src_corelib_kernel_qobject.cpp

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// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
QObject *obj = new QPushButton;
obj->metaObject()->className();             // returns "QPushButton"
 
QPushButton::staticMetaObject.className();  // returns "QPushButton"
 
 
QPushButton::staticMetaObject.className();  // returns "QPushButton"
 
QObject *obj = new QPushButton;
obj->metaObject()->className();             // returns "QPushButton"
 
 
QObject *obj = new QTimer;          // QTimer inherits QObject
 
QTimer *timer = qobject_cast<QTimer *>(obj);
// timer == (QObject *)obj
 
QAbstractButton *button = qobject_cast<QAbstractButton *>(obj);
// button == nullptr
 
 
QTimer *timer = new QTimer;         // QTimer inherits QObject
timer->inherits("QTimer");          // returns true
timer->inherits("QObject");         // returns true
timer->inherits("QAbstractButton"); // returns false
 
// QVBoxLayout inherits QObject and QLayoutItem
QVBoxLayout *layout = new QVBoxLayout;
layout->inherits("QObject");        // returns true
layout->inherits("QLayoutItem");    // returns true (even though QLayoutItem is not a QObject)
 
 
qDebug("MyClass::setPrecision(): (%s) invalid precision %f",
       qPrintable(objectName()), newPrecision);
 
 
class MainWindow : public QMainWindow
{
public:
    MainWindow();
 
protected:
    bool eventFilter(QObject *obj, QEvent *ev) override;
 
private:
    QTextEdit *textEdit;
};
 
MainWindow::MainWindow()
{
    textEdit = new QTextEdit;
    setCentralWidget(textEdit);
 
    textEdit->installEventFilter(this);
}
 
bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
    if (obj == textEdit) {
        if (event->type() == QEvent::KeyPress) {
            QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);
            qDebug() << "Ate key press" << keyEvent->key();
            return true;
        } else {
            return false;
        }
    } else {
        // pass the event on to the parent class
        return QMainWindow::eventFilter(obj, event);
    }
}
 
 
myObject->moveToThread(QApplication::instance()->thread());
 
 
class MyObject : public QObject
{
    Q_OBJECT
 
public:
    MyObject(QObject *parent = nullptr);
 
protected:
    void timerEvent(QTimerEvent *event) override;
};
 
MyObject::MyObject(QObject *parent)
    : QObject(parent)
{
    startTimer(50);     // 50-millisecond timer
    startTimer(1000);   // 1-second timer
    startTimer(60000);  // 1-minute timer
 
    using namespace std::chrono;
    startTimer(milliseconds(50));
    startTimer(seconds(1));
    startTimer(minutes(1));
 
    // since C++14 we can use std::chrono::duration literals, e.g.:
    startTimer(100ms);
    startTimer(5s);
    startTimer(2min);
    startTimer(1h);
}
 
void MyObject::timerEvent(QTimerEvent *event)
{
    qDebug() << "Timer ID:" << event->timerId();
}
 
 
QPushButton *button = parentWidget->findChild<QPushButton *>("button1");
 
 
QListWidget *list = parentWidget->findChild<QListWidget *>();
 
 
QList<QWidget *> widgets = parentWidget.findChildren<QWidget *>("widgetname");
 
 
QList<QPushButton *> allPButtons = parentWidget.findChildren<QPushButton *>();
 
 
monitoredObj->installEventFilter(filterObj);
 
 
class KeyPressEater : public QObject
{
    Q_OBJECT
    ...
 
protected:
    bool eventFilter(QObject *obj, QEvent *event) override;
};
 
bool KeyPressEater::eventFilter(QObject *obj, QEvent *event)
{
    if (event->type() == QEvent::KeyPress) {
        QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
        qDebug("Ate key press %d", keyEvent->key());
        return true;
    } else {
        // standard event processing
        return QObject::eventFilter(obj, event);
    }
}
 
 
KeyPressEater *keyPressEater = new KeyPressEater(this);
QPushButton *pushButton = new QPushButton(this);
QListView *listView = new QListView(this);
 
pushButton->installEventFilter(keyPressEater);
listView->installEventFilter(keyPressEater);
 
 
MyWindow::MyWindow()
{
    QLabel *senderLabel = new QLabel(tr("Name:"));
    QLabel *recipientLabel = new QLabel(tr("Name:", "recipient"));
}
 
 
if (receivers(SIGNAL(valueChanged(QByteArray))) > 0) {
    QByteArray data;
    get_the_value(&data);       // expensive operation
    emit valueChanged(data);
}
 
 
QLabel *label = new QLabel;
QScrollBar *scrollBar = new QScrollBar;
QObject::connect(scrollBar, SIGNAL(valueChanged(int)),
                 label,  SLOT(setNum(int)));
 
 
// WRONG
QObject::connect(scrollBar, SIGNAL(valueChanged(int value)),
                 label, SLOT(setNum(int value)));
 
 
class MyWidget : public QWidget
{
    Q_OBJECT
 
public:
    MyWidget();
 
signals:
    void buttonClicked();
 
private:
    QPushButton *myButton;
};
 
MyWidget::MyWidget()
{
    myButton = new QPushButton(this);
    connect(myButton, SIGNAL(clicked()),
            this, SIGNAL(buttonClicked()));
}
 
 
QObject::connect: Cannot queue arguments of type 'MyType'
(Make sure 'MyType' is registered using qRegisterMetaType().)
 
