I'd like to shutdown system if a button pressed more than 3 seconds.
how can I detect click duration while the button is pressed?
I can do it using pressed() and released() but not at press time.
You have to use a QTimer:
#include <QtWidgets>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QPushButton button("Press me");
QTimer timer;
timer.setSingleShot(true);
timer.setInterval(3 * 1000);
QObject::connect(&button, &QPushButton::pressed, &timer, [&timer](){
timer.start();
});
QObject::connect(&button, &QPushButton::released, &timer, [&timer](){
timer.stop();
});
QObject::connect(&timer, &QTimer::timeout, [](){
qDebug() << "shutdown";
});
button.show();
return a.exec();
}
Related
Whenever the application window is minimized or maximized i want to link that signal with a function.
This is the code.
int main(int argc, char *argv[])
{
QApplication application(argc, argv);
Renderer w(model ); // This is QWidget
w.show();
QObject::connect(&w, &QWindow::windowStateChanged, [&](Qt::WindowState state) {
});
// how will i define the QObject::connect
return application.exec();
}
What would be the parameters for the QObject::connect function ?
You cannot use the connect function to connect to different slots based on the given value. You can however simply call the functions based on the value by checking the value in your lambda.
At least, you could if you had the signal. However, your connect suggests that w is - or inherits - a QWindow. You can obviously only connect to signals your class provides. As your Renderer is a QWidget, you have to check that class.
The documentation of QWidget tells us, that there is no windowStateChanged signal, but it states:
When the window state changes, the widget receives a changeEvent() of type QEvent::WindowStateChange.
So therefor we can create our own signal and connect to that. This can look similar to the following working example:
#ifndef RENDERER_H
#define RENDERER_H
#include <QWidget>
#include <QEvent>
class Renderer : public QWidget {
Q_OBJECT
signals:
void stateChanged(bool isMaximized);
protected:
void changeEvent(QEvent *e)
{
if(e->type() == QEvent::WindowStateChange) {
emit stateChanged(windowState() & ~Qt::WindowMaximized);
}
QWidget::changeEvent(e);
}
};
#endif // RENDERER_H
int main(int argc, char *argv[])
{
QApplication application(argc, argv);
Renderer w; // This is QWidget
w.show();
QObject::connect(&w, &Renderer::stateChanged, [&](bool maximized) {
qDebug() << "Maximized?" << maximized;
});
return application.exec();
}
I was able to solve by using QApplication::focusWindow()
int main(int argc, char *argv[])
{
QApplication application(argc, argv);
Renderer w; // This is QWidget
w.show();
QObject::connect(QApplication::focusWindow(), &Renderer::stateChanged, [&](bool maximized) {
qDebug() << "Maximized?" << maximized;
});
return application.exec();
}
Is it expected behaviour that QDialog::show() does not show the window if a parent has been set?
Background: I want to use QMetaObject::connectSlotsByName() to react to one of the dialogs signals, which means the parent-object needs to own it. Without the line marked as "This is the line in question" I get a runtime-message "QMetaObject::connectSlotsByName: No matching signal for on_child_accepted()". But with the line, the child dialog no longer appears.
#include <QtCore/QDebug>
#include <QtWidgets/QApplication>
#include <QtWidgets/QDialog>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QVBoxLayout>
class Parent : public QDialog
{
Q_OBJECT
public:
Parent(QWidget *parent = nullptr, Qt::WindowFlags f = Qt::WindowFlags())
: QDialog{parent, f}
{
b.setText(tr("Show child"));
connect(&b, &QPushButton::clicked, [&]() {
c.show();
});
l.addWidget(&b);
setLayout(&l);
c.setParent(this); // This is the line in question
c.setObjectName("child");
QMetaObject::connectSlotsByName(this);
}
private slots:
void on_child_accepted()
{
qDebug() << "I got called";
}
private:
QPushButton b;
QDialog c;
QVBoxLayout l;
};
#include "main.moc"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Parent w;
w.show();
return a.exec();
}
This test fails on Qt 5.11 for Windows, from the MSYS2 64-bit build.
Any suggestions? Thank you in advance.
The dialog did actually appear, but not where you think it did: it is a non-window child widget of your window - it was transparent and yet it obscured most of the "Show Dialog" button, consuming all mouse events, and furthermore it was already shown since all children are shown when the parent is shown - so the button appeared to be non-functional for both of those reasons.
Setting a widget's parent clears its Qt::Window flag. Setting the dialog's background helps visualize the problem. Thus, you need to make the dialog a window after setting its parent.
The following reproduces your bug, and also demonstrates the fix.
