UnsafeMutablePointer<OpaquePointer>Swift如何与某些Core Foundation框架一起使用？为什么要UnsafeMutablePointer<OpaquePointer>？
给定，一般：一些UnsafeMutablePointer<SomeType>地方typealias SomeType = OpaquePointer
具体示例API
// SOURCE: import ApplicationServices.PrintCore
typealias PMPrinter = OpaquePointer
func PMSessionGetCurrentPrinter(_ printSession: PMPrintSession, _ currentPrinter: UnsafeMutablePointer<PMPrinter>)
func PMPrinterGetPaperList(PMPrinter, UnsafeMutablePointer<Unmanaged<CFArray>?>)
特定示例用例：获取打印机支持的纸张列表
let printInfo = NSPrintInfo.shared()
let printSession = PMPrintSession(printInfo.pmPrintSession())
var currentPrinterOptional: PMPrinter? = nil
PMSessionGetCurrentPrinter(printSession, ¤tPrinterOptional!)
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 }
编译不运行。看起来（可能）合理的语法无法编译。
上面的示例获得以下（预期）运行时“致命错误：在展开Optional值时意外发现nil”。
一些选择其他尝试：
// Compile Error: Address of variable 'currentPrinter' taken before is is initialized
var currentPrinter: PMPrinter
PMSessionGetCurrentPrinter(printSession, ¤tPrinter)
// Compile Error: Nil cannot initialze specified type 'PMPrinter' (aka 'OpaquePointer')
var currentPrinter: PMPrinter = nil
PMSessionGetCurrentPrinter(printSession, ¤tPrinter)
// 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, ¤tPrinterOptional!)
苹果：核心印刷⇗
苹果：将Swift与Cocoa和Objective-C结合使用⇗
尽管文档提供了有用的信息，但难以理解的UnsafeMutablePointer<PMPrinter>使用typealias的可行实现UnsafeMutablePointer<OpaquePointer>。
PMPrinter并且PMPaper在PrintCore框架中定义为指向“不完整类型”的指针
typedef struct OpaquePMPrinter* PMPrinter;
typedef struct OpaquePMPaper* PMPaper;
这些作为导入到Swift中OpaquePointer，使用起来有些麻烦。
的第二个参数PMSessionGetCurrentPrinter()是指向非可选 PMPrinter变量的指针，在Swift中，必须在将其作为inout参数传递之前对其进行初始化。初始化空指针的一种可能方法是使用unsafeBitCast。
PMPaper从数组中获取对象的最简单方法似乎是使用CFArrayGetValueAtIndex()而不是将其桥接到Swift数组。会传回UnsafeRawPointer，可以转换成OpaquePointer。
这在我的测试中有效：
let printInfo = NSPrintInfo.shared()
let printSession = PMPrintSession(printInfo.pmPrintSession())
var currentPrinter = unsafeBitCast(0, to: PMPrinter.self)
PMSessionGetCurrentPrinter(printSession, ¤tPrinter);
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)
}
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