Qt Widgets: Beyond QSizeGrip::moveEvent() for Window Resizing

Purpose

• It avoids unnecessary updates during a resize operation.

When it's called

• This function is called whenever the QSizeGrip widget itself is moved. This can happen due to various reasons, but it's important in the context of resizing.

What it does

1. • The function first checks if a resize operation is already in progress using an internal flag.
   • If a resize is happening (!d->p.isNull()), it exits without further action. This is because the QSizeGrip itself is being moved as part of the resize, and no additional update is necessary.

2. Updates corner detection (if needed)
   

   • If no resize is in progress, it recalculates the corner position of the QSizeGrip relative to its top-level widget. This corner position is used to determine the appropriate cursor to display during resize.

3. Updates cursor (optional)
   

   • (For Qt versions without QT_NO_CURSOR and excluding macOS)
     • Based on the calculated corner position, it sets the cursor to indicate the resize direction (diagonal arrows for diagonally positioned grips).

In simpler terms

• It potentially updates the cursor to visually indicate the resizing direction.
• It avoids redundant updates while the resize is happening.
• This function ensures the QSizeGrip stays informed about its position within the top-level widget during a resize.

Additional points

• QSizeGrip itself relies on other event handlers like mousePressEvent and mouseMoveEvent to handle the actual resizing functionality.

---

#include <QApplication>
#include <QWidget>
#include <QHBoxLayout>
#include <QSizeGrip>

class MyWindow : public QWidget {
  Q_OBJECT

public:
  MyWindow() {
    // Create layout and widgets
    QHBoxLayout* layout = new QHBoxLayout(this);
    QLabel* label = new QLabel("This window is resizable!");
    layout->addWidget(label);

    // Add QSizeGrip
    QSizeGrip* sizeGrip = new QSizeGrip(this);
    layout->addWidget(sizeGrip);
    setLayout(layout);

    // Optional: Set minimum size (optional)
    setMinimumSize(200, 100);
  }

protected:
  // No need to override moveEvent here (for this example)

private:
  // ... (other member variables)
};

int main(int argc, char *argv[]) {
  QApplication app(argc, argv);
  MyWindow window;
  window.show();
  return app.exec();
}

1. We create a MyWindow class inheriting from QWidget.
2. A layout is created to arrange a label and the QSizeGrip.
3. The QSizeGrip is added to the layout and becomes a child of the window.
4. (Optional) A minimum size is set for the window.
5. The moveEvent function is not overridden in this example because the default behavior is sufficient.

How moveEvent is involved

• This ensures the grip stays in the bottom-right corner and updates the cursor if needed (depending on Qt version).
• Inside moveEvent, the grip recalculates its position relative to the new window position.
• This triggers the internal logic of QSizeGrip, potentially including the moveEvent function.
• Since QSizeGrip is a child of the window, it also gets moved along with the window.
• When the window is resized by dragging the edges (excluding the grip area), the window itself is moved.

Running the code

• Dragging the window edges will resize the window, and the grip will automatically adjust its position.
• This code creates a simple window with a label and a resize grip in the bottom-right corner.

• This is a basic example. You can explore other event handlers of QSizeGrip for more advanced resize behavior.

---

1. • You can create a custom widget that inherits from QWidget and implements the resize behavior.
   • This widget can handle mouse events like mousePressEvent and mouseMoveEvent to track user interaction and update the window size accordingly.
   • You can position this custom widget in the bottom-right corner of your window to mimic the QSizeGrip behavior.

2. Subclassing QWidget and Overriding resizeEvent()
   

   • You can inherit from QWidget for your main window class.
   • Override the QWidget::resizeEvent(QResizeEvent* event) function.
   • Inside this function, you can access the new and old size information from the QResizeEvent object.
   • Based on this information, you can perform custom logic to resize child widgets or adjust the layout of your window.

3. Using Layout Managers Effectively
   

   • Qt provides various layout managers like QHBoxLayout and QVBoxLayout.
   • By setting appropriate margins, spacing, and stretch factors for child widgets within the layout, you can achieve automatic resizing behavior when the window is resized.

Choosing the right approach

• For basic automatic resizing based on the window size, using layout managers effectively can be a good solution.
• If you need more control over the resizing behavior or want a custom resizing widget, creating a custom widget or subclassing QWidget is a better option.
• If you only need a basic visual cue for resizing without custom behavior, the default QSizeGrip is sufficient.