如何用flutter计算在后台处理pdf数据

Question

我正在制作一个可以在pdf package 中显示和打印图像的应用程序。

用pdf package 创建一个 pdf 并printing package。

要打印，必须将Uint8List数据发送到 pdf 的save() pdf 。 但是，加载屏幕停止了，因为我的 pdf 的大小不小。

我认为你应该使用compute()来解决这个问题。

绘制 pdf

 Future<pdfWidget.Document> _drawEnsembleScorePrint(BuildContext context) async {
    DrawScore drawScore = DrawScore();
    pdfWidget.Document scorePdf = pdfWidget.Document(version: PdfVersion.pdf_1_5, compress: true);

    print('drawEnsemble print start');
    for(int nCurPage=0; nCurPage<scorePngList.length; nCurPage++) {
      File imageFile = File(scorePngList[nCurPage].path);
      final image = pdfWidget.MemoryImage(imageFile.readAsBytesSync());

      final watermarkingImage = await imageFromAssetBundle('assets/images/ELF_CI.png');
      final underElfimage = await imageFromAssetBundle('assets/images/ELF_CI2.png');

      scorePdf.addPage(
        pdfWidget.Page(
            pageFormat: PdfPageFormat.a4,

            ///2480 * 3508
            build: (pdfWidget.Context pContext) {
              return pdfWidget.Stack(
                children: [

                  drawScore.drawWatermarking(context, watermarkingImage, 1.0),

                  pdfWidget.Container(
                    width: 2480,
                    height: 3508,
                    child: pdfWidget.Image(image, fit: pdfWidget.BoxFit.fill),
                  ),


                  pdfWidget.Positioned(
                    top: 100,
                    left: 2190,
                    child: pdfWidget.Container(
                      width: 201,
                      height: 100,
                      color: PdfColor.fromInt(0xFFFFFFFF),
                    ),
                  ),

                  // pdfWidget.Positioned(
                  //   top: 100,
                  //   left: 2190,
                  //   child: pdfWidget.Container(
                  //     width: 201,
                  //     height: 100,
                  //     alignment: pdfWidget.Alignment.centerRight,
                  //     child: pdfWidget.Text(
                  //       httpCommunicate.nDispNumber.toString(),
                  //       style: pdfWidget.TextStyle(
                  //         font: context.read<CustomFontProvider>().SOURCE_HANSANS_MEDIUM,
                  //         fontSize: 48,
                  //       ),
                  //     ),
                  //   ),
                  // ),

                  // drawScore.drawWatermarking(context, watermarkingImage, httpCommunicate),

                  pdfWidget.Positioned(
                    top: 3424,
                    left: 24,
                    child: pdfWidget.Container(
                      width: 2432,
                      height: 83,
                      alignment: pdfWidget.Alignment.topCenter,
                      child: pdfWidget.Text(
                        '${nCurPage + 1} / ${scorePngList.length}',
                        style: pdfWidget.TextStyle(
                          // font: context.read<CustomFontProvider>().SOURCE_HANSANS_NORMAL,
                          fontSize: 36,
                        ),
                      ),
                    ),
                  ),

                  pdfWidget.Positioned(
                    top: 3424,
                    left: 24,
                    child: pdfWidget.Container(
                      width: 2432,
                      height: 83,
                      alignment: pdfWidget.Alignment.topLeft,
                      child: pdfWidget.Text(
                        strUserID,
                        style: pdfWidget.TextStyle(
                            // font: context.read<CustomFontProvider>().SOURCE_HANSANS_NORMAL,
                            fontSize: 36,
                            color: PdfColor.fromInt(0xFF000000)
                        ),
                      ),
                    ),
                  ),

                  pdfWidget.Positioned(
                    top: 3424,
                    left: 24,
                    child: pdfWidget.Container(
                      width: 2432,
                      height: 83,
                      alignment: pdfWidget.Alignment.topRight,
                      child: pdfWidget.Image(underElfimage, width: 190, height: 72),
                    ),
                  ),


                ],
              );
            }
        ),
      );
    }
    return scorePdf;
  }

调用 draw pdf func

_realDrawEnsembleScore(BuildContext context, HttpCommunicate httpCommunicate) async {

    DrawScore drawScore = DrawScore();

    pdfWidget.Document scorePdf = pdfWidget.Document(version: PdfVersion.pdf_1_5, compress: true);

      if(httpCommunicate.bIsPreViewScore == true){
        // scorePdf = await compute(_drawEnsemblePreview, 0);
        scorePdf = await _drawEnsemblePreview(context);
      }
      else{
        // scorePdf = await compute(_drawEnsembleScorePrint, 0);
        scorePdf = await _drawEnsembleScorePrint(context);
      }

    return scorePdf;
  }

调用 realDrawEnsembleScore 函数

Future<pdfWidget.Document> addScorePDF(BuildContext context, HttpCommunicate httpCommunicate, bool bIsTiff)async{
    DrawScore drawScore = DrawScore();
    pdfWidget.Document scorePdf = pdfWidget.Document(version: PdfVersion.pdf_1_5, compress: true);
    final cutpaperImage = await imageFromAssetBundle('assets/images/CutPaper.png');
    final watermarkingImage = await imageFromAssetBundle('assets/images/ELF_CI.png');
    final underElfimage = await imageFromAssetBundle('assets/images/ELF_CI2.png');

    if(!bIsTiff){
      scorePdf = await _realDrawScore(context, httpCommunicate);
    }
    else{
      scorePdf = await _realDrawEnsembleScore(context, httpCommunicate);
    }

    return scorePdf;

  }

计算代码

pdfWidget.Document? scorePdf;
scorePdf = await context.read<PDFProvider>().addScorePDF(context, httpCommunicate, true);
Uint8List uintData = await compute(_testfunction, 0);

////
Future<Uint8List> _testfunction(int x) async {
    pdfWidget.Document scorePdf = pdfWidget.Document(version: PdfVersion.pdf_1_5, compress: true);

    print("_test function start");

    Uint8List data;

    data = g_scorePdf!.save();
    return data;
  }

但是_testfunction永远不会被调用。 g_scorePdf不是 null，但Save()会抛出错误。

Answer 1

您需要确保您呼叫的 function 是顶级 function 。 我也会避免将 BuildContext 作为参数发送给隔离。

Future<Uint8List> _testfunction(int x) async {
  pdfWidget.Document scorePdf = pdfWidget.Document(version: PdfVersion.pdf_1_5, compress: true);

  for (int i = 0; i < 3; i++) {
    scorePdf.addPage(
      pdfWidget.Page(build: (pdfWidget.Context context) {
        return pdfWidget.Stack(
          children: [
            pdfWidget.Positioned(
              top: 1000,
              left: 1000,
              child: pdfWidget.Text('top 1000, left 1000'),
            ),
          ],
        );
      }),
    );
  }
  debugPrint('Done');

  return await scorePdf.save();
}

class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({super.key});

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  Future<Uint8List> addPdfTest(BuildContext context) async {
    Uint8List data = await compute(_testfunction, 1);

    return data;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      extendBody: true,
      body: Center(
        child: GestureDetector(
          onTap: () async {
            await addPdfTest(context);
          },
          child: Container(color: Colors.red),
        ),
      ),
    );
  }
}

您可以在此处和此处阅读有关使用计算的更多信息。

pdf package 任何其他感兴趣的人的链接： https://pub.dev/packages/pdf