在 swift 中将 UInt32 拆分为 [UInt8]

Question

我想将 UInt32 添加到我使用 [UInt8] 的字节缓冲区。在 java 中，有一个方便的 ByteBuffer class，它有像 putInt() 这样的方法来处理这种情况。在 swift 中如何做到这一点？

我想我可以按以下方式解决这个问题：

let example: UInt32 = 72 << 24 | 66 << 16 | 1 << 8 | 15
var byteArray = [UInt8](count: 4, repeatedValue: 0)

for i in 0...3 {
    byteArray[i] = UInt8(0x0000FF & example >> UInt32((3 - i) * 8))
}

虽然这很冗长，有没有更简单的方法？

Answer 1

你的循环可以更简洁地写成

let byteArray = 24.stride(through: 0, by: -8).map {
    UInt8(truncatingBitPattern: example >> UInt32([=10=]))
}

或者，创建一个 UnsafeBufferPointer 并将其转换到数组：

let example: UInt32 = 72 << 24 | 66 << 16 | 1 << 8 | 15

var bigEndian = example.bigEndian
let bytePtr = withUnsafePointer(&bigEndian) {
    UnsafeBufferPointer<UInt8>(start: UnsafePointer([=11=]), count: sizeofValue(bigEndian))
}
let byteArray = Array(bytePtr)

print(byteArray) // [72, 66, 1, 15]

Swift3（Xcode8 beta 6）的更新：

var bigEndian = example.bigEndian
let count = MemoryLayout<UInt32>.size
let bytePtr = withUnsafePointer(to: &bigEndian) {
    [=12=].withMemoryRebound(to: UInt8.self, capacity: count) {
        UnsafeBufferPointer(start: [=12=], count: count)
    }
}
let byteArray = Array(bytePtr)

Answer 2

您可以从一种 UnsafeMutablePointer 类型转换为另一种类型：

var arr = UnsafeMutablePointer<UInt32>.alloc(1)
arr.memory = example
var arr2 = UnsafeMutablePointer<UInt8>(arr)

Answer 3

我对 Xcode 8 Beta 6 也有类似的问题：写下这一行

var value = String(cString: sqlite3_column_text(stmt, index))

   to


let txt = UnsafePointer<Int8>(sqlite3_column_text(stmt, index))

并解决问题

Answer 4

改进了@Martin R 的回答。在 UInt16、UInt32 和 UInt64 上工作：

protocol UIntToBytesConvertable {
    var toBytes: [Byte] { get }
}

extension UIntToBytesConvertable {
    func toByteArr<T: Integer>(endian: T, count: Int) -> [Byte] {
        var _endian = endian
        let bytePtr = withUnsafePointer(to: &_endian) {
            [=10=].withMemoryRebound(to: Byte.self, capacity: count) {
                UnsafeBufferPointer(start: [=10=], count: count)
            }
        }
        return [Byte](bytePtr)
    }
}

extension UInt16: UIntToBytesConvertable {
    var toBytes: [Byte] {
        return toByteArr(endian: self.littleEndian,
                         count: MemoryLayout<UInt16>.size)
    }
}

extension UInt32: UIntToBytesConvertable {
    var toBytes: [Byte] {
        return toByteArr(endian: self.littleEndian,
                         count: MemoryLayout<UInt32>.size)
    }
}

extension UInt64: UIntToBytesConvertable {
    var toBytes: [Byte] {
        return toByteArr(endian: self.littleEndian,
                         count: MemoryLayout<UInt64>.size)
    }
}

Answer 5

改进了@Martin R 的回答。

func toByteArrary<T>(value: T)  -> [UInt8] where T: UnsignedInteger, T: FixedWidthInteger{
  var bigEndian = value.bigEndian
  let count = MemoryLayout<T>.size
  let bytePtr = withUnsafePointer(to: &bigEndian) {
    [=10=].withMemoryRebound(to: UInt8.self, capacity: count) {
        UnsafeBufferPointer(start: [=10=], count: count)
    }
  }

  return Array(bytePtr)
}

Answer 6

另一种选择是直接扩展 FixedWidthInteger 协议，因此任何 UnsignedInteger 自动免费获得该功能。这是建立在@Benson 的回答上的样子：

extension FixedWidthInteger where Self: UnsignedInteger {

    var bytes: [UInt8] {
        var _endian = littleEndian
        let bytePtr = withUnsafePointer(to: &_endian) {
            [=10=].withMemoryRebound(to: UInt8.self, capacity: MemoryLayout<Self>.size) {
                UnsafeBufferPointer(start: [=10=], count: MemoryLayout<Self>.size)
            }
        }
        return [UInt8](bytePtr)
    }

}

我们可以通过单元测试来验证这一点：

func test_bytes() {
    XCTAssertEqual(UInt8.min.bytes, [0])
    XCTAssertEqual(UInt8.max.bytes, [255])

    XCTAssertEqual(UInt16.min.bytes, [0, 0])
    XCTAssertEqual(UInt16.max.bytes, [255, 255])

    XCTAssertEqual(UInt32.min.bytes, [0, 0, 0, 0])
    XCTAssertEqual(UInt32.max.bytes, [255, 255, 255, 255])

    XCTAssertEqual(UInt64.min.bytes, [0, 0, 0, 0, 0, 0, 0, 0])
    XCTAssertEqual(UInt64.max.bytes, [255, 255, 255, 255, 255, 255, 255, 255])
}

Answer 7

我有类似的需求，正在尝试学习一些有关手动内存管理的知识。我是不是遗漏了什么，或者使用更新版本的 Swift 变得更容易了吗？（我正在使用 Swift 5）

从 UInt32

创建字节数组 ([UInt8])

let example: UInt32 = 1

let byteArray = withUnsafeBytes(of: example.bigEndian) {
    Array([=10=])
}

print(byteArray) // [0, 0, 0, 1]

或者如果你想追加到现有数组，可以在闭包中完成：

var existingArray: [UInt8] = [1, 2, 3]
let example: UInt32 = 1

withUnsafeBytes(of: example.bigEndian) {
    existingArray.append(contentsOf: [=11=])
}

print(existingArray)  // [1, 2, 3, 0, 0, 0, 1]

在 swift 中将 UInt32 拆分为 [UInt8]

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