解决jsPDF导出html,黑屏，白屏，不全问题

1.项目中用到导出pdf，用到jsPDF,canvas,
2.网上很多方法，存在黑屏白屏，只能解决当前可视区域的问题
3.其中参考 https://blog.csdn.net/qq_36706878/article/details/111289963和https://blog.csdn.net/m0_37879526/article/details/103827235?utm_medium=distribute.pc_relevant_bbs_down.none-task--2~all~first_rank_v2~rank_v29-5.nonecase&depth_1-utm_source=distribute.pc_relevant_bbs_down.none-task--2~all~first_rank_v2~rank_v29-5.nonecase  ，这两位博友的博客，
使用方法：引入

<script src="https://cdn.bootcss.com/jspdf/1.5.3/jspdf.debug.js"></script>

<script src="https://cdn.bootcss.com/html2canvas/0.5.0-beta4/html2canvas.min.js"></script>

方法名

downloadC() {//方法名绑定你的导出按钮

                    var _this = this;

                    var targetDom = $("#export_content");//你的目标元素id

                    var copyDom = targetDom.clone();

                    copyDom.width(targetDom.width() + "px");

                    copyDom.height(targetDom.height() + "px");

                    $('body').append(copyDom);

                    console.log('copyDom',copyDom)

                    html2canvas(copyDom, {

                        onrendered:function(canvas) {

                            var contentWidth = canvas.width;

                            var contentHeight = canvas.height;

                            var pageHeight = contentWidth / 592.28 * 841.89;

                            var leftHeight = contentHeight;

                            var position = 0;

                            var f="p";

                            // var imgWidth = 595.28;

                            // var imgHeight = 592.28/contentWidth * contentHeight;

                            var pageHeight = contentWidth / 592.28 * 841.89;

                            //未生成pdf的html页面高度

                            var leftHeight = contentHeight;

                            //页面偏移

                            var position = 0;

                            //a4纸的尺寸[595.28,841.89]，html页面生成的canvas在pdf中图片的宽高

                            var imgWidth = 595.28;

                            var imgHeight = 592.28/contentWidth * contentHeight;

                

                            var pageData = canvas.toDataURL('image/jpeg', 1.0);

                

                            var pdf = new jsPDF('', 'pt', 'a4');

                            

                            //有两个高度需要区分，一个是html页面的实际高度，和生成pdf的页面高度(841.89)

                            //当内容未超过pdf一页显示的范围，无需分页

                            if (leftHeight < pageHeight) {

                                pdf.addImage(pageData, 'JPEG', 0, 0, imgWidth, imgHeight);

                            } else {    // 分页

                                while(leftHeight > 0) {

                                    pdf.addImage(pageData, 'JPEG', 0, position, imgWidth, imgHeight)

                                    leftHeight -= pageHeight;

                                    position -= 841.89;

                                    // position += 841.89;

                                    //避免添加空白页

                                    if(leftHeight > 0) {

                                        pdf.addPage();

                                    }

                                }

                            }

                            // contentHeight > contentWidth ? f = 'p' : f = 'l';

                            // var pageData = canvas.toDataURL('image/png', 1.0);

                            // _this.aaa = canvas.toDataURL();

                            // var pdf = new jsPDF(f, 'px', [imgWidth, imgHeight]);

                            // pdf.addImage(pageData, 'png', 0, 0, imgWidth, imgHeight);

                            pdf.save('content.pdf');

                            copyDom.remove();

                        }

       });

            },