Alternatives to QScroller::finalPosition() for Tracking Scrolling Behavior in Qt Widgets

Purpose

• Kinetic scrolling refers to the smooth, momentum-based scrolling behavior that continues even after the user lifts their finger or stops interacting with the scroll bar.
• In Qt Widgets, QScroller::finalPosition() is a function used to retrieve the target position (in pixels) that a kinetic scroll operation is aiming to reach.

Key Points

• The returned value is only meaningful when the scroller is in an active scrolling state. If the scroller is inactive (not currently scrolling), the result is undefined.
• The function returns a QPointF object, which holds the final position for both the x and y axes.

Usage

1. Include Necessary Header
   Make sure you have included the QScroller header file in your code:

   #include <QtWidgets/qscroller.h>
   

2. Obtain a QScroller Object
   You might have a QScroller object associated with a scrolling widget in your Qt application. The exact way to obtain this object depends on your specific widget setup. Here are some common scenarios:

   • If you're using a standard scrolling widget like QListView or QScrollArea, the scroller might be accessible through a property or method. Consult the documentation for your specific widget class to determine the appropriate way to retrieve the QScroller object.

3. Check Scroller State (Optional)
   While not strictly necessary, you can optionally check the scroller's state using QScroller::scrollerState() before calling finalPosition(). This ensures that you only attempt to read the final position when the scroller is actively scrolling.

4. Call finalPosition()
   Once you have a valid QScroller object and (optionally) confirmed its active state, call finalPosition() to get the target scrolling position:

   QPointF targetPosition = myScroller->finalPosition();
   

   The targetPosition will contain the x and y coordinates (in pixels) that the kinetic scroll is aiming to reach.

Example

#include <QtWidgets/qapplication.h>
#include <QtWidgets/qwidget.h>
#include <QtWidgets/qscrollarea.h>
#include <QtWidgets/qscroller.h>

class MyWidget : public QWidget {
    Q_OBJECT

public:
    MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
        // ... (create scroll area and other content)
    }

private slots:
    void onScrollFinished() {
        QScroller *scroller = scrollArea->horizontalScroller();
        if (scroller->scrollerState() == QScroller::Active) {
            QPointF targetPosition = scroller->finalPosition();
            // Do something with the target position, e.g., print it
            qDebug() << "Target scroll position (X, Y):" << targetPosition;
        } else {
            qDebug() << "Scroller is not in active state";
        }
    }

private:
    QScrollArea *scrollArea;
};

In this example, the onScrollFinished() slot is triggered when the kinetic scrolling animation finishes. It checks the scroller's state and, if active, retrieves the finalPosition().

---

#include <QtWidgets/QApplication>
#include <QtWidgets/qwidget.h>
#include <QtWidgets/qscrollarea.h>
#include <QtWidgets/qscroller.h>
#include <QtWidgets/qlabel.h>
#include <QtCore/qdebug.h>

class ScrollingContent : public QWidget {
    Q_OBJECT

public:
    ScrollingContent(QWidget *parent = nullptr) : QWidget(parent) {
        // Create some large content that needs scrolling
        for (int i = 0; i < 100; ++i) {
            QLabel *label = new QLabel(QString("Item %1").arg(i + 1), this);
            label->setAlignment(Qt::AlignCenter);
            label->setMinimumHeight(40);
            QVBoxLayout *layout = new QVBoxLayout(this);
            layout->addWidget(label);
        }

        setLayout(layout);
        setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    }
};

class MyWidget : public QWidget {
    Q_OBJECT

public:
    MyWidget(QWidget *parent = nullptr) : QWidget(parent) {
        scrollArea = new QScrollArea(this);
        content = new ScrollingContent(scrollArea);
        scrollArea->setWidget(content);
        scrollArea->setWidgetResizable(true);  // Allow content to resize scroll area

        // Connect signal to track scroll events (optional)
        connect(scrollArea->horizontalScroller(), &QScroller::scrollingFinished, this, &MyWidget::onScrollFinished);

        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(scrollArea);
        setLayout(layout);
    }

private slots:
    void onScrollFinished() {
        QScroller *scroller = scrollArea->horizontalScroller();
        if (scroller->scrollerState() == QScroller::Active) {
            QPointF targetPosition = scroller->finalPosition();
            qDebug() << "Target scroll position (X, Y):" << targetPosition;

            // Example: Optionally adjust some UI element based on target position
            int scrollThreshold = content->width() - scrollArea->viewport()->width();
            if (targetPosition.x() > scrollThreshold) {
                qDebug() << "Scrolled near the end of content";
                // You could load more content here or perform another action
            }
        } else {
            qDebug() << "Scroller is not in active state";
        }
    }

private:
    QScrollArea *scrollArea;
    ScrollingContent *content;
};

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

In this example:

• The example also includes an optional check to see if the scroll position is near the end of the content (based on the content's width and the viewport's width). This could be used to trigger actions like loading more content.
• The onScrollFinished slot demonstrates how to use finalPosition() to get the target scroll position after a kinetic scroll finishes.
• The MyWidget class creates a QScrollArea and embeds the ScrollingContent within it.
• The ScrollingContent class creates a large amount of content that requires scrolling.

---

Tracking Scroller State and Value Changes

• Within the slot triggered by scrollerState(), you can access the current scroll position using value(). This provides the current scroll value, which might be sufficient for some use cases.
• Connect to the scrollerState() signal to detect when the scrolling animation finishes (becomes QScroller::Idle).
• If you only need to react to the completion of a scroll and don't necessarily require the exact target position, you can combine the scrollerState() and value() signals of QScroller.

connect(scrollArea->horizontalScroller(), &QScroller::scrollerState, this, [this](QScroller::ScrollerState state) {
    if (state == QScroller::Idle) {
        int currentPosition = scrollArea->horizontalScroller()->value();
        // Do something with the current scroll position
        qDebug() << "Scroll finished at position:" << currentPosition;
    }
});

Using QAbstractScrollArea Signals

• You can connect to these signals to track the scroll position throughout the scrolling animation, not just at the end.
• These signals emit whenever the horizontal or vertical scroll value changes, respectively.
• Qt's QAbstractScrollArea class, which QScrollArea inherits from, provides signals like horizontalValueChanged() and verticalValueChanged().

connect(scrollArea, &QAbstractScrollArea::horizontalValueChanged, this, [this](int value) {
    // Handle scroll updates here
    qDebug() << "Horizontal scroll position:" << value;
});

Custom Implementation (Advanced)

• This approach is more complex and requires a deeper understanding of Qt's scrolling mechanisms.
• This might involve overriding event handlers or subclassing QScroller to capture specific events related to the scrolling behavior.
• For more granular control, you could implement your own logic to track the scroll position and momentum.

• For very specific tracking or custom behaviors, a custom implementation using event handling or subclassing could be explored (but this is for advanced users).
• If you require continuous updates on the scroll position throughout the animation, QAbstractScrollArea signals like horizontalValueChanged() would be a better choice.
• If you only need to know when scrolling finishes and the general area of the target position, scrollerState() and value() might suffice.
• The best alternative depends on your specific needs.