PDF to UIImage (with capability to resize)

PDFToUIImage.swift

import Foundation
import UIKit

/// Renders the first page of the given PDF file to a `UIImage`. If
/// `maxLength` is given, then the resulting image is scaled to a
/// matching size.
///
/// Example: If `maxLength` is set to `100.0`, this might result in
/// the following dimensions (since aspect ratio is preserved):
///
///  +------50-----+
///  |             |
///  |             |
///  |             |
///  |             |
///  100           |
///  |             |
///  |             |
///  |             |
///  |             |
///  +-------------+
///
///  +-----------100-----------+
///  |                         |
///  30                        |
///  |                         |
///  +-------------------------+
///
/// Inspired by Paul Hudson's post on PDF rendering:
/// https://www.hackingwithswift.com/example-code/core-graphics/how-to-render-a-pdf-to-an-image
///
/// - Parameters:
///   - url: The file URL to the PDF document
///   - maxLength: The max length of the resulting UIImage, in points at main screen scale
/// - Returns: An image if the PDF can be rendered
func drawPDF(_ url: URL, maxLength: CGFloat?) -> UIImage? {
  guard let document = CGPDFDocument(url as CFURL) else { return nil }
  guard let page = document.page(at: 1) else { return nil }

  let pageRect = page.getBoxRect(.mediaBox)
  let (width, height): (CGFloat, CGFloat) = {
    guard let maxLength = maxLength else {
      return (pageRect.width, pageRect.height)
    }
    let aspectRatio = pageRect.width / pageRect.height
    if aspectRatio >= 1 {
      return (maxLength, maxLength / aspectRatio)
    } else {
      return (maxLength / (1 / aspectRatio), maxLength)
    }
  }()
  let scale = width / pageRect.width
  let renderer = UIGraphicsImageRenderer(size: .init(width: width, height: height))
  
  return renderer.image { ctx in
    ctx.cgContext.translateBy(x: 0.0, y: height)
    ctx.cgContext.scaleBy(x: scale, y: -scale)
    ctx.cgContext.drawPDFPage(page)
  }
}