 
 
disconnect(myObject, nullptr, nullptr, nullptr);
 
 
myObject->disconnect();
 
 
disconnect(myObject, SIGNAL(mySignal()), nullptr, nullptr);
 
 
myObject->disconnect(SIGNAL(mySignal()));
 
 
disconnect(myObject, nullptr, myReceiver, nullptr);
 
 
myObject->disconnect(myReceiver);
 
 
if (signal == QMetaMethod::fromSignal(&MyObject::valueChanged)) {
    // signal is valueChanged
}
 
 
void on_<object name>_<signal name>(<signal parameters>);
 
 
void on_button1_clicked();
 
 
class MyClass : public QObject
{
    Q_OBJECT
    Q_CLASSINFO("Author", "Pierre Gendron")
    Q_CLASSINFO("URL", "http://www.my-organization.qc.ca")
 
public:
    ...
};
 
Q_PROPERTY(QString title READ title WRITE setTitle USER true)
 
 
class MyClass : public QObject
{
    Q_OBJECT
 
public:
    MyClass(QObject *parent = nullptr);
    ~MyClass();
 
    enum Priority { High, Low, VeryHigh, VeryLow };
    Q_ENUM(Priority)
    void setPriority(Priority priority);
    Priority priority() const;
};
 
 
class QItemSelectionModel : public QObject
{
    Q_OBJECT
 
public:
    ...
    enum SelectionFlag {
        NoUpdate       = 0x0000,
        Clear          = 0x0001,
        Select         = 0x0002,
        Deselect       = 0x0004,
        Toggle         = 0x0008,
        Current        = 0x0010,
        Rows           = 0x0020,
        Columns        = 0x0040,
        SelectCurrent  = Select | Current,
        ToggleCurrent  = Toggle | Current,
        ClearAndSelect = Clear | Select
    };
 
    Q_DECLARE_FLAGS(SelectionFlags, SelectionFlag)
    Q_FLAG(SelectionFlags)
    ...
}
 
 
QPushButton *button = parentWidget->findChild<QPushButton *>("button1", Qt::FindDirectChildrenOnly);
 
 
QListWidget *list = parentWidget->findChild<QListWidget *>(Qt::FindDirectChildrenOnly);
 
 
QList<QPushButton *> childButtons = parentWidget.findChildren<QPushButton *>(Qt::FindDirectChildrenOnly);
 
QLabel *label = new QLabel;
QLineEdit *lineEdit = new QLineEdit;
QObject::connect(lineEdit, &QLineEdit::textChanged,
                 label,  &QLabel::setText);
 
void someFunction();
QPushButton *button = new QPushButton;
QObject::connect(button, &QPushButton::clicked, someFunction);
 
QByteArray page = ...;
QTcpSocket *socket = new QTcpSocket;
socket->connectToHost("qt-project.org", 80);
QObject::connect(socket, &QTcpSocket::connected, [=] () {
        socket->write("GET " + page + "\r\n");
    });
 
disconnect(myObject, &MyObject::mySignal(), nullptr, nullptr);
 
QObject::disconnect(lineEdit, &QLineEdit::textChanged,
                 label,  &QLabel::setText);
 
static const QMetaMethod valueChangedSignal = QMetaMethod::fromSignal(&MyObject::valueChanged);
if (isSignalConnected(valueChangedSignal)) {
    QByteArray data;
    data = get_the_value();       // expensive operation
    emit valueChanged(data);
}
 
void someFunction();
QPushButton *button = new QPushButton;
QObject::connect(button, &QPushButton::clicked, this, someFunction, Qt::QueuedConnection);
 
QByteArray page = ...;
QTcpSocket *socket = new QTcpSocket;
socket->connectToHost("qt-project.org", 80);
QObject::connect(socket, &QTcpSocket::connected, this, [=] () {
        socket->write("GET " + page + "\r\n");
    }, Qt::AutoConnection);
 
class MyClass : public QWidget
{
    Q_OBJECT
 
public:
    MyClass(QWidget *parent = nullptr);
    ~MyClass();
 
    bool event(QEvent* ev) override
    {
        if (ev->type() == QEvent::PolishRequest) {
            // overwrite handling of PolishRequest if any
            doThings();
            return true;
        } else  if (ev->type() == QEvent::Show) {
            // complement handling of Show if any
            doThings2();
            QWidget::event(ev);
            return true;
        }
        // Make sure the rest of events are handled
        return QWidget::event(ev);
    }
};
 
 
{
const QSignalBlocker blocker(someQObject);
// no signals here
}
 
const bool wasBlocked = someQObject->blockSignals(true);
// no signals here
someQObject->blockSignals(wasBlocked);
 
{
    QObject *obj;
    ...
    int id = obj->startTimer(100ms);
    if (id > Qt::TimerId::Invalid)
        // The timer has been started successfully
 
    if (id > 0) // Equivalent, albeit less readable
        // The timer has been started successfully
}