How to use UnsafeMutablePointer in Swift?

Question

How does one use an UnsafeMutablePointer in Swift with some Core Foundation framework? Why have an UnsafeMutablePointer

Answer 1

PMPrinter and PMPaper are defined in the PrintCore framework as pointer to an "incomplete type"

typedef struct OpaquePMPrinter*         PMPrinter;
typedef struct OpaquePMPaper*           PMPaper;

Those are imported into Swift as OpaquePointer, and are a bit cumbersome to use.

The second argument to PMSessionGetCurrentPrinter() is a pointer to a non-optional PMPrinter variable, and in Swift it must be initialized before being passed as an inout argument. One possible way to initialize a null-pointer is to use unsafeBitCast.

The easiest way to get the PMPaper objects from the array seems to be to use CFArrayGetValueAtIndex() instead of bridging it to a Swift array. That returns a UnsafeRawPointer which can be converted to an OpaquePointer.

This worked in my test:

let printInfo = NSPrintInfo.shared()
let printSession = PMPrintSession(printInfo.pmPrintSession())

var currentPrinter = unsafeBitCast(0, to: PMPrinter.self)
PMSessionGetCurrentPrinter(printSession, &currentPrinter);

var paperListUnmanaged: Unmanaged<CFArray>?
PMPrinterGetPaperList(currentPrinter, &paperListUnmanaged)
guard let paperList = paperListUnmanaged?.takeUnretainedValue() else {
    fatalError()
}
for idx in 0..<CFArrayGetCount(paperList) {
    let paper = PMPaper(CFArrayGetValueAtIndex(paperList, idx))!
    var width = 0.0, height = 0.0
    PMPaperGetWidth(paper, &width)
    PMPaperGetHeight(paper, &height)
    print(width, height)
}