Qt Widgets: Alternatives to Manually Hiding Scrollbars

• Reimplementation
  In QScrollBar, the hideEvent() function is reimplemented to handle specific behavior when the scrollbar is hidden.
• Inheritance
  QScrollBar inherits from QWidget, which provides the base functionality for widgets in Qt. QWidget itself has a hideEvent() function that gets called when the widget is hidden.

What it Does

The code inside QScrollBar::hideEvent() focuses on handling the scrollbar's internal state related to dragging:

• Stops Repeat Action
  It also sets the scrollbar's repeat action to SliderNoAction. This prevents any automatic scrolling that might occur due to holding the mouse button down on the slider.
• Releases Pressed State
  If the slider was pressed, the function resets the pressedControl variable to QStyle::SC_None, signifying that the press is no longer active.
• Checks for Pressed State
  It checks a private member variable (often denoted by d prefix in Qt), likely pressedControl, which indicates if the scrollbar's slider (the draggable part) is currently pressed.

In simpler terms

When you hide a scrollbar, this function ensures that any ongoing dragging interaction with the slider is stopped. It cleans up the scrollbar's internal state to reflect that it's no longer visible and user interaction is not expected.

• You wouldn't typically call hideEvent() directly in your code. It's an internal function that Qt's event system triggers when necessary.
• The specific implementation details might vary slightly between Qt versions, but the overall functionality remains similar.

---

#include <QtWidgets>

class MyWidget : public QWidget {
  Q_OBJECT

public:
  MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
    // Create a scrollbar
    scrollbar = new QScrollBar(Qt::Horizontal, this);

    // Layout the scrollbar
    QHBoxLayout *layout = new QHBoxLayout(this);
    layout->addWidget(scrollbar);
  }

signals:

public slots:
  void hideScrollbar() {
    scrollbar->hide();
  }

private:
  QScrollBar *scrollbar;

protected:
  bool event(QEvent *event) override {
    return QWidget::event(event);
  }
};

1. We define a MyWidget class that inherits from QWidget.
2. Inside the constructor, a horizontal scrollbar is created and added to a horizontal layout within the widget.
3. A hideScrollbar slot is defined to programmatically hide the scrollbar when triggered.
4. The event function is reimplemented from QWidget but doesn't modify the behavior in this example.

Functionality

• Even though we don't see the implementation details of hideEvent here, it would internally handle any ongoing dragging interaction with the slider and reset its state as explained earlier.
• When the scrollbar is hidden, Qt's event system triggers the hideEvent function within QScrollBar.
• Clicking a button connected to the hideScrollbar slot would hide the scrollbar.

---

1. Hiding the Scrollbar
   

• Use the hide() function on the QScrollBar object:

myScrollBar->hide();

• Set the scrollbar policy to Qt::ScrollBarAlwaysOff:

myScrollBar->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
myScrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);  // For QAbstractScrollArea

1. Reacting to Scrollbar Visibility Changes
   

• Connect to the visibilityChanged signal of the scrollbar:

connect(myScrollBar, &QScrollBar::visibilityChanged, this, &MyClass::handleScrollbarVisibility);

In the handleScrollbarVisibility slot, you can perform actions based on whether the scrollbar is visible or hidden.

1. Custom Scrollbar Behavior
   

• If you need more control over the scrollbar's behavior when hidden, consider subclassing QScrollBar and reimplementing relevant functions like mousePressEvent and mouseReleaseEvent to handle interactions differently when the scrollbar is hidden.

Choosing the Right Approach

The best approach depends on your specific needs:

• For highly customized behavior, subclassing QScrollBar might be necessary.
• If you need to react to visibility changes and perform actions accordingly, use the visibilityChanged signal.
• If you simply want to hide the scrollbar, use hide() or set the scrollbar policy.