// https://github.com/KubaO/stackoverflown/tree/master/questions/dialog-show-parenting-53208641
#include <QtWidgets>
class Parent : public QDialog {
Q_OBJECT
QVBoxLayout layout{this};
QDialog child;
QPushButton cShow{tr("Show child")}, cNonWindow{tr("Renew non-window child")},
cWindow{tr("Renew window child")};
Q_SLOT void on_child_accepted() {}
void reChild(bool makeWindow) {
child.~QDialog();
new (&child) QDialog;
Q_ASSERT(child.isWindow());
child.setParent(this);
child.setObjectName("child");
child.setStyleSheet("QWidget { background: blue }");
if (makeWindow) {
child.setWindowFlag(Qt::Dialog);
Q_ASSERT(child.isWindow());
} else {
Q_ASSERT(!child.isWindow());
child.show(); // The child gets shown when we're shown
}
QMetaObject::invokeMethod(this, &Parent::updateChild, Qt::QueuedConnection);
}
void updateChild() {
if (!child.isWindow()) child.move(50, cWindow.y() + cWindow.height() / 2);
this->update(); // Work around a refresh bug (affects OS X on 5.11 at least)
}
public:
Parent(QWidget *parent = nullptr, Qt::WindowFlags f = {}) : QDialog{parent, f} {
connect(&cShow, &QPushButton::clicked, [&]() { child.show(); });
connect(&cNonWindow, &QPushButton::clicked, [&] { reChild(false); });
connect(&cWindow, &QPushButton::clicked, [&] { reChild(true); });
for (auto *w : {&cShow, &cNonWindow, &cWindow}) layout.addWidget(w);
cNonWindow.click();
QMetaObject::connectSlotsByName(this);
}
};
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
Parent w;
w.show();
return a.exec();
}
#include "main.moc"
It is very straightforward to connect to QMenu::triggered or QMenu::hovered signals by calling QObject::connect and pass the appropriate QAction.
However, I do not know how to use QMenu::aboutToHide signal, as there is no action passed to that signal.
How to use QMenu::aboutToHide and QMenu::aboutToShow signals or those are just virtual functions that can be overridden?
The signals in the world of Qt are not functions, never invoke them. The signals notify that something has happened with the QObject and send information if necessary.
In the case of triggered and hovered it is necessary to send the QAction because several QActions in a QMenu, then the developer thought that it is necessary to know with which QAction was interacting. On the other hand with aboutToShow and aboutToHide the signal does not send anything because it wants to notify is that if the QMenu was shown or hidden, respectively. Is there any need to know that QMenu was shown or hidden if he did it ? no, because the sender did it, I do not use other properties that we do not have at hand.
Example of use:
#include <QApplication>
#include <QMainWindow>
#include <QMenuBar>
#include <QDebug>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QMainWindow w;
QMenu *foo_menu = w.menuBar()->addMenu("Foo Menu");
for(const QString & name: {"action1", "action2", "action3"}){
foo_menu->addAction(name);
}
QObject::connect(foo_menu, &QMenu::aboutToShow, [](){
qDebug()<<"aboutToShow";
});
QObject::connect(foo_menu, &QMenu::aboutToHide, [](){
qDebug()<<"aboutToHide";
});
QObject::connect(foo_menu, &QMenu::triggered, [](QAction *action){
qDebug()<< "triggered: " <<action->text();
});
QObject::connect(foo_menu, &QMenu::hovered, [](QAction *action){
qDebug()<< "hovered: " <<action->text();
});
w.show();
return a.exec();
}
And what happens if you have several QMenu that connect to the same slot? How do I know QMenu was shown or hidden?
The solution is to use sender() which is a method that belongs to the QObject class that returns the object that emitted the signal, in this case the QMenu.
Example:
#include <QApplication>
#include <QMainWindow>
#include <QMenuBar>
#include <QDebug>
class MainWindow: public QMainWindow{
public:
MainWindow(QWidget *parent=nullptr):
QMainWindow(parent)
{
for(const QString & name_of_menubar: {"bar1", "bar2", "bar3"}){
QMenu *menu = menuBar()->addMenu(name_of_menubar);
connect(menu, &QMenu::aboutToShow, this, &MainWindow::on_aboutToShow);
connect(menu, &QMenu::aboutToHide, this, &MainWindow::on_aboutToHide);
for(const QString & name: {"action1", "action2", "action3"}){
menu->addAction(name);
}
}
}
private slots:
void on_aboutToShow(){
if(QMenu *menu = qobject_cast<QMenu *>(sender()))
qDebug()<<"aboutToShow" << menu->title();
}
void on_aboutToHide(){
if(QMenu *menu = qobject_cast<QMenu *>(sender()))
qDebug()<<"aboutToHide" << menu->title();
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
I've created a Qt Quick 2.5 app and connected signals and slots to my C++ module,
I only can send signals from C++ and activate QML slots, but I can't receive QML signals on Qt side.
