如何在 Swift 中使用 UnsafeMutablePointer<OpaquePointer>？

Question

如何在 Swift 中使用 UnsafeMutablePointer<OpaquePointer> 和一些 Core Foundation 框架？为什么要有UnsafeMutablePointer<OpaquePointer>？

给定，一般：一些UnsafeMutablePointer<SomeType> 其中typealias SomeType = OpaquePointer

特定示例 API

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

特定用例示例：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, &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 }

观察到的错误

编译的东西不会运行。看起来（也许）合理的语法无法编译。

上面的示例得到以下（预期的）运行时“致命错误：在展开可选值时意外发现 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, &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!)

资源

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

虽然文档提供了有用的信息，但类型别名为 UnsafeMutablePointer<OpaquePointer> 的 UnsafeMutablePointer<PMPrinter> 的可行实现一直难以捉摸。

Answer 1

PMPrinter 和 PMPaper 在 PrintCore 框架中定义 作为指向“不完整类型”的指针

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

这些以OpaquePointer 的形式导入 Swift，并且有点 使用起来很麻烦。

PMSessionGetCurrentPrinter() 的第二个参数是指向 一个 非可选 PMPrinter 变量，在 Swift 中它必须是 在作为 inout 参数传递之前初始化。一种可能的方式 初始化空指针是使用unsafeBitCast。

从数组中获取PMPaper 对象的最简单方法似乎是 是使用CFArrayGetValueAtIndex() 而不是将它桥接到一个 斯威夫特数组。这将返回一个 UnsafeRawPointer 可以转换 到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, &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)
}