ScrollView: only get visible drawing area?

Tobias Fensch

Hello,

I am using a QML-ScrollView and want to draw something in C++ with a QQuickPaintedItem.

At the moment, in the Paint-Method of the QQuickPaintedItem, my drawing area takes up the entire scroll area. This has some ugly side effects as mentioned here: ScrollView: contentWidth limited?

Here my question: Is it possible to get only the visible scroll area as the drawing area? And than repaint the visible area on scroll changed event?

I am using Qt5.15

regards,
Tobias

Tobias Fensch

To solve this, I use a ListView to divide the area in segments, 8.000 pixel each.

ListView {
    id: graphicView
    anchors.fill: parent
    orientation: Qt.Horizontal
    spacing: 0
    // always repaint items on scroll
    reuseItems: false

    // the width of the scrollable area
    readonly property real displayWidth: 64000
    // define a max width for segmentation to avoid the contentWidth limitation of qml
    readonly property int segmentWith: 8000
    readonly property int segmentCount: Math.max(1, Math.round(displayWidth / segmentWith))
    onSegmentCountChanged: {
        console.log("OsziGraphic: displayWidth = " + displayWidth + ", segmentCount = " + segmentCount);
    }

    model: graphicView.segmentCount
    delegate:
        OsziGraphicViewModel {
            height: ListView.view.height
            width: graphicView.displayWidth / graphicView.segmentCount
            segmentIndex: index
        }
}

In the backend I inherit from QQuickPaintedItem to add properties for the current segment index and the render area to draw in. I translate the rendered coordinates to draw as if the entire area is used.

void QQuickSegmentedPaintedItem::paint(QPainter *painter)
{
    if (m_segmentIndex == -1)
        return;

     // calculate the current render rect inside the drawing area
     m_renderRect = QRect(m_segmentIndex * boundingRect().width(), 0, boundingRect().width(), boundingRect().height());

     // move the drawing area relative to the current render area
     painter->translate(-m_renderRect.left(), 0);
}

This works nicely for my needs. I hope you get the idea.

cheers!