在Swift中将UInt32拆分为[UInt8]

Question

我希望将UInt32添加到字节缓冲区中，我使用[UInt8]。在Java中，有方便的ByteBuffer类，其中包含像putInt()这样的方法，适用于这种情况。在Swift中该怎么做呢？

我想可以按照以下方式解决：

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))
}

虽然这样说话很啰嗦，但有没有更简单的方法呢？

- Kelo

Answer 1

你的循环可以更简洁地编写为

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

或者，创建一个UnsafeBufferPointer并将其转换为数组：

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

var bigEndian = example.bigEndian
let bytePtr = withUnsafePointer(&bigEndian) {
    UnsafeBufferPointer<UInt8>(start: UnsafePointer($0), count: sizeofValue(bigEndian))
}
let byteArray = Array(bytePtr)

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

Swift 3更新（Xcode 8 beta 6）：

var bigEndian = example.bigEndian
let count = MemoryLayout<UInt32>.size
let bytePtr = withUnsafePointer(to: &bigEndian) {
    $0.withMemoryRebound(to: UInt8.self, capacity: count) {
        UnsafeBufferPointer