是否有一种 Swift 预处理器方法来检测桥接头包含？

Question

我找到的唯一答案是在this，我对此并不满意。

我正在添加一个标准 MD5 转换器作为字符串扩展：

/* ###################################################################################################################################### */
/**
 From here: https://stackoverflow.com/q/24123518/879365
 I am not making this public, because it requires the common crypto in the bridging header.
 */
fileprivate extension String {
    /* ################################################################## */
    /**
     - returns: the String, as an MD5 hash.
     */
    var md5: String {
        let str = self.cString(using: String.Encoding.utf8)
        let strLen = CUnsignedInt(self.lengthOfBytes(using: String.Encoding.utf8))
        let digestLen = Int(CC_MD5_DIGEST_LENGTH)
        let result = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: digestLen)
        CC_MD5(str!, strLen, result)

        let hash = NSMutableString()

        for i in 0..<digestLen {
            hash.appendFormat("%02x", result[i])
        }

        result.deallocate()
        return hash as String
    }
}

它要求我将以下内容添加到我的桥接头中：

#import <CommonCrypto/CommonCrypto.h>

由于我想将此添加到一套可重复使用的工具中，我想看看是否有一种方法可以在编译时检测是否使用了通用加密库。

有没有办法将其设置为条件编译？

如果不是，这没什么大不了的；只是意味着我需要将其设置为单独的源文件。

Answer 1

值得注意的是，如果您使用dlsym 访问它，您可以在没有桥接头的情况下调用CC_MD5。

import Foundation

typealias CC_MD5_Type = @convention(c) (UnsafeRawPointer, UInt32, UnsafeMutableRawPointer) -> UnsafeMutableRawPointer

let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
let CC_MD5 = unsafeBitCast(dlsym(RTLD_DEFAULT, "CC_MD5")!, to: CC_MD5_Type.self)

var md5 = Data(count: 16)
md5.withUnsafeMutableBytes {
    _ = CC_MD5("abc", 3, $0)
}

assert(md5 == Data(bytes: [0x90, 0x01, 0x50, 0x98, 0x3C, 0xD2, 0x4F, 0xB0, 0xD6, 0x96, 0x3F, 0x7D, 0x28, 0xE1, 0x7F, 0x72]))

Answer 2

这是我找到的解决方案。这是我原始变体的混搭，也是 Rob 的出色答案。它很有魅力（我用它来建立对RFC2617 Digest authentication的响应）。由于使用了内置的钩子，我不再需要桥接头，并且可以将它添加到我的字符串扩展集中。

非常经典的正确答案案例来自与我正在寻找的完全不同的地方。当这种情况发生时，我喜欢它。

来吧：

public extension String {
    /* ################################################################## */
    /**
     From here: https://stackoverflow.com/q/24123518/879365, but modified from here: https://stackoverflow.com/a/55639723/879365
     - returns: an MD5 hash of the String
     */
    var md5: String {
        var hash = ""
        
        // Start by getting a C-style string of our string as UTF-8.
        if let str = self.cString(using: .utf8) {
            // This is a cast for the MD5 function. The convention attribute just says that it's a "raw" C function.
            typealias CC_MD5_Type = @convention(c) (UnsafeRawPointer, UInt32, UnsafeMutableRawPointer) -> UnsafeMutableRawPointer
            
            // This is a flag, telling the name lookup to happen in the global scope. No dlopen required.
            let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
            
            // This loads a function pointer with the CommonCrypto MD5 function.
            let CC_MD5 = unsafeBitCast(dlsym(RTLD_DEFAULT, "CC_MD5")!, to: CC_MD5_Type.self)
            
            // This is the length of the hash
            let CC_MD5_DIGEST_LENGTH = 16
            
            // This is where our MD5 hash goes. It's a simple 16-byte buffer.
            let result = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: CC_MD5_DIGEST_LENGTH)
            
            // Execute the MD5 hash. Save the result in our buffer.
            _ = CC_MD5(str, CUnsignedInt(str.count), result)
            
            // Turn it into a normal Swift String of hex digits.
            for i in 0..<CC_MD5_DIGEST_LENGTH {
                hash.append(String(format: "%02x", result[i]))
            }
            
            // Don't need this anymore.
            result.deallocate()
        }
        
        return hash
    }
}