How to use UnsafeMutablePointer <OpaquePointer> in Swift?

Question

How to use UnsafeMutablePointer<OpaquePointer>in Swift with some Core Foundation base? Why is there UnsafeMutablePointer<OpaquePointer>?

Given the general: some UnsafeMutablePointer<SomeType>, wheretypealias SomeType = OpaquePointer

API Specific Example

// SOURCE: import ApplicationServices.PrintCore
typealias PMPrinter = OpaquePointer
func PMSessionGetCurrentPrinter(_ printSession: PMPrintSession, _ currentPrinter: UnsafeMutablePointer<PMPrinter>)
func PMPrinterGetPaperList(PMPrinter, UnsafeMutablePointer<Unmanaged<CFArray>?>)

Case Study Usage: Listing Documents Supported by the Printer

let printInfo = NSPrintInfo.shared()
let printSession = PMPrintSession(printInfo.pmPrintSession())
var currentPrinterOptional: PMPrinter? = nil
PMSessionGetCurrentPrinter(printSession, &currentPrinterOptional!)
guard let currentPrinter = currentPrinterOptional else { return }

// Get the array of pre-defined PMPapers this printer supports.
// PMPrinterGetPaperList(PMPrinter, UnsafeMutablePointer<Unmanaged<CFArray>?>)
var paperListUnmanaged: Unmanaged<CFArray>?
PMPrinterGetPaperList(currentPrinter, &paperListUnmanaged)
guard let paperList = paperListUnmanaged?.takeUnretainedValue() as [AnyObject]? else { return }

Observed errors

Which compilers do not start. It seems that the (possibly) sensible syntax is not compiling.

The above (expected) Runtime "fatal error: unexpectedly found nil while unpacking an optional value" was received in the above example.

Some choose other attempts:

// Compile Error: Address of variable 'currentPrinter' taken before is is initialized
var currentPrinter: PMPrinter
PMSessionGetCurrentPrinter(printSession, &currentPrinter)

// Compile Error: Nil cannot initialze specified type 'PMPrinter' (aka 'OpaquePointer')
var currentPrinter: PMPrinter = nil
PMSessionGetCurrentPrinter(printSession, &currentPrinter)

// Compile Error: Variable 'currentPrinterPtr' used before being initialized
var currentPrinterPtr: UnsafeMutablePointer<PMPrinter>
PMSessionGetCurrentPrinter(printSession, currentPrinterPtr)

// Compile OK: actually compiles
// Runtime Error: unexpectedly found nil while unwrapping an Optional value
var currentPrinterOptional: PMPrinter? = nil
PMSessionGetCurrentPrinter(printSession, &currentPrinterOptional!)

Resources

Apple: Core Printing ⇗
Apple: Using Swift with Cocoa and Objective-C ⇗

, UnsafeMutablePointer<PMPrinter> typealias UnsafeMutablePointer<OpaquePointer> .

Answer 1

PMPrinter PMPaper PrintCore " "

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

Swift OpaquePointer, .

PMSessionGetCurrentPrinter() - PMPrinter, Swift inout. unsafeBitCast.

PMPaper CFArrayGetValueAtIndex() , Swift . UnsafeRawPointer, OpaquePointer.

:

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)
}