Understanding Focus and Using QGuiApplication::focusObject() in Qt GUI

What it Does

• QGuiApplication::focusObject() returns a pointer to the currently focused Qt object within your application. This object could be a widget like a QPushButton, QLineEdit, or any other widget that can receive keyboard input.
• QWindow::focusObject() is not a member function of QWindow in Qt. It might be a confusion with QGuiApplication::focusObject().

Understanding Focus in Qt

• Only one widget can have focus at a time within your application.
• Focus refers to the widget that has the user's current attention. This means it's the widget that will receive keyboard events like key presses and releases.

When to Use QGuiApplication::focusObject()

• You might use QGuiApplication::focusObject() in situations where you need to:
  • Determine which widget currently has focus. This can be useful for implementing custom behavior based on the focused widget.
  • Access specific properties or methods of the focused widget. However, it's generally better to establish a parent-child relationship or use signals and slots for communication between widgets.

Example

#include <QApplication>
#include <QPushButton>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QPushButton button1("Button 1");
    QPushButton button2("Button 2");

    QObject::connect(&button1, &QPushButton::clicked, []() {
        QObject* focusedObject = QGuiApplication::focusObject();
        if (focusedObject) {
            qDebug() << "Button 1 clicked, focused object:" << focusedObject->objectName();
        }
    });

    button1.show();
    button2.show();

    return app.exec();
}

In this example:

• Clicking Button 1 will print the name of the currently focused object to the console.

• The focus can change due to various user interactions like clicking on another widget or using keyboard shortcuts. Be mindful of this when relying on the focused object.
• QGuiApplication::focusObject() returns nullptr if no widget has focus.

---

Checking Focus Before Performing an Action

#include <QApplication>
#include <QPushButton>
#include <QLineEdit>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QPushButton copyButton("Copy");
    QLineEdit textInput;

    QObject::connect(&copyButton, &QPushButton::clicked, [&textInput]() {
        if (QGuiApplication::focusObject() == &textInput) {
            // Get the text from the line edit and perform copy operation
            QString text = textInput.text();
            // Simulate copying (replace with your actual copy logic)
            qDebug() << "Copied text:" << text;
        } else {
            qDebug() << "Copy button clicked, but text input is not focused.";
        }
    });

    textInput.show();
    copyButton.show();

    return app.exec();
}

• The copyButton only performs the copy operation if the textInput widget is currently focused. This ensures the user has intended to copy the text they entered.

#include <QApplication>
#include <QPushButton>
#include <QLineEdit>

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QPushButton deleteButton("Delete");
    QLineEdit textInput;

    textInput.setReadOnly(true);  // Initially, text input is read-only

    QObject::connect(&textInput, &QLineEdit::gotFocus, [&textInput]() {
        textInput.setReadOnly(false);  // Allow editing when focused
    });

    QObject::connect(&textInput, &QLineEdit::lostFocus, [&textInput]() {
        if (textInput.text().isEmpty()) {
            textInput.setReadOnly(true);  // Make read-only again if empty
        }
    });

    QObject::connect(&deleteButton, &QPushButton::clicked, [&textInput]() {
        if (QGuiApplication::focusObject() == &textInput && !textInput.text().isEmpty()) {
            // Clear the text input since it's focused and has content
            textInput.clear();
        }
    });

    textInput.show();
    deleteButton.show();

    return app.exec();
}

• The deleteButton only clears the text if the textInput is focused and has content.
• If the user focuses on the textInput but doesn't enter any text and then clicks away, it becomes read-only again.
• When the user clicks on the textInput, it becomes editable.
• The textInput is initially read-only.

---

1. Signals and Slots
   

   • Establish a parent-child relationship between widgets.
   • Use signals emitted by the focused widget to react to focus changes.
   • This approach promotes better code organization and avoids relying solely on a global function.

   #include <QApplication>
   #include <QPushButton>
   
   class MyWidget : public QWidget {
       Q_OBJECT
   
   public:
       MyWidget(QWidget* parent = nullptr) : QWidget(parent) {
           // ... (widget setup)
       }
   
   signals:
       void focused();
       void lostFocus();
   
   private slots:
       void onFocus() {
           emit focused();
       }
   
       void onBlur() {
           emit lostFocus();
       }
   };
   
   int main(int argc, char *argv[]) {
       QApplication app(argc, argv);
   
       MyWidget widget;
       QObject::connect(&widget, &MyWidget::focused, []() {
           qDebug() << "Widget gained focus!";
       });
       QObject::connect(&widget, &MyWidget::lostFocus, []() {
           qDebug() << "Widget lost focus.";
       });
   
       widget.show();
       return app.exec();
   }
   

   In this example, the MyWidget class emits signals (focused and lostFocus) when it gains or loses focus. You can connect these signals to slots in your application logic to handle focus changes.

2. • Check if a specific widget has focus using QWidget::hasFocus().
   • This method is useful for targeted checks within your application.

   #include <QApplication>
   #include <QPushButton>
   
   int main(int argc, char *argv[]) {
       QApplication app(argc, argv);
   
       QPushButton button1("Button 1");
       QPushButton button2("Button 2");
   
       if (button1.hasFocus()) {
           qDebug() << "Button 1 is currently focused.";
       } else if (button2.hasFocus()) {
           qDebug() << "Button 2 is currently focused.";
       } else {
           qDebug() << "No button is focused.";
       }
   
       // ... (application logic)
   
       return app.exec();
   }
   

3. QWidget::focusWidget() (Qt versions >= 5.15)
   

   • Get the widget that has focus within a specific parent widget hierarchy.
   • This can be useful for traversing a widget tree and finding the focused child.

   #include <QApplication>
   #include <QWidget>
   #include <QVBoxLayout>
   
   int main(int argc, char *argv[]) {
       QApplication app(argc, argv);
   
       QWidget container;
       QVBoxLayout* layout = new QVBoxLayout(&container);
   
       QPushButton button1("Button 1");
       QPushButton button2("Button 2");
   
       layout->addWidget(&button1);
       layout->addWidget(&button2);
   
       QWidget* focusedWidget = container.focusWidget();
       if (focusedWidget) {
           qDebug() << focusedWidget->objectName() << " is focused within the container.";
       } else {
           qDebug() << "No widget is focused within the container.";
       }
   
       // ... (application logic)
   
       return app.exec();
   }