I'm new to QT please help me with how to implement reactive programming in Qt
If you want to use QML, then reactive programming will be very easy. Item properties in QML are naturally of reactive nature (just write a "java script" expression which uses other properties and the reactive connection will be established). Since Qt6, property bindings are available also in C++, see https://www.youtube.com/watch?v=pN0pRBUqrrc
Moreover, you can use property bindings by KDAB https://www.kdab.com/signals-slots-properties-bindings/
There is a project called rqxt that you may want to look into. Check out the samples folder.
For example, this is a sample for signals and slots:
#include <QApplication>
#include <QDebug>
#include <QKeyEvent>
#include <QLineEdit>
#include <QPushButton>
#include <QVBoxLayout>
#include <QWidget>
#include <rxqt.hpp>
int main(int argc, char* argv[])
{
QApplication a(argc, argv);
auto widget = std::unique_ptr<QWidget>(new QWidget());
auto layout = new QVBoxLayout;
widget->setLayout(layout);
{
auto e0 = new QLineEdit("Edit here");
auto e1 = new QLineEdit;
e1->setEnabled(false);
layout->addWidget(e0);
layout->addWidget(e1);
rxqt::from_signal(e0, &QLineEdit::textChanged)
.map([](const QString& s) { return "[[[" + s + "]]]"; })
.subscribe([e1](const QString& s) { e1->setText(s); });
rxqt::from_event(e0, QEvent::KeyPress)
.subscribe([](const QEvent* e) {
auto ke = static_cast<const QKeyEvent*>(e);
qDebug() << ke->key();
});
}
widget->show();
return a.exec();
}
Related
My child widget does not get keyPressEvents, while if I put the same widget as top level window, it does. I try to set it get focus, but it has no effect on this. Code is below, showing what I try to get to work.
#include <QApplication>
#include <QKeyEvent>
#include <QLCDNumber>
#include <QLabel>
#include <QVBoxLayout>
class DigitSummer: public QLCDNumber {
Q_OBJECT
public:
DigitSummer(QWidget *parent = nullptr) : QLCDNumber(parent) {
}
protected:
void keyPressEvent(QKeyEvent *event) override {
display(intValue() + event->text().toInt());
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
#if 1 // this version does not work, number does not increase
QWidget widget;
widget.setLayout(new QVBoxLayout());
widget.layout()->addWidget(new QLabel("Press digits!"));
DigitSummer summer; // in stack: must be after widget to avoid child delete
widget.layout()->addWidget(&summer);
widget.setFocusProxy(&summer); // I notice no effect!
widget.show();
#else // this version works, number grows with keypresseas
DigitSummer summer;
summer.show();
#endif
return a.exec();
}
#include "main.moc"
And for completenes, .pro file for the same:
QT += core gui widgets
TARGET = QtMCVE
TEMPLATE = app
DEFINES += QT_DEPRECATED_WARNINGS
CONFIG += c++11
QMAKE_CXXFLAGS += -Wall -Wextra
SOURCES += main.cpp
How to fix the widget to receive key events?
This related question suggests installing event filter, but I don't want to do that, there must be a self-contained way to fix the widget itself.
I think you need to set the focus policy for the widget before it will accept keyboard input. In your ctor try...
setFocusPolicy(Qt::StrongFocus);
Having said that, I'm really not sure why the behaviour would differ for top-level and non-top-level widgets.
Working version of the question code:
#include <QApplication>
#include <QKeyEvent>
#include <QLCDNumber>
#include <QLabel>
#include <QVBoxLayout>
class DigitSummer: public QLCDNumber {
Q_OBJECT
public:
DigitSummer(QWidget *parent = nullptr) : QLCDNumber(parent) {
setFocusPolicy(Qt::StrongFocus);
}
protected:
void keyPressEvent(QKeyEvent *event) override {
display(intValue() + event->text().toInt());
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QWidget widget;
widget.setLayout(new QVBoxLayout());
widget.layout()->addWidget(new QLabel("Press digits!"));
widget.layout()->addWidget(new DigitSummer);
widget.show();
return a.exec();
}
#include "main.moc"
I've recently started learning Qt and I'm a beginner of it now. So as first example for myself I wrote the following simple example.
