canvas - Empty PDF report is generated when we have multiple graphs using html2canvas and jsPDF library

Question

I have two flot charts in One page , and with the code below, I want to use jsPDF to create report of them in two different pages (but one PD file) here id my code: Lets use this sample graphs :

function flot1() {
        var oilPrices = [[1167692400000, 61.05], [1167778800000, 58.32], [1167865200000, 57.35], [1167951600000, 56.31], [1168210800000, 55.55], [1168297200000, 55.64]
            , [1168383600000, 54.02], [1168470000000, 51.88], [1168556400000, 52.99], [1168815600000, 52.99], [1168902000000, 51.21], [1168988400000, 52.24], [1169074800000, 50.48]];

        var exchangeRates = [[1167606000000, 0.7580], [1167692400000, 0.7580], [1167778800000, 0.75470], [1167865200000, 0.75490], [1167951600000, 0.76130], [1168038000000, 0.76550],
            [1168124400000, 0.76930], [1168210800000, 0.76940], [1168297200000, 0.76880], [1168383600000, 0.76780], [1168470000000, 0.77080], [1168556400000, 0.77270],
            [1168642800000, 0.77490], [1168729200000, 0.77410], [1168815600000, 0.77410], [1168902000000, 0.77320], [1168988400000, 0.77270], [1169074800000, 0.77370],
            [1169161200000, 0.77240], [1169247600000, 0.77120]];

        var data = [
            { data: oilPrices, label: "Oil price ($)" },
            { data: exchangeRates, label: "USD/EUR exchange rate", yaxis: 2 }
        ];

        var options = {
            canvas: true,
            xaxes: [{ mode: "time" }],
            yaxes: [{ min: 0 }, {
                position: "right",
                alignTicksWithAxis: 1,
                tickFormatter: function (value, axis) {
                    return value.toFixed(axis.tickDecimals) + "€";
                }
            }],
            legend: { position: "sw" }
        }

        plot = $.plot("#placeholder_2", data, options);
    }

    function flot2() {
        plot = $.plot($("#placeholder"), [{ label: 'Test', data: [[0, 0], [1, 1]] }], { yaxis: { max: 1 } });
    }

var doc = new jsPDF();
html2canvas($("#placeholder").get(0), {
    onrendered: function (canvas) {
         var imgData = canvas.toDataURL('image/png');

         doc.addImage(imgData, 'PNG', 10, 10, 180, 115);
              }
            });

var element = $("#placeholder_2").get(0);
html2canvas(element, {
    onrendered: function (canvas2) {
         var imgData2 = canvas2.toDataURL('image/png');
         doc.addPage()
         doc.addImage(imgData2, 'PNG', 10, 10, 180, 60);
              }
           });

doc.save('sample-file.pdf');

the problem is :

when I save pdf with ONE graph, it works fine, but after puting doc.save() at the end of canvas renders , the created PDF file is empty.

NOTE: I use html2canvas and jsPDF libraries.

Answer 1

Then problem was because of async execution of converting image and creating pdf.

One solution is using Asynchronous functions.because the "doc.save()" was executing before Html2Canvas finished creating canvas from graph, so it was empty.

I created a fiddle here. this is the code for Asynchronous execution using Promise API in JavaScript.(promise.all)

 var p1 = new Promise(function (resolve, reject) {
     var element = $("#placeholder").get(0);
       html2canvas(element,
                        {
                            onrendered: function (canvas) {
                                resolve(canvas.toDataURL('image/png'));
                            }
                        });
        });


 var p2 = new Promise(function (resolve, reject) {
     var element = $("#placeholder_2").get(0);
       html2canvas(element,
                    {
                        onrendered: function (canvas) {
                            resolve(canvas.toDataURL('image/png'));
                        }
                    });
        });

  Promise.all([p1, p2]).then(function (screens) {
            var doc = new jsPDF();
            doc.addImage(screens[0], 'PNG', 10, 10, 180, 115);
            doc.addPage();
            doc.addImage(screens[1], 'PNG', 10, 10, 180, 60);
            doc.save('sample-file.pdf');

    });