Effectively Tracking Panning in Your Qt Widgets Application with QPanGesture::lastOffset

Understanding Pan Gestures in Qt Widgets

• QPanGesture
  A class in Qt Widgets that helps you detect and handle pan gestures. It provides properties and signals to track the user's finger movement.
• Panning
  A common interaction in touch-enabled applications where the user drags their finger across the screen to move content.

QPanGesture::lastOffset Property

• Value
  
  • When a pan gesture starts (first movement), lastOffset is a zero-sized point (QPointF(0.0, 0.0)).
  • As the user drags their finger, lastOffset is updated with the difference between the current and previous finger positions.
  • This allows you to track the incremental movement during the pan gesture.
• Data Type
  QPointF, which represents a 2D point with floating-point coordinates (x and y).
• Purpose
  This property stores the change in the user's finger position since the previous gesture event.

Accessing and Using lastOffset

• Setter
  The setLastOffset(const QPointF &value) function is provided, but it's generally not recommended to modify lastOffset directly. Qt manages this property internally.
• Getter
  Use the lastOffset() const member function to retrieve the current value of lastOffset.

Example Usage Scenario

#include <QtWidgets>

class MyWidget : public QWidget {
    Q_OBJECT

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

        // Create a QPanGesture object and install it on the widget
        panGesture = new QPanGesture(this);
        panGesture->setObjectName("imagePanGesture");
        connect(panGesture, &QPanGesture::StateChanged, this, &MyWidget::handlePanStateChanged);
    }

private slots:
    void handlePanStateChanged(Qt::GestureState state) {
        if (state == Qt::GestureState::Started) {
            // Pan gesture has started, reset lastOffset
            lastPanOffset = QPointF(0.0, 0.0);
        } else if (state == Qt::GestureState::Updated) {
            // Pan gesture is ongoing, calculate the new image position based on lastOffset
            QPointF currentOffset = panGesture->lastOffset();
            QPointF newImagePosition = currentImagePosition + currentOffset - lastPanOffset;

            // Update the image position in your widget (implementation details omitted)
            updateImagePosition(newImagePosition);

            // Store the current offset for the next update
            lastPanOffset = currentOffset;
        }
    }

private:
    QPanGesture *panGesture;
    QPointF currentImagePosition; // Store the current position of the image
    QPointF lastPanOffset; // Store the offset from the previous pan update
};

In this example:

1. A QPanGesture object named panGesture is created and installed on the widget.
2. The handlePanStateChanged slot is connected to the QPanGesture::StateChanged signal.
3. When the pan gesture starts (state == Qt::GestureState::Started), lastPanOffset is reset.
4. During pan updates (state == Qt::GestureState::Updated), the current lastOffset is used to calculate the new position of the image, considering the previous offset stored in lastPanOffset.
5. The image position is updated in the widget, and lastPanOffset is updated with the current offset for the next update.

---

#include <QtWidgets>
#include <QGraphicsScene> // Needed for scene and graphics view
#include <QGraphicsPixmapItem> // Needed for displaying the image

class MyWidget : public QWidget {
    Q_OBJECT

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

        // Create a QGraphicsScene and a QGraphicsView to display the image
        scene = new QGraphicsScene(this);
        graphicsView = new QGraphicsView(scene, this);
        graphicsView->setDragAutoScroll(false); // Disable auto-scrolling during pan

        // Load the image (replace with your image loading logic)
        imagePixmap = new QPixmap("path/to/your/image.png");
        imageItem = scene->addPixmap(*imagePixmap);

        // Create and install the QPanGesture
        panGesture = new QPanGesture(this);
        panGesture->setObjectName("imagePanGesture");
        connect(panGesture, &QPanGesture::StateChanged, this, &MyWidget::handlePanStateChanged);
    }

private slots:
    void handlePanStateChanged(Qt::GestureState state) {
        if (state == Qt::GestureState::Started) {
            // Pan gesture has started, reset lastOffset
            lastPanOffset = QPointF(0.0, 0.0);
        } else if (state == Qt::GestureState::Updated) {
            // Pan gesture is ongoing, calculate the new image position
            QPointF currentOffset = panGesture->lastOffset();
            QPointF newImagePosition = imageItem->pos() + currentOffset - lastPanOffset;

            // Constrain image movement within scene boundaries (optional)
            newImagePosition.setX(qBound(0.0, newImagePosition.x(), scene->width() - imagePixmap->width()));
            newImagePosition.setY(qBound(0.0, newImagePosition.y(), scene->height() - imagePixmap->height()));

            // Update the image position in the scene
            imageItem->setPos(newImagePosition);

            // Store the current offset for the next update
            lastPanOffset = currentOffset;
        }
    }

private:
    QGraphicsScene *scene;
    QGraphicsView *graphicsView;
    QPixmap *imagePixmap;
    QGraphicsPixmapItem *imageItem;
    QPanGesture *panGesture;
    QPointF lastPanOffset;
};

• Constraining Image Movement (Optional)
  The qBound function is used to constrain the image's position within the scene's boundaries, preventing it from going off-screen during panning. You might need to adjust these constraints based on your specific requirements.
• Image Loading and Display
  The code assumes you have a QPixmap object loaded with your image. You'll need to replace path/to/your/image.png with the actual path. The QGraphicsPixmapItem is added to the scene, making the image visible within the QGraphicsView.
• Graphics Scene and View
  We use QGraphicsScene and QGraphicsView to display the image within a dedicated area. You'll need to include the <QGraphicsScene> and <QGraphicsPixmapItem> headers.

---

Tracking Absolute Position

• Implementation
  
  • Store the initial finger position (startPos) when the pan gesture starts.
  • In the QPanGesture::StateChanged slot (state == Qt::GestureState::Updated), calculate the difference between the current finger position (currentPos) obtained from the gesture and startPos. This difference represents the total pan movement so far.
• Concept
  Instead of using the offset (difference from previous position), you directly track the current absolute position of the user's finger during the pan gesture.

void handlePanStateChanged(Qt::GestureState state) {
    if (state == Qt::GestureState::Started) {
        startPos = panGesture->globalPos(); // Store initial finger position
    } else if (state == Qt::GestureState::Updated) {
        QPointF currentPos = panGesture->globalPos();
        QPointF totalPan = currentPos - startPos;

        // Update the image position based on totalPan (implementation details omitted)
        updateImagePosition(totalPan);
    }
}

• Considerations
  
  • This approach calculates the total pan movement since the gesture began, not the incremental movement between updates. You might need to adjust your calculations based on your specific use case.
  • It's less efficient for smooth, continuous panning as it requires keeping track of the initial position throughout the gesture.

• Considerations
  
  • This approach is more complex and requires a deeper understanding of touch event handling in Qt.
  • It's generally recommended to use the built-in QPanGesture class unless you have very specific needs that it doesn't address.
• Implementation
  
  • Override the event handler in your widget class to detect touch events (e.g., QTouchEvent).
  • Within the event handler, track the touch position changes and calculate the offsets between events manually.
• Concept
  If you have very specific requirements for pan tracking, you could create a custom gesture handler that directly tracks the touch events and calculates the offsets yourself.