Swift 5.0：不推荐使用“withUnsafeBytes”：使用 `withUnsafeBytes<R>(...)

Question

我之前在 Swift 4.2 中使用过这段代码来生成一个 id：

public static func generateId() throws -> UInt32 {
    let data: Data = try random(bytes: 4)
    let value: UInt32 = data.withUnsafeBytes { $0.pointee } // deprecated warning!
    return value // + some other stuff 
}

withUnsafeBytes 在 Swift 5.0 上已弃用。我该如何解决这个问题？

Answer 1

解决此警告的另一种方法是使用 bindMemory(to:)。

var rawKey = Data(count: rawKeyLength)
let status = rawKey.withUnsafeMutableBytes { rawBytes -> Int32 in
    guard let rawBytes = rawBytes.bindMemory(to: UInt8.self).baseAddress else {
        return Int32(kCCMemoryFailure)
    }
    return CCSymmetricKeyUnwrap(alg, ivBytes, iv.count, keyBytes, key.count, wrappedKeyBytes, wrappedKey.count, rawBytes, &rawKeyLength)
}

Answer 2

我在尝试制作压缩流教程时遇到此错误。为了让它工作，我添加了一个将原始缓冲区指针转换为 UnsafePointer 的步骤

我正在编写的教程中的原始代码。

--> 其中输入：数据

--> 其中流：压缩流

//Method that shows the deprecation alert
return input.withUnsafeBytes { (srcPointer: UnsafePointer<UInt8>) in

//holder
var output = Data()

//Source and destination buffers
stream.src_ptr = srcPointer  //UnsafePointer<UInt8>
stream.src_size = input.count
… etc. 
}

带有转换的代码以使上述代码与有效方法一起工作

return input.withUnsafeBytes { bufferPtr in

//holder
var output = Data()

//Get the Raw pointer at the initial position of the UnsafeRawBuffer
let base: UnsafeRawPointer? = bufferPtr.baseAddress

//Unwrap (Can be combined with above, but kept it separate for clarity)
guard let srcPointer = base else {
   return output
}

//Bind the memory to the type
let count = bufferPtr.count
let typedPointer: UnsafePointer<UInt8> = srcPointer.bindMemory(to: UInt8.self, capacity: count)

// Jump back into the original method
stream.src_ptr = typedPointer  //UnsafePointer<UInt8>
}

Answer 3

在 Swift 5 中，Data 的 withUnsafeBytes() 方法使用（无类型的）UnsafeRawBufferPointer 调用闭包，您可以通过 load() 原始内存中的值：

let value = data.withUnsafeBytes { $0.load(as: UInt32.self) }

（比较 Swift 论坛中的 How to use Data.withUnsafeBytes in a well-defined manner?）。请注意，这要求内存在 4 字节边界上对齐。有关替代方案，请参阅round trip Swift number types to/from Data。

另请注意，从 Swift 4.2 开始，您可以简单地使用新的 Random API 创建一个随机 32 位整数：

let randomId = UInt32.random(in: .min ... .max)

Answer 4

在 Xcode 10.2、Swift 5 上，使用 $0.load(as:) 对我不起作用，无论是读取指针还是写入指针。

相反，使用$0.baseAddress?.assumingMemoryBound(to:) 似乎效果很好。

从指针缓冲区读取的示例（代码与问题无关）：

var reachability: SCNetworkReachability?
data.withUnsafeBytes { ptr in
    guard let bytes = ptr.baseAddress?.assumingMemoryBound(to: Int8.self) else {
        return
    }
    reachability = SCNetworkReachabilityCreateWithName(nil, bytes)
}

写入缓冲区指针的示例（代码与问题无关）：

try outputData.withUnsafeMutableBytes { (outputBytes: UnsafeMutableRawBufferPointer) in
    let status = CCKeyDerivationPBKDF(CCPBKDFAlgorithm(kCCPBKDF2),
                                      passphrase,
                                      passphrase.utf8.count,
                                      salt,
                                      salt.utf8.count,
                                      CCPseudoRandomAlgorithm(kCCPRFHmacAlgSHA1),
                                      rounds,
                                      outputBytes.baseAddress?.assumingMemoryBound(to: UInt8.self),
                                              kCCKeySizeAES256)
    guard status == kCCSuccess else {
        throw Error.keyDerivationError
    }
}

问题中的代码如下所示：

let value = data.withUnsafeBytes { 
    $0.baseAddress?.assumingMemoryBound(to: UInt32.self)
}

在'withUnsafeBytes' is deprecated: use withUnsafeBytes<R>(…) 警告持续存在的情况下，它看起来像the compiler can get confused when the closure has only one line。使闭包有两行或多行可能会消除歧义。