Draw on a PDF using Swift on macOS - swift

Question

My goal is to write text on a PDF, like an annotation.
I achieved it transforming the PDFPage to a NSImage, I drew on the NSImage then I saved the PDF formed by the images.
let image = NSImage(size: pageImage.size)
image.lockFocus()
let rect: NSRect = NSRect(x: 50, y: 50, width: 60, height: 20)
"Write it on the page!".draw(in: rect, withAttributes: someAttributes)
image.unlockFocus()
let out = PDFPage(image: image)
The problem is obviously that out (the new page of the output PDF) is a PDFPage of images and not a regular one. So the output PDF is very big in size and you can't copy and paste anything on it. It's just a sequence of images.
My question is if there's a way to add simple text on a PDF page programmatically without using NSImage. Any idea?
Note: There's this class in iOS programming UIGraphicsBeginPDFPageWithInfo which could be very helpful in my case. But I can't find the similar class for macOS development.

Answer 1

You can create a PDF graphics context on macOS and draw a PDFPage into it. Then you can draw more objects into the context using either Core Graphics or AppKit graphics.
Here's a test PDF I created by printing your question:
And here's the result from drawing that page into a PDF context, then drawing more text on top of it:
Here's the code I wrote to transform the first PDF into the second PDF:
import Cocoa
import Quartz
let inUrl: URL = URL(fileURLWithPath: "/Users/mayoff/Desktop/test.pdf")
let outUrl: CFURL = URL(fileURLWithPath: "/Users/mayoff/Desktop/testout.pdf") as CFURL
let doc: PDFDocument = PDFDocument(url: inUrl)!
let page: PDFPage = doc.page(at: 0)!
var mediaBox: CGRect = page.bounds(for: .mediaBox)
let gc = CGContext(outUrl, mediaBox: &mediaBox, nil)!
let nsgc = NSGraphicsContext(cgContext: gc, flipped: false)
NSGraphicsContext.current = nsgc
gc.beginPDFPage(nil); do {
page.draw(with: .mediaBox, to: gc)
let style = NSMutableParagraphStyle()
style.alignment = .center
let richText = NSAttributedString(string: "Hello, world!", attributes: [
NSFontAttributeName: NSFont.systemFont(ofSize: 64),
NSForegroundColorAttributeName: NSColor.red,
NSParagraphStyleAttributeName: style
])
let richTextBounds = richText.size()
let point = CGPoint(x: mediaBox.midX - richTextBounds.width / 2, y: mediaBox.midY - richTextBounds.height / 2)
gc.saveGState(); do {
gc.translateBy(x: point.x, y: point.y)
gc.rotate(by: .pi / 5)
richText.draw(at: .zero)
}; gc.restoreGState()
}; gc.endPDFPage()
NSGraphicsContext.current = nil
gc.closePDF()