Understanding QSizeGrip::mouseReleaseEvent() in Qt Widgets

Inheritance and Reimplementation

• QSizeGrip reimplements this function to handle the left mouse button release event specifically.
• Qt provides a default implementation for mouseReleaseEvent() in QWidget.
• QSizeGrip inherits from QWidget.

Functionality

• Otherwise:
  • It calls the parent class's mouseReleaseEvent() implementation, likely for handling any generic mouse release behavior.
• If it is the left button:
  • Internal variables (gotMousePress and p) used for tracking the resize operation are reset. These likely keep track of the initial mouse press position to calculate the resize offset.
• It checks if the pressed button is the left mouse button (Qt::LeftButton).
• This function is called when the user releases the mouse button after clicking and dragging on the size grip.

In essence

• By resetting internal variables, it prepares the size grip to handle a new resize interaction.
• This function finalizes the resize operation initiated by a left mouse button press on the size grip.

• This function works in conjunction with other reimplemented functions in QSizeGrip like mousePressEvent() and mouseMoveEvent() to handle the entire resize interaction.

---

#include <QtWidgets>

class MainWindow : public QWidget {
  Q_OBJECT

public:
  MainWindow(QWidget *parent = nullptr) : QWidget(parent) {
    // ... other widget initialization

    // Create the size grip and set its geometry
    QSizeGrip *grip = new QSizeGrip(this);
    grip->setGeometry(rect().bottomRight() - grip->size());
  }

protected:
  void mousePressEvent(QMouseEvent *event) override {
    if (event->button() == Qt::LeftButton) {
      // Handle initial mouse press for potential resize
    }
    QWidget::mousePressEvent(event);
  }

  void mouseMoveEvent(QMouseEvent *event) override {
    if (event->buttons() & Qt::LeftButton) {
      // Handle mouse drag for resizing
    }
    QWidget::mouseMoveEvent(event);
  }
};

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

1. We create a QSizeGrip object and set its geometry to the bottom-right corner of the main window.
2. We override the mousePressEvent and mouseMoveEvent functions of the main window to handle the user interaction for resizing.
3. Inside mousePressEvent, we check if the left mouse button is pressed. This could be where you might call grip->startTracking() (if such a function exists) to indicate the beginning of a resize operation.
4. In mouseMoveEvent, we handle the left mouse button drag to update the window size based on the mouse movement.

• This is a simplified example, and the actual implementation might involve more logic and error handling based on your specific requirements.

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Reimplementing QWidget's mouse events

• In mouseReleaseEvent(), finalize the resize operation (optional).
• Use the difference to update the window geometry (width and height) during dragging.
• In mouseMoveEvent(), calculate the difference between the current and initial mouse positions.
• In mousePressEvent(), check for the left button press and record the initial mouse position.
• You can directly override the mousePressEvent(), mouseMoveEvent(), and mouseReleaseEvent() functions in your main widget class.

This method offers more control over the resizing behavior but requires more manual implementation compared to QSizeGrip.

Using a custom QLayout

• This approach provides a more integrated solution for managing the layout and resizing of your window.
• Override the layout's virtual functions like minimumSize(), sizeHint(), and setGeometry() to handle the resizing logic based on the child widgets and desired behavior.
• You can create a custom layout class that inherits from QLayout.

It's generally more complex to implement a custom layout but can be useful for achieving specific resizing behaviors that are difficult with standard layouts.

• Libraries like Qt Extended offer additional widgets and functionalities, including custom grip widgets that might provide more features or customization options compared to the basic QSizeGrip.