How to get values out of these callbacks when reading a PDF?

Question

I am trying to read a PDF file. Below callbacks also print the messages but there's nothing I can get out of the PDF.

    let pdfBundlePath = Bundle.main.path(forResource: "sample", ofType: "pdf")
    let pdfURL = URL.init(fileURLWithPath: pdfBundlePath!)
    let pdf = CGPDFDocument(pdfURL as CFURL)        

    let operatorTableRef = CGPDFOperatorTableCreate()

    CGPDFOperatorTableSetCallback(operatorTableRef!, "BT") { (scanner, info) in
        print("Begin text object")
    }
    CGPDFOperatorTableSetCallback(operatorTableRef!, "ET") { (scanner, info) in
        print("End text object")
    }
    CGPDFOperatorTableSetCallback(operatorTableRef!, "Tf") { (scanner, info) in
        print("Select font")
    }
    CGPDFOperatorTableSetCallback(operatorTableRef!, "Tj") { (scanner, info) in
        print("Show text")
    }
    CGPDFOperatorTableSetCallback(operatorTableRef!, "TJ") { (scanner, info) in
        print("Show text, allowing individual glyph positioning")
    }

        let page = pdf!.page(at: 1)
        let stream = CGPDFContentStreamCreateWithPage(page!)
        let scanner = CGPDFScannerCreate(stream, operatorTableRef, nil)
        CGPDFScannerScan(scanner)
        CGPDFScannerRelease(scanner)
        CGPDFContentStreamRelease(stream)

Output:

Begin text object
Select font
Show text, allowing individual glyph positioning
End text object

// the same output for at least 10 or more times.

But I am not sure how to get the actual string out of this? Any suggestion would be appreciated.

Answer 1

I have pdf with "hello, world" text (created with export as pdf from TextEdit)

This callback function

CGPDFOperatorTableSetCallback(operatorTableRef!, "TJ") { (scanner, info) in
    print("Show text, allowing individual glyph positioning")
    var pa: CGPDFArrayRef?
    withUnsafeMutablePointer(to: &pa, { (ppa) -> () in
        let r = CGPDFScannerPopArray(scanner, ppa)
        print("TJ", r)
        if r {
            let count = CGPDFArrayGetCount(ppa.pointee!)
            var j = 0
            for i in 0..<count {
                var str: CGPDFStringRef?
                let r = CGPDFArrayGetString(ppa.pointee!, i, &str)
                if r {
                    let string = String(cString: CGPDFStringGetBytePtr(str!)!)
                    print(string, i, j)
                    j += 1
                }
            }
        }
    })
}

prints me

Show text, allowing individual glyph positioning
TJ true
h 0 0
e 2 1
l 4 2
l 6 3
o 8 4
, 10 5
  12 6
w 14 7
o 16 8
rl 18 9
d 20 10

I think it demonstrates, that getting the String is possible :-), at least for Latin alphabet.

for Tj operator, the callback function could be as simple as

CGPDFOperatorTableSetCallback(operatorTableRef!, "Tj") { (scanner, info) in
        print("Show text")
        var text: CGPDFStringRef?
        withUnsafeMutablePointer(to: &text, { (p) -> () in
            let r = CGPDFScannerPopString(scanner, p)
            if r {
                let string = String(cString: CGPDFStringGetBytePtr(p.pointee!)!)
                print(string)
            }
        })
    }

WARNING! to properly show all characters, it is neccesary use font information, but that is a different story. For Latin characters this solutions should work as is.

To be able to 'extract' all strings, all text-showing operators must be implemented

UPDATE Because PDFKit is available on both apple platforms (from iOS11) I suggest to use it for text extraction. The process is much straightforward