The example is named Calculator. It now only has two buttons an a line edit. It's here:
:
My Calculator.h is this:
#ifndef CALCULATOR_H
#define CALCULATOR_H
#include<QDialog>
#include "ui_Calculator.h"
class Calculator : public QDialog, public Ui::Calculator
{
Q_OBJECT
public:
Calculator(QWidget* parent = 0);
private slots:
void myslot();
};
#endif // CALCULATOR_H
And the Calculator.cpp is this:
#include <QtWidgets>
#include "calculator.h"
Calculator::Calculator(QWidget *parent)
:QDialog(parent)
{
setupUi(this);
connect(oneButton,SIGNAL(clicked(bool)), this, SLOT(myslot()));
}
void Calculator::myslot(){
lineEdit -> setText("1");
}
And this is the main.cpp:
#include <QApplication>
#include <QDialog>
#include "ui_Calculator.h"
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
Ui::Calculator ui;
QDialog* dialog = new QDialog;
ui.setupUi(dialog);
dialog -> show();
return app.exec();
}
The program runs fine without any error. But when I click on 1 button, nothing will be printed/shown in the line edit. Why please?
And what part of my program should I change to solve the issue please?
You are setting up the wrong class in your main.
You should use your custom Calculator class and not QDialog.
setupUi only initializes your elements but your code in Calculator never gets called. Your main should look like this:
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
Calculator calc; //using your Calculator class.
calc.show();
return app.exec();
}
And don't include ui_calculator.h but calculator.h
I am having a bit of difficulty with some code. I am super rather new to Qt so it is entirely possible that I am simply ignorant to the problem I am having.
Basically, I am blocking out a program so that I can add the specifics of it later. I want to be able to create a grid of buttons, and when one of those buttons is pressed, another shape to replace it.
I am able to make my button grid, have it be scrollable, and have the button call it its position on the grid when pressed. However, when I try and use those coordinates to add another button to the grid, Qt crashes.
Here's my code so far:
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <cmath>
#include <QLabel>
#include <QMainWindow>
#include <QVBoxLayout>
#include <QGridLayout>
#include <QApplication>
#include <QPushButton>
#include <QScrollArea>
#include <QDebug>
#include <QString>
#include <QSignalMapper>
#include <QStringList>
#include <QLayoutItem>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
populateViewGrid(); //I wanted to see if I could add in a scrollbar
//from outside the main window. Could this be causing
// the issue?
}
void MainWindow::populateViewGrid()
{
QScrollArea*scrollArea = new QScrollArea(this);
QWidget*central = new QWidget(this);
QGridLayout*gridLayout = new QGridLayout(central);
QSignalMapper *signalMapper = new QSignalMapper(central);
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 3; j++)
{
QString position= QString("%1,%2").arg(i).arg(j);
QPushButton* button = new QPushButton("addTrack",central);
gridLayout->addWidget(button, i, j);
connect(button, SIGNAL(clicked()),signalMapper, SLOT(map()));
signalMapper->setMapping(button, position);
}
}
connect(signalMapper, SIGNAL(mapped(QString)),this, SLOT(addTrack(QString )));
central->setLayout(gridLayout);
scrollArea->setWidget(central);
setCentralWidget(scrollArea);
}
void MainWindow::addTrack(QString position)
{
QStringList query = position.split(",");
int x;
x=query.at(0).toInt();
int y;
y=query.at(1).toInt() ;
QPushButton *Ifthisworks=new QPushButton(this);
//This first line is where is crashes. I know this due to having the code
//laced with qDebugs. From all of my google searches and such, it seems that
// something simple should be wrong and I can't find it.
