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Page-Layout-Customization.md

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Page Layout Customization

NOTE: setting PdfViewerParams.layoutPages dynamically does not refresh the viewer. You should call PdfViewerController.invalidate, setState or some equivalent function.

Horizontal Scroll View

By default, the pages are laid out vertically. You can customize the layout logic by PdfViewerParams.layoutPages:

layoutPages: (pages, params) {
  final height =
      pages.fold(0.0, (prev, page) => max(prev, page.height)) +
          params.margin * 2;
  final pageLayouts = <Rect>[];
  double x = params.margin;
  for (var page in pages) {
    pageLayouts.add(
      Rect.fromLTWH(
        x,
        (height - page.height) / 2, // center vertically
        page.width,
        page.height,
      ),
    );
    x += page.width + params.margin;
  }
  return PdfPageLayout(
    pageLayouts: pageLayouts,
    documentSize: Size(x, height),
  );
},

Underflow Alignment

When the laid-out document is smaller than the viewport, PdfViewerParams.underflowAnchor controls how the document is aligned on the underflowing axis:

params: const PdfViewerParams(
  underflowAnchor: PdfPageAnchor.topLeft,
),

If PdfViewerParams.underflowAnchor is null, pdfrx keeps the legacy behavior and centers only the axes where the document is smaller than the viewport. Set it to a PdfPageAnchor value when you need deterministic top, bottom, left, right, or corner alignment.

Facing Pages

The following code will show pages in "facing-sequential-layout" that is often used in PDF viewer apps:

/// Page reading order; true to L-to-R that is commonly used by books like manga or such
var isRightToLeftReadingOrder = false;
/// Use the first page as cover page
var needCoverPage = true;

...

layoutPages: (pages, params) {
  final width = pages.fold(
      0.0, (prev, page) => max(prev, page.width));

  final pageLayouts = <Rect>[];
  final offset = needCoverPage ? 1 : 0;
  double y = params.margin;
  for (int i = 0; i < pages.length; i++) {
    final page = pages[i];
    final pos = i + offset;
    final isLeft = isRightToLeftReadingOrder
        ? (pos & 1) == 1
        : (pos & 1) == 0;

    final otherSide = (pos ^ 1) - offset;
    final h = 0 <= otherSide && otherSide < pages.length
        ? max(page.height, pages[otherSide].height)
        : page.height;

    pageLayouts.add(
      Rect.fromLTWH(
        isLeft
            ? width + params.margin - page.width
            : params.margin * 2 + width,
        y + (h - page.height) / 2,
        page.width,
        page.height,
      ),
    );
    if (pos & 1 == 1 || i + 1 == pages.length) {
      y += h + params.margin;
    }
  }
  return PdfPageLayout(
    pageLayouts: pageLayouts,
    documentSize: Size(
      (params.margin + width) * 2 + params.margin,
      y,
    ),
  );
},