Method paintEvent

YACReader/continuous_page_widget.cpp:201–295

}

void ContinuousPageWidget::paintEvent(QPaintEvent *event)
{
    if (!continuousViewModel || continuousViewModel->numPages() == 0 || !provider) {
        return;
    }

    QPainter painter(this);
    const qreal dpr = devicePixelRatioF();
    const int effectivePixelWidth = std::max(1, qRound(width() * dpr));

    // A width change makes every cached/in-flight scale obsolete. Bump the
    // generation so stale results that are already running get discarded.
    if (scaledPageCache.effectiveWidth != effectivePixelWidth) {
        scaledPageCache.invalidateForWidth(effectivePixelWidth);
        ++cacheGeneration;
        pendingScaleRequests.clear();
        scalePool.clear();
    }

    const int numPages = continuousViewModel->numPages();

    const QRect paintRect = event->rect();
    int firstPage = qBound(0, continuousViewModel->pageAtY(paintRect.top()), numPages - 1);
    int lastPage = qBound(0, continuousViewModel->pageAtY(paintRect.bottom()), numPages - 1);

    // Cache management and prefetch follow the real viewport, not this paint's
    // dirty rect (which can be a single page from an async delivery), so a
    // partial repaint never evicts a still-visible page.
    int prefetchMin = 0;
    int prefetchMax = -1;
    managedPageRange(prefetchMin, prefetchMax);
    scaledPageCache.keepOnlyRange(prefetchMin, prefetchMax);

    // Drive the background decoder to keep the visible + prefetch window ready.
    // This is the only place decoding is requested, and it never blocks.
    provider->requestRange(prefetchMin, prefetchMax);

    auto targetPixelSizeFor = [dpr](const QSize &scaled) {
        return QSize(std::max(1, qRound(scaled.width() * dpr)),
                     std::max(1, qRound(scaled.height() * dpr)));
    };

    const int w = width();
    for (int i = firstPage; i <= lastPage && i < numPages; ++i) {
        const int y = continuousViewModel->yPositionForPage(i);
        const QSize scaled = continuousViewModel->scaledPageSize(i);
        // center horizontally if page is narrower than widget
        int x = (w - scaled.width()) / 2;
        if (x < 0) {
            x = 0;
        }
        const QRect pageRect(x, y, scaled.width(), scaled.height());

        const QImage *img = provider->image(i);
        if (img && !img->isNull()) {
            const QSize targetPixelSize = targetPixelSizeFor(scaled);
            const QImage *drawable = cachedScaledImage(i, img, scaled, targetPixelSize, dpr);
            if (drawable) {