Question

How we can move the window by pressing and holding the mouse in window client area.

Answer 1

Use QWidget::move() to move the window around. You detect when the user presses (with mousePressEvent()) and holds (with mouseMoveEvent()) the mouse and use those delta values to move the top-left corner os the window accordingly.

Holding the mouse means to set a flag in mousePressEvent()and to check that the flag is set in mouseMoveEvent(). Call globalPos() on the QMouseEvent passed to get the position of the cursor and save it so that you know the last cursor position between calls to mouseMoveEvent(). Then you calculate the difference between the last position and the current position and move the window the delta distance.

As an example add these to your window class:

class Window : public QWidget {

// ..

protected:
  void mousePressEvent(QMouseEvent *event);
  void mouseReleaseEvent(QMouseEvent *event);  
  void mouseMoveEvent(QMouseEvent *event);                                          

private:
  bool down;
  QPoint lastPos;
};

And one way to implement them would be:

void Window::mousePressEvent(QMouseEvent *event) {
  if (event->button() == Qt::LeftButton) {
    down = true;
    lastPos = event->globalPos();  
  }

  QWidget::mousePressEvent(event);  
}

void Window::mouseReleaseEvent(QMouseEvent *event) {
  down = false;  
  QWidget::mouseReleaseEvent(event);  
}

void Window::mouseMoveEvent(QMouseEvent *event) {
  if (down) {
    QPoint curPos = event->globalPos();
    if (curPos != lastPos) {
      QPoint diff = (lastPos - curPos);
      move(pos() - diff);
      lastPos = curPos;
    }
  }

  QWidget::mouseMoveEvent(event);
}

Now left clicking and holding in the client area will move the window around.