Qt Window Resizing: QWindow::resize() and Alternatives

Purpose

• It takes two arguments:
  • width: The new width of the window in pixels.
  • height: The new height of the window in pixels.
• The QWindow::resize() function is used to change the size of a Qt window.

Usage

#include <QtWidgets>

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

    // Create a window
    QWidget window;

    // Set the initial size (optional)
    window.resize(400, 300);  // Width 400 pixels, Height 300 pixels

    // ... (add widgets and layout to the window)

    window.show();

    // Later, to resize the window programmatically:
    window.resize(600, 400);  // Change size to 600x400

    return app.exec();
}

Considerations

• Resizing the window might trigger layout updates for child widgets within the window. This ensures proper positioning and scaling based on the new size. You can override the QWidget::resizeEvent() handler in custom widgets to control this behavior.
• For more control over the content area size, consider using QWidget::setFixedSize(), QWidget::setMinimumSize(), or QWidget::setMaximumSize().
• QWindow::resize() directly resizes the window frame, including decorations like title bar and borders. The actual content area might be smaller depending on the window manager settings.

• Qt provides various layout managers (e.g., QHBoxLayout, QVBoxLayout, QGridLayout) to help you arrange and manage child widgets within the window. These layouts automatically adjust the positions and sizes of widgets when the window is resized.
• QWindow is the base class for most Qt window types like QWidget, QMainWindow, and QDialog. So, resize() can be used with any of these window types.

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Resizing the window on startup

#include <QtWidgets>

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

    // Create a window
    QWidget window;

    // Set the initial size during window creation
    window.resize(800, 600);

    // ... (add widgets and layout to the window)

    window.show();

    return app.exec();
}

In this example, window.resize() is called during window creation, setting the initial size to 800x600 pixels.

Resizing the window based on user interaction (e.g., a button click)

#include <QtWidgets>

class MyWindow : public QWidget {
    Q_OBJECT

public:
    MyWindow(QWidget *parent = nullptr) : QWidget(parent) {
        // Create a button
        QPushButton* resizeButton = new QPushButton("Resize Window");

        // Connect button click to resize slot
        connect(resizeButton, &QPushButton::clicked, this, &MyWindow::resizeWindow);

        // ... (add layout and other widgets)

        // Add the button to the layout
        // ...
    }

public slots:
    void resizeWindow() {
        window()->resize(650, 450); // Resize to a new size
    }
};

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

    MyWindow window;
    window.show();

    return app.exec();
}

Here, a QPushButton triggers the resizeWindow slot when clicked, which resizes the window to 650x450 pixels.

Resizing the window while maintaining aspect ratio

#include <QtWidgets>

class MyWindow : public QWidget {
    Q_OBJECT

public:
    MyWindow(QWidget *parent = nullptr) : QWidget(parent) {
        // ... (add widgets and layout)
    }

protected:
    void resizeEvent(QResizeEvent* event) override {
        QWidget::resizeEvent(event);

        // Maintain aspect ratio (adjust height based on width)
        int newWidth = event->size().width();
        int newHeight = newWidth * 3 / 4; // Adjust ratio as needed

        resize(newWidth, newHeight);
    }
};

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

    MyWindow window;
    window.show();

    return app.exec();
}

This example overrides the resizeEvent handler to maintain a specific aspect ratio (3:4) for the window. It calculates the appropriate height based on the new width and resizes the window accordingly.

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Setting Minimum and Maximum Sizes

• Use QWidget::setMinimumSize() and QWidget::setMaximumSize() to define the minimum and maximum allowable sizes for the window. This prevents the user (or code) from resizing the window beyond these limits.

window.setMinimumSize(300, 200);
window.setMaximumSize(800, 600);

Setting Fixed Size

• Use QWidget::setFixedSize() to set a fixed, unchangeable size for the window. This is useful for windows that should always have a specific size.

window.setFixedSize(500, 400);

Layout Management

• Leverage Qt's layout managers like QHBoxLayout, QVBoxLayout, or QGridLayout to arrange and size child widgets within the window. These layouts automatically adjust the positions and sizes of widgets when the window is resized. This ensures your UI adapts gracefully to different window sizes.

Size Hints

• Override the QWidget::sizeHint() method in your custom widgets to provide a recommended size based on the widget's content. This can be helpful when using layout managers.

class MyWidget : public QWidget {
  // ...
public:
  QSize sizeHint() const override {
    return QSize(200, 100); // Recommended size for the widget
  }
};

Choosing the Right Approach

The best approach depends on your specific needs:

• If you have custom widgets with specific size requirements, provide size hints using sizeHint().
• For dynamic layouts that adapt to different window sizes, use layout managers.
• If you need a fixed size that cannot be changed, use setFixedSize().
• If you want to prevent the window from becoming too small or too large, use setMinimumSize() and setMaximumSize().