WPF FlowDocument determine what page a Block would be on

Question

If I have a var flow = new FlowDocument() and I add several blocks to its BlockCollection:

if (m_Blocks == null || m_Blocks.Count <= 0)
  return flow;

flow.Blocks.AddRange(m_Blocks.ToList());
var paginatorSource = flow as IDocumentPaginatorSource;
paginatorSource.DocumentPaginator.ComputePageCount();
var pageCount = paginatorSource.DocumentPaginator.PageCount;

I am able to determine how many pages this document consists of.

Question

If the number of pages is greater than 1, how do I go about finding out which blocks of the document weren't able to fit on the first page? I can't find anything on the Block class that helps me determine this. But since the paginator can ComputePageCount(), I figured this was possible.

Answer 1

I took a brute-force approach to what I was attempting to accomplish. Basically, I needed to repeat a Paragraph at the top of each page. This is not the most performant way, I'm assuming, but it got the job done:

protected FlowDocument RenderBody()
{
  var flow = new FlowDocument
  {
    PageWidth = PaperSizeDPI.Width,
    PageHeight = PaperSizeDPI.Height,
    ColumnGap = 0,
    ColumnWidth = PaperSizeDPI.Width,
    PagePadding = new Thickness(MarginDPI.Left, MarginDPI.Top + HeaderHeightDPI, MarginDPI.Right, MarginDPI.Bottom + FooterHeightDPI)
  };

  if (m_Blocks == null || m_Blocks.Count <= 0)
    return flow;

  // Get page count before deciding to deal with repeat blocks.
  // Adding blocks to the FlowDocument is required before invoking
  // `DocumentPaginator.ComputePageCount()`.
  flow.Blocks.AddRange(m_Blocks.ToList());

  int GetPageCount()
  {
    var paginatorSource = (IDocumentPaginatorSource) flow;
    paginatorSource.DocumentPaginator.ComputePageCount();
    return paginatorSource.DocumentPaginator.PageCount;
  }

  // If there is only one page, no roll-over of headers necessary
  if (GetPageCount() <= 1) return flow;

  // If we don't have a header to repeat, we assume the report is correct already
  if (PrintTemplateConverter.BuildRepeatableHeader == null) return flow;

  // Build the FlowDocument, one block at a time, checking if the page count
  // changes so that we can inject a repeatable header
  flow.Blocks.Clear();
  var pageCount = 1;

  var blocks = m_Blocks.ToList();

  for (var blockIndex = 0; blockIndex < blocks.Count; blockIndex++)
  {
    flow.Blocks.Add(blocks[blockIndex]);
    if (GetPageCount() <= pageCount) continue;

    // If this block moved us to a new page, inject repeatable header in its spot.
    // `flow.Blocks.AddRange(...)` ignores duplicate blocks (by reference).
    // This is why I go through the "pain" of making a whole new Header block
    // instance.
    var blocksSoFar = flow.Blocks.ToList();
    flow.Blocks.Clear();
    blocksSoFar.Insert(blockIndex, PrintTemplateConverter.BuildRepeatableHeader());

    flow.Blocks.AddRange(blocksSoFar);
    pageCount = GetPageCount();
  }

  return flow;
}

I hope this helps someone in the future!