QLayoutItem * existingitem = gridLayout->itemAtPosition(x, y);
if(existingitem) {
gridLayout->removeItem(existingitem);
delete existingitem;
}
// before I included the above to remove the button from the grid point, the
//program would crash here.
gridLayout->addWidget(Ifthisworks, x, y);
}
MainWindow::~MainWindow()
{
delete ui;
}
main.cpp
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <cmath>
#include <QLabel>
#include <QMainWindow>
#include <QVBoxLayout>
#include <QGridLayout>
#include <QApplication>
#include <QPushButton>
#include <QMainWindow>
#include <QScrollArea>
#include <QSignalMapper>
#include <QHash>
//unrelated question, do I need the above in my code? I know not all of them
//used, but do I need includes in this file as well?
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
void populateViewGrid();
QGridLayout *gridLayout;
public slots:
void addTrack(QString);
private:
QScrollArea*scrollArea;
QWidget * central;
QPushButton *Ifthisworks;
QSignalMapper *signalMapper;
QPushButton *clockViews;
Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H
If you could help me understand how to not make Qt crash there and also add a button, that would be fantastic =)
So some background real quick incase you are looking at my code and are scratching your head. I'm a mechanical engineer who should have probably been an electrical or computer engineer and knows enough about coding to get myself into these kinds of messes. For the most part, I searched for what I wanted Qt to do and hacked it all together to hopefully make it work. Any bit of deeper understanding you can share would be more than welcome.
Thank you for your time.
You are initializing a local variable called gridLayout in your MainWindow::populateViewGrid() method:
QGridLayout*gridLayout = new QGridLayout(central);
Then in your MainWindow::addTrack(QString position) method, you are trying to access the member variable called gridLayout which is never initialized.
To fix this, simply initialize the member variable instead of creating a local variable in your MainWindow::populateViewGrid() method:
gridLayout = new QGridLayout(central);
You are doing the same mistake with your other member variables as well. Fix them the same way.
I would go for a different implementation.
Move gridlayout, signalmapper,... to be class members. I personally like also to keep a list of my widgets QList<QPushButton*> for manually deleting or keeping a button cache.
Make the signalmapper to map to index in list or QWidget*. For the example i`ll go with the QWidget* pointer.
QPushButton *newButton = new QPushButton("The new Thing");
QPushButton *oldButton = static_cast<QPushButton*>(widgetPointer);
gridLayout->replaceWidget(oldButton ,newButton);
buttonList->takeAt(buttonList->indexOf(oldButton))->deleteLater()); //if you keep a list..
buttonList->insert(buttonList->indexOf(oldButton),newButton);
I am writing a QtQuick desktop application. I use both c++ (for functionality) and QML (for UI) in it.
I use QQuickView to show the interface written in QML.
I want this application to reside in System Tray when minimised.
I mean a functionality similar to this example. http://qt-project.org/doc/qt-4.8/desktop-systray.html .
I am trying to implement this feature but could not find a way to do this in my Qt Quick application.
Here is my main.cpp code:
#include <QGuiApplication>
#include <QQmlEngine>
#include <QQmlContext>
#include <QQmlFileSelector>
#include <QQuickView>
#include "myapp.h"
int main(int argc, char* argv[])
{
QGuiApplication app(argc,argv);
app.setApplicationName(QFileInfo(app.applicationFilePath()).baseName());
QDir::setCurrent(qApp->applicationDirPath());
MyApp myappObject;
QQuickView view;
view.connect(view.engine(), SIGNAL(quit()), &app, SLOT(quit()));
view.rootContext()->setContextProperty("myappObject", &myappObject);
new QQmlFileSelector(view.engine(), &view);
view.setSource(QUrl("qrc:///myapp.qml"));
view.setResizeMode(QQuickView::SizeRootObjectToView);
view.show();
return app.exec();
}
Please help by providing any hint/pointers to do this.
