Reacting to Font Changes in Qt Applications: QFontDialog's currentFontChanged()

Purpose

• This signal is emitted by a QFontDialog object whenever the currently selected font within the font dialog changes.

Functionality

1. Connection
   You connect a slot (a function) to this signal using Qt's signal-slot mechanism.
2. Emission
   When the user interacts with the font dialog controls (e.g., selecting a different font family, size, or style), the currentFontChanged() signal is emitted.
3. Slot Execution
   The connected slot function is then invoked, receiving the new QFont object as an argument.

In essence, this signal provides a way to be notified and react to font changes within the font dialog in real-time.

Example Usage

#include <QApplication>
#include <QtWidgets>

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

    QFontDialog fontDialog;
    QFont currentFont;

    // Connect the currentFontChanged() signal to a slot
    QObject::connect(&fontDialog, &QFontDialog::currentFontChanged, [&currentFont](const QFont& font) {
        currentFont = font;
        // Update your application's UI or perform other actions based on the new font
        qDebug() << "Current font changed to:" << font.family() << font.pointSize();
    });

    fontDialog.exec(); // Show the font dialog

    return app.exec();
}

In this example:

1. A QFontDialog object (fontDialog) is created.
2. A variable currentFont is used to store the currently selected font.
3. The currentFontChanged() signal is connected to a lambda slot.
4. The lambda slot updates the currentFont variable and optionally performs actions based on the new font (e.g., printing it to the debug console in this case).
5. The fontDialog.exec() method displays the font dialog modally (blocks the main thread until the user closes it).

• The slot function receives the new QFont object as an argument, allowing you to access its properties like family, size, style, weight, etc.
• By connecting to currentFontChanged(), you can keep track of the user's font selections within the dialog and dynamically adjust your application's UI or perform other operations accordingly.

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Updating a QLabel with the Selected Font

This example shows how to update the font of a QLabel widget whenever the user selects a new font in the font dialog:

#include <QApplication>
#include <QtWidgets>

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

    QFontDialog fontDialog;
    QLabel label("This is some text");

    // Connect the currentFontChanged() signal to a slot
    QObject::connect(&fontDialog, &QFontDialog::currentFontChanged, [&label](const QFont& font) {
        label.setFont(font);
    });

    fontDialog.exec();
    label.show();

    return app.exec();
}

• The font dialog is displayed, and then the label is shown after the dialog is closed.
• The currentFontChanged() signal is connected to a slot that sets the font of the label to the newly selected font.
• A QLabel widget (label) is created with some initial text.

Applying the Selected Font to a QTextEdit

This example demonstrates how to apply the selected font from the font dialog to the text within a QTextEdit widget:

#include <QApplication>
#include <QtWidgets>

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

    QFontDialog fontDialog;
    QTextEdit textEdit;

    // Connect the currentFontChanged() signal to a slot
    QObject::connect(&fontDialog, &QFontDialog::currentFontChanged, [&textEdit](const QFont& font) {
        textEdit.setFont(font);
    });

    fontDialog.exec();
    textEdit.setPlainText("Enter your text here");
    textEdit.show();

    return app.exec();
}

Here:

• The font dialog is displayed, followed by showing the textEdit with some placeholder text.
• The currentFontChanged() signal is connected to a slot that sets the font of the textEdit.
• A QTextEdit widget (textEdit) is created for entering text.

Customizing the Font Dialog (Optional)

While not directly related to currentFontChanged(), you can optionally customize the behavior of the font dialog using methods like setOptions():

fontDialog.setOptions(QFontDialog::DontUseNativeDialog | QFontDialog::MonospacedFonts);

This code snippet sets the font dialog to not use the native system dialog and to only show monospaced fonts. Refer to the Qt documentation for more customization options:

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Property Changes

• In the slot function, check if the changed property is font to react to font modifications.
• You can connect to the propertyChanged() signal emitted by the widget whenever a property changes.
• Qt widgets have built-in properties like font and fontPointSize.

Example

#include <QApplication>
#include <QtWidgets>

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

    QLabel label("This is some text");
    QFont initialFont = label.font();

    // Connect to propertyChanged() signal
    QObject::connect(&label, &QLabel::propertyChanged, [&label, initialFont](const QMetaProperty* property) {
        if (property->name() == "font") {
            QFont newFont = label.font();
            if (newFont != initialFont) {
                // Font has changed, perform actions
                qDebug() << "Font changed to:" << newFont.family() << newFont.pointSize();
                initialFont = newFont; // Update initialFont for future comparisons
            }
        }
    });

    label.show();  // Show the label

    // ... (user interaction that might change the font)

    return app.exec();
}

• If the font has changed, it performs actions based on the new font.
• The slot checks if the changed property is font.
• We connect to the propertyChanged() signal of the label.
• We store the initial font of the label.

User Interaction Events

• If font changes are triggered by specific user interactions (e.g., clicking a button that sets the font), you can connect slots directly to those events.

Example

QPushButton* setFontButton = new QPushButton("Set Font");

// Connect button click to a slot that sets the font
QObject::connect(setFontButton, &QPushButton::clicked, [&label] {
    // Code to open a custom font selection dialog or set font programmatically
    label.setFont(/* new font */);
});

Custom Font Selection Widget

• You can implement your own logic to update the font of the target widget(s) based on user selections.
• If you need more control over font selection, consider creating a custom widget that allows users to choose fonts.

Choosing the Right Approach

The best approach depends on your specific requirements.

• For more granular control, consider creating a custom font selection widget.
• If font changes are triggered by specific user interactions, connecting to those events directly can be simpler.
• If you need to react to any font changes within a widget, including programmatic modifications, using propertyChanged() might be a good choice.