macOS下Swift生成Hello World PDF出现空白页的解决方法

问题描述 #

目前公开的Swift编写PDF内容的示例几乎都针对iOS平台，要实现macOS端的PDF生成功能时，无法使用UIGraphicsRenderer等iOS专属UI接口。
最初编写的仅输出「Hello World」文本的测试程序，生成的PDF可以正常创建保存，但没有任何内容，错误代码如下：

import Foundation
import PDFKit

func printPDF() {
    
    let localURL = FileManager.default.currentDirectoryPath
    let url = URL(fileURLWithPath: localURL).appendingPathComponent("tv-schedule-for-today.pdf")
 
    var pdfDoc: PDFDocument? = PDFDocument()
    
    let page: PDFPage? = PDFPage()
    var mediaBox: CGRect = page!.bounds(for: .mediaBox)
    
    guard let currentContext = CGContext(url as CFURL, mediaBox: &mediaBox, nil) else {
        return
    }
    var context: CGContext? = currentContext

    context!.beginPDFPage(nil)

    let attributes = [
          NSAttributedString.Key.font: NSFont.boldSystemFont(ofSize: 72)
        ]
    let text = "Hello World!"
    text.draw(at: CGPoint(x: 200, y: 300), withAttributes: attributes)

    context!.endPDFPage()

    pdfDoc!.insert(page!, at: 0)
        
    context!.closePDF()

    pdfDoc!.write(to: url as URL)
    
    context = nil

}

问题原因 #

实现时存在两处遗漏：

• 没有将CGContext与NSGraphicsContext关联
• 没有调用page.draw启动渲染

修复后可正常运行的代码 #

import Foundation
import PDFKit

func printPDF() {
    
    let localURL = FileManager.default.currentDirectoryPath
    let url = URL(fileURLWithPath: localURL).appendingPathComponent("tv-schedule-for-today.pdf")
 
    let pdfDoc: PDFDocument? = PDFDocument(url: url)
    //var pageCount = 0  // 后续自定义分页使用
    
    let page: PDFPage? = pdfDoc!.page(at: 0)!
    
    var mediaBox: CGRect = page!.bounds(for: .mediaBox)
    
    guard let currentContext = CGContext(url as CFURL, mediaBox: &mediaBox, nil) else {
        return
    }
    var context: CGContext? = currentContext
    
    let nscontext = NSGraphicsContext(cgContext: context!, flipped: false)
    NSGraphicsContext.current = nscontext

    context!.beginPDFPage(nil)
    
    do {
        page!.draw(with: .mediaBox, to: context!)

        let attributes = [
              NSAttributedString.Key.font: NSFont.boldSystemFont(ofSize: 36)
            ]
        let text = "I'm a simpler PDF!"
        text.draw(at: CGPoint(x: 100, y: 100), withAttributes: attributes)
    
    }

    context!.endPDFPage()

    NSGraphicsContext.current = nil

    context!.closePDF()
    
    context = nil

}

内容的提问来源于stack exchange，提问作者Joe Bell