Thanks.
I was facing the same challenge today and ended up using the following solution within main(). Works great for me when using Qt 5.3. You should of course implement a better way to check whether the first root object is your application window object or not.
#include <QApplication>
#include <QQmlApplicationEngine>
#include <QMessageBox>
#include <QAction>
#include <QMenu>
#include <QSystemTrayIcon>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
if (!QSystemTrayIcon::isSystemTrayAvailable()) {
QMessageBox::critical(0, QObject::tr("Systray"),
QObject::tr("I couldn't detect any system tray "
"on this system."));
return 1;
}
QApplication::setQuitOnLastWindowClosed(false);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
QObject *root = 0;
if (engine.rootObjects().size() > 0)
{
root = engine.rootObjects().at(0);
QAction *minimizeAction = new QAction(QObject::tr("Mi&nimize"), root);
root->connect(minimizeAction, SIGNAL(triggered()), root, SLOT(hide()));
QAction *maximizeAction = new QAction(QObject::tr("Ma&ximize"), root);
root->connect(maximizeAction, SIGNAL(triggered()), root, SLOT(showMaximized()));
QAction *restoreAction = new QAction(QObject::tr("&Restore"), root);
root->connect(restoreAction, SIGNAL(triggered()), root, SLOT(showNormal()));
QAction *quitAction = new QAction(QObject::tr("&Quit"), root);
root->connect(quitAction, SIGNAL(triggered()), qApp, SLOT(quit()));
QMenu *trayIconMenu = new QMenu();
trayIconMenu->addAction(minimizeAction);
trayIconMenu->addAction(maximizeAction);
trayIconMenu->addAction(restoreAction);
trayIconMenu->addSeparator();
trayIconMenu->addAction(quitAction);
QSystemTrayIcon *trayIcon = new QSystemTrayIcon(root);
trayIcon->setContextMenu(trayIconMenu);
trayIcon->setIcon(QIcon(":/resources/DatagnanLogoColor.png"));
trayIcon->show();
}
return app.exec();
}
Copy the Windos class (window.cpp/window.h) from systray example to your project, port it to Qt5 if necessary and open both from your main file:
int main(int argc, char* argv[])
{
// ...
QQuickView view;
// ...
view.show();
Window window;
window.show();
return app.exec();
}
I am trying to record sound by QAudioInput. According to the doc in this website QAudioInput. But when I ran, it exported an empty-raw file. After checking, It seems like the function QTimer::singleShot didn't working ( I added statement qWarning << "Done" in void stopRecording() and It didn't display "Done" so I thought it had some mistake in QTimer::singleShot function ).
This is my code used to check function QTimer::singleShot
----Check.pro----
QT += core
QT -= gui
TARGET = Check
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app
SOURCES += main.cpp
HEADERS += test.h
-----test.h------
#ifndef TEST_H
#define TEST_H
#include <QCoreApplication>
#include <QTimer>
#include <iostream>
#include <QObject>
#include <test.h>
#include <QDebug>
using namespace std;
class Object: public QObject {
Q_OBJECT
private slots:
void func() { cout << "Hello"; }
};
#endif // TEST_H
----main.cpp----
#include <QCoreApplication>
#include <QTimer>
#include <iostream>
#include <QObject>
#include <test.h>
#include <QDebug>
using namespace std;
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Object *o = new Object;
QTimer::singleShot(10000, o, SLOT(func()));
return 0;
}
And this code doesn't working, too. Can anyone explain me? I am newbie at Qt-programming.
Your program exits right after it's set the timer - it has no time to fire.
For the timer to work, you need an event loop running. Without the event loop, no events get processed.
Change the last line of your main to
return a.exec();
Also change your test slot by adding << std::endl or flush std::cout otherwise you might see no output on the console.
Your program should then work as expected (except it won't ever finish since nothing will cause the event loop to stop - just interrupt it).