将String转换为UnsafePointer< xmlChar>的快捷方法在Swift 3(libxml2)中

问题描述

我正在为 libxml2 C库开发一个Swift 3包装器.

I'm working on a Swift 3 wrapper for the libxml2 C-library.

有两种方便的方法可以将String转换为UnsafePointer<xmlChar>，反之亦然.在libxml2中，xmlChar被声明为unsigned char.

There are two convenience methods to convert String to UnsafePointer<xmlChar> and vice versa. In libxml2 xmlChar is declared as unsigned char.

• UnsafePointer<xmlChar>到String并不复杂

func stringFrom(xmlchar: UnsafePointer<xmlChar>) -> String {
    let string = xmlchar.withMemoryRebound(to: CChar.self, capacity: 1) {
        return String(validatingUTF8: $0)
    }
    return string ?? ""
}

对于String到UnsafePointer<xmlChar>，我尝试了很多事情，例如

For String to UnsafePointer<xmlChar> I tried many things for example

let bytes = string.utf8CString.map{ xmlChar($0) }
return UnsafePointer<xmlChar>(bytes)

但是这不起作用，我发现的唯一可行的解​​决方案是

but this doesn't work, the only working solution I figured out is

func xmlCharFrom(string: String) -> UnsafePointer<xmlChar> {
    let pointer = (string as NSString).utf8String
    return unsafeBitCast(pointer, to: UnsafePointer<xmlChar>.self)
}

有没有一种更好的 swiftier 方式，而无需将桥接转换为NSString和unsafeBitCast?

Is there a better, swiftier way without the bridge cast to NSString and unsafeBitCast?

推荐答案

我能想到的最快的方法是只使用bitPattern:初始化程序:

Swiftiest way I can think of is to just use the bitPattern: initializer:

let xmlstr = str.utf8CString.map { xmlChar(bitPattern: $0) }

这将为您提供xmlChar的Array.挂在上面，当需要将UnsafePointer传递给某些东西时，请使用Array的withUnsafeBufferPointer方法:

This will give you an Array of xmlChars. Hang onto that, and use Array's withUnsafeBufferPointer method when you need to pass an UnsafePointer to something:

xmlstr.withUnsafeBufferPointer { someAPIThatWantsAPointer($0.baseAddress!) }

不要让UnsafePointer从闭包中逸出，因为它在外面不会有效.

Don't let the UnsafePointer escape from the closure, as it won't be valid outside it.

这是如何妥协的?不用让函数返回指针，而是让它闭包.

How's this for a compromise? Instead of having your function return a pointer, have it take a closure.

func withXmlString<T>(from string: String, handler: (UnsafePointer<xmlChar>) throws -> T) rethrows -> T {
    let xmlstr = string.utf8CString.map { xmlChar(bitPattern: $0) }

    return try xmlstr.withUnsafeBufferPointer { try handler($0.baseAddress!) }
}

或者，作为String上的扩展名:

Or, as an extension on String:

extension String {
    func withXmlString<T>(handler: (UnsafePointer<xmlChar>) throws -> T) rethrows -> T {
        let xmlstr = self.utf8CString.map { xmlChar(bitPattern: $0) }

        return try xmlstr.withUnsafeBufferPointer { try handler($0.baseAddress!) }
    }
}

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