使用以下代码，当请求/pdf时，express将停止服务所有其他页面，直到生成pdf为止。如何使PDF生成同步？感谢。
app.get('/pdf', function (req, res) {
var publication_id = req.query['id'] // e.g. http://localhost:3000/pdf?id=I1519673917344
var booklet = req.query['booklet'] // e.g. http://localhost:3000/pdf?id=I15196739173440&booklet=true
const canvasWidth = 450
const canvasHeight = 636
const pageWidth = canvasWidth/1.34
const pageHeight = canvasHeight/1.34
var canvases = []
var _publication
const tasks = [
function findPublication(callback) {
Publication.findOne({ 'id': publication_id }, function (err, publication) {
if (err) return console.error(err)
_publication = publication
console.log('found pub')
callback(null)
})
},
function makeCanvases(callback) {
for (var i = 1; i < 9; i++) {
var canvas = new fabric.StaticCanvas('c') // random name
canvas.setWidth(canvasWidth)
canvas.setHeight(canvasHeight)
if ( _publication && _publication.pages.hasOwnProperty('p' + i) ) { // if not empty
var pages = _publication.pages
canvas.loadFromJSON(pages['p' + i]);
}
canvases.push(canvas)
}
console.log('made canvases')
callback(null)
},
function makePdf(callback) {
res.writeHead(200, {
'Content-Type': 'application/pdf',
'Access-Control-Allow-Origin': '*',
'Content-Disposition': 'filename=' + publication_id + '.pdf'
});
if (booklet != 'true') {
doc = new PDFDocument({size:[pageWidth, pageHeight]})
var i = 0
canvases.forEach(function(canvas) {
SVGtoPDF(doc, canvas.toSVG(), 0, 0, {fontCallback: fonts })
if (i != canvases.length - 1) {
doc.addPage()
}
i++
})
doc.pipe(res).on('finish', function() {
console.log('single page pdf was successfully created')
})
doc.end()
} else {
doc = new PDFDocument({size:[pageWidth*2, pageHeight]})
// all the -1 to have a normal page number
SVGtoPDF(doc, canvases[8-1].toSVG(), 0, 0, {fontCallback: fonts });
SVGtoPDF(doc, canvases[1-1].toSVG(), pageWidth, 0, {fontCallback: fonts });
doc.addPage()
SVGtoPDF(doc, canvases[2-1].toSVG(), 0, 0, {fontCallback: fonts });
SVGtoPDF(doc, canvases[7-1].toSVG(), pageWidth, 0, {fontCallback: fonts });
doc.addPage()
SVGtoPDF(doc, canvases[6-1].toSVG(), 0, 0, {fontCallback: fonts });
SVGtoPDF(doc, canvases[3-1].toSVG(), pageWidth, 0, {fontCallback: fonts });
doc.addPage()
SVGtoPDF(doc, canvases[4-1].toSVG(), 0, 0, {fontCallback: fonts });
SVGtoPDF(doc, canvases[5-1].toSVG(), pageWidth, 0, {fontCallback: fonts });
doc.pipe(res).on('finish', function() {
console.log('booklet pdf was successfully created')
})
doc.end()
}
}
]
async.series(tasks, (err) => {
if (err) {
return next(err);
}
})
console.log('serving pdf')
})