Event Handling in Qt - c++

Question

I am new to Qt GUI programming and come from a .NET/Winforms background as far as GUI development goes. I am using the Qt Creator IDE.
How can I handle a button press event in Qt to do something like the following:
if (button.clicked = true)
{
startProgram();
}
Also, how would I browse for a configuration file which would populate values into all of my line edit textboxes when opened? I am programming this in C++.
EDIT: I am taking a console app that someone else wrote and building a GUI around it. I want to access one of the functions in there from a button click event. However, I can't figure out how to get the functions of the original app to be in scope of the GUI which I have created when I try to use SIGNALS and SLOTS.

Answer 1

A simple example could be something like this. Say you have a class like ProgramStarter:
#include <QObject>
class ProgramStarter : public QObject {
Q_OBJECT
public slots:
void startProgram() {
//Do stuff
}
};
and something like this where ProgramStarter is used:
#include <QtGui/QApplication>
#include <QPushButton>
#include <QObject>
#include "programstarter.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QPushButton *testButton = new QPushButton("Push me");
testButton->resize(100, 40);
testButton->show();
ProgramStarter *program = new ProgramStarter;
QObject::connect(testButton, SIGNAL(clicked()), program, SLOT(startProgram()));
return app.exec();
}
Qt uses signals and slots to communicate between objects. The core idea is that signals are emitted when events occur and these signals can be connected to slots. In this case the button emits a clicked() signal when the button is pressed and the clicked() signal is connected to the startProgram() slot. So when the button is pressed, startProgram() is called.
Qt has a meta-object system for enabling the use of signals and slots, among other things. This is the reason why ProgramStarter inherits QObject and why the Q_OBJECT macro is in the header. I highly suggest reading this if you want to understand how signals and slots and the meta-object system work in Qt: http://doc.qt.io/qt-5/signalsandslots.html

Answer 2

It's a little long to explain but you connect the signal from the button to the slot with the function.
connect(button, SIGNAL(triggered()), this, SLOT(doSomething()));
You really need to read and understand signals and slots before trying to write code
edit: You are trying to call functions in a compiled separate running program?
Are you confusing Qt Signals with C/Unix signals? You could start the separate app from within your Qt app, get its PID and then send it a 'c' signal.
Or if you have the source of the app you could include it's code into your Qt app and then call any of the functions by simply wrapping them with a Qt slot handler.

Answer 3

In order to access the functions defined in the console application, you need to include the headers (.h/.hpp) that declare these functions, and link their implementation. I you have the source code of the implementations (.cpp), you need to add them to the sources to be compiled (they will be compiled in object files (.o) that will be linked in the final executable). If the console application is a front-end to a set of functions defined in a library (.lib, .a), you need to link this library.
After that, you can use Qt's signal/slot mechanism to connect the "button clicked" signal to a slot calling the function you want to execute, as described in other answers.

Answer 4

It seems like your problem is not really to do with Qt or GUI programming at all, but rather a general design issue. Others have told you how to be notified when the button is pressed - it's up to you to hook that notification up to whatever code you want to call. Some possibilities:
If you just need to call a function then include the appropriate header file and call it.
If you have an object you want to call a method on, then you will need to provide a reference to that object to the class which receives the notification.
If your console app is a separate program, you can use QProcess to launch and communicate with it.

Answer 5

I guess your problem is that you cannot call the console app's function, because it hasn't been defined to be a slot. So here is how you might do it.
Let's assume you have a console app class which might look like this:
class ConsoleApp
{
public:
void run()
{
// Code which you want to trigger by a button in your UI.
}
}
Then we implement a Qt based wrapper class which inherits from QObject class and, hence, is able to send and receive signals. This class simply provides a custom slot which delegates execution to the ConsoleApp class.
class ConsoleAppWrapper : public QObject
{
Q_OBJECT
public slots:
void startProgram()
{
m_consoleApp.run();
}
private:
ConsoleApp m_consoleApp;
}
Ok, and now we want a button which, when pressed, will trigger a call of the 'run()' method.
int main(int argc, const char** argv)
{
QApplication app(argc, argv);
// Define wrapper for console app.
ConsoleAppWrapper wrapper;
// Define button and connect its 'clicked()' signal
// to the wrapper's 'startProgram()' slot.
QPushButton startProgramButton("Start Program");
connect(&startProgramButton, SIGNAL(clicked()), &wrapper, SLOT(startProgram()));
// Show the button.
startProgramButton.setVisible(true);
// Start Qt's event loop.
app.exec();
}
This should give you a 'Start Program' button which, when pressed, will call ConsoleApp::run() method.
While this example is not Qt Designer based I hope this example helps you hunderstand how signals and slots work in Qt.
Cheers,
Jonny

Answer 6

If you have a button created with the Designer you can use on_myButton_clicked() slot in the parent widget class, just like in WinForms.