Dynamic Footer Space Allocation in MultiPage

[tomaspalaoro]

I'm using the MultiPage widget. I have a scenario where the footer should only appear on the last page, so I'm conditionally rendering it like so:

footer: (context) {
  if (context.pageNumber == context.pagesCount) {
    return pw.Container(
      alignment: pw.Alignment.center,
      margin: const pw.EdgeInsets.only(top: 20),
      child: pw.Text('Footer content'),
    );
  }
  return pw.SizedBox.shrink();
},

In my implementation, it seems that the maximum footer height is pre-calculated and reserved on every page, even if the footer content is only present on the last page. Is there a way to dynamically allocate footer space so that pages without footer content do not reserve that extra space?

[matteo-convert]

you can put it in build for rendering only at the end

btw yes the package seem to miss some good features in multipage like page report

[tomaspalaoro]

Thanks for the suggestion. However, when my footer, which contains a list of widgets, is wrapped in an Expanded widget, it results in the following error:

Cannot have a spanning widget flexible

Here's an example

build: (context) => [
          pw.Text("content"),
          pw.Expanded(
              child: pw.Align(
                  alignment: pw.Alignment.bottomCenter,
                  child: pw.Column(
                    children: footerContentList(),
                  )))
        ],

List<pw.Widget> footerContentList() {
    return [
      pw.Text("content example 1"),
      pw.Text("content example 2"),
      pw.Text("content example 2")
    ];
  }

[leocirto]

Interesting discussion. The vertical space is allotted to the footer beforehand, and is based on the largest footer. I believe the Spacer( ) widget can help you push a fake footer inside Build to the bottom of the page.

Example:

  Future<Uint8List> _builPdfPreview(final PdfPageFormat format) async {
      final pdf = pw.Document( );
      const bool useFakeFooter = false;
   // const bool useFakeFooter = true;
      pdf.addPage(
        pw.MultiPage(
            pageFormat: format,
            margin: const pw.EdgeInsets.symmetric( horizontal: 50, vertical: 0 ),
            build: (final pw.Context ctx) {
              return [
                 for( int k = 0; k < 40; k++ )  pw.Text( '$k ---------' ),
                 if( useFakeFooter ) ... [
                   pw.Spacer( ),
                   pw.Footer(
                      padding: const pw.EdgeInsets.all( 0 ),
                      margin:  const pw.EdgeInsets.all( 0 ),
                      leading: pw.Text('useFakeFooter = $useFakeFooter Footer.\n\n\n\n'),
                   // NOTE: CANNOT ACCESS pageNumber AND pagesCount USING THIS CONTEXT, LOOK:
                   //       Null check operator used on a null value
                   // leading: pw.Text('Fake Footer. Page ${ctx.pageNumber}/${ctx.pagesCount}\n\n\n\n'),
                   ),
                 ]
              ];
            },
            footer: ( useFakeFooter == true) ? null : ( final pw.Context ctx ){
              if (ctx.pageNumber != ctx.pagesCount) {
                 return pw.Footer(
                     padding: const pw.EdgeInsets.all( 0 ),
                     margin:  const pw.EdgeInsets.all( 0 ),
                     leading: null,
                 );
              }
              return pw.Footer(
                     padding: const pw.EdgeInsets.all( 0 ),
                     margin:  const pw.EdgeInsets.all( 0 ),
                     leading: pw.Text('useFakeFooter = $useFakeFooter. Page ${ctx.pageNumber}/${ctx.pagesCount}\n\n\n\n'),
              );
            }
        )
      );
      return pdf.save();
  }

Outputs:

True Footer	Fake Footer (footer inside build)

[matteo-convert]

Yes it works with my example because my footer is wrapped in a container with fixed height