Here is main.cpp:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
AlfredApp(engine.rootObjects().first());
return app.exec();
}
here is my QML code which holds the signal:
MouseArea {
id: mainButtonMouseArea
objectName: "mainButtonMouseArea"
anchors.fill: parent
signal signalClicked()
onClicked: {
console.log("clicked")
signalClicked()
}
}
I always get the console message from QML when I click on the MouseArea.
Here is my c++ constructor:
AlfredApp::AlfredApp(QObject* viewRootObject, QObject* parent)
: QObject(parent), d(new Private)
{
d->viewRootObject = viewRootObject;
d->viewMainButton = viewRootObject->findChild<QObject*>("mainButton");
d->viewMainButtonIcon = viewRootObject->findChild<QObject*>("mainButtonIcon");
d->viewMainButtonMouseArea = viewRootObject->findChild<QObject*>("mainButtonMouseArea");
// Signals/Slots connection
connect(d->viewMainButtonMouseArea, SIGNAL(signalClicked()),
this, SLOT(mainButtonClicked()));
connect(this, SIGNAL(signalListening()),
d->viewMainButtonIcon, SLOT(listening()));
connect(this, SIGNAL(signalProcessing()),
d->viewMainButtonIcon, SLOT(processing()));
}
Here is my slot that never gets called:
void AlfredApp::mainButtonClicked()
{
qDebug() << "Main Button Clicked";
}
BTW, are there some qml code examples that have slots/function that respond normally to C++ signals
Just for testing, could you connect the signal and the slot in main.cpp?
Something like this:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/qml/main.qml")));
QObject *rootObj = engine.rootObjects().first();
QObject *item = rootObj->findChild<QObject*>("mainButtonMouseArea");
AlfredApp alfredapp(rootObj);
QObject::connect(item, SIGNAL(signalClicked()),
&alfredapp, SLOT(mainButtonClicked()));
return app.exec();
}
This should work. If not, try the idea suggested by #hyde.
Here you have the sample code.
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QDialog dlg;
dlg.exec();
return a.exec();
}
That's all my code, but when I close the window, The process isn't exit, it seems that drop in the loop a.exec().
Generally speaking, calling any exec is a bad idea, other than QCoreApplication::exec() or QDrag::exec(). The presence of exec() and waitForXxx() methods is an enticing trap for the unwary. Those methods are "easy" to use, but that ease comes at a price of hard to track bugs. Don't use them.
You should simply show the dialog:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QMessageBox msg;
msg.setText("Hello");
msg.addButton(QMessageBox::Close);
msg.show();
return a.exec();
}
If you wish to wait for the dialog to be accepted or rejected, you should use the dialog's clickedButton slot. QMessageBox has a long-standing bug that makes the accepted and rejected signals useless :(
// https://github.com/KubaO/stackoverflown/tree/master/questions/messagebox-show-25545652
#include <QtGui>
#if QT_VERSION >= QT_VERSION_CHECK(5,0,0)
#include <QtWidgets>
#endif
#include <functional>
[...]
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QMessageBox msg;
msg.setText("Continue?");
msg.addButton(QMessageBox::Yes);
msg.addButton(QMessageBox::No);
auto onClick = [&msg]() {
auto role = msg.buttonRole(msg.clickedButton());
if (role == QMessageBox::NoRole)
QApplication::quit();
if (role == QMessageBox::YesRole) {
auto label = new QLabel("I'm running");
label->setAttribute(Qt::WA_DeleteOnClose);
label->show();
}
};
#if QT_VERSION >= QT_VERSION_CHECK(5,0,0)
QObject::connect(&msg, &QMessageBox::buttonClicked, onClick);
#else
QObject::connect(&msg, SIGNAL(buttonClicked(QAbstractButton*)),
new FunctorSlot{onClick, &msg}, SLOT(call()));
#endif
msg.show();
return app.exec();
}
#include "main.moc"
For Qt 4, you need the following helper:
// Qt 4 only
struct FunctorSlot : public QObject {
Q_OBJECT
public:
std::function<void()> callable;
template <typename Fun>
FunctorSlot(Fun && fun, QObject * parent = {}) :
QObject{parent}, callable{std::forward<Fun>(fun)} {}
Q_SLOT void call() {
callable();
}
};
Possible solution:
QApplication a(argc, argv);
QDialog dlg;
QTimer::singleShot( &dlg, 0, SLOT(exec()) );
return a.exec();
It will work well. First - application event loop will be started. Then dialog event loop will be executed. After closing of dialog, both dialog and application loop will be finished. Application loop will be terminated automatically (by default), when last window is closed.
But, as noted by #thuga - there are no reason to call exec(). It is enough to call show() method.