0reen 1ar'" eing added to t/e

33 list of descriptions of Order,ist o4ect33 list



Refletion on Example 1

• Signals and slots in a non)GUIprogram

• User)defined classes *ith signals andslots



!ext

+e *ill

• see ho* to use signals and slots in a

GUI program

•use signals and slots from the e'istingQt classes



Re#ui$ements fo$ t%e GUI p$og$am

%he program should generate thefollo*ing GUI

and clicing on the button Clear shouldclear the line edit



Details of t%e GUI

Q,aelQ,ine+dit

QPus/9utton

Q:9o;,aout

,efer to the A-I documentation forfurther details of these classes.

Q=idget



Example : (GUI p$og$am)

++include necessar, pre-processor directives .ereint &ain!int argc/ c.ar 0argv12*

Q3pplication a!argc/ argv$Q4idget 5$Q)abel 0label ' ne5 Q)abel!6Enter Te7t:6$Q)ineEdit 0input ' ne5 Q)ineEdit!68ello6$QPus.Button 0clear ' ne5 QPus.Button!6Clear6$Q9Bo7)a,out 0la,out ' ne5 Q9Bo7)a,out!$la,out->add4idget!label$la,out->add4idget!input$la,out->add4idget!clear$

5set)a,out!la,out$

QObject::connect!clear/ ";=3)!cliced!/ input/")OT!clear!$

5s.o5!$return ae7ec!$

'etsa$e

$eated

*a+out isused to

plae t%e ,idgets

Signal and slotonneted



Refletion on Example

• Signals and slots in a GUI program

• Used signals and slots that alreadye'ist in Qt classes



Summa$+

%his presentation

• e'plained object to object

communication in Qt using signalsand slots

• contains simple e'amples that shouldhelp you understand the basics ofsignals and slots



a+ .o$,a$d//

/ou should e'plore signal and slots byusing them in your console and GUIapplications.

%hat0s the best *ay to understand them1

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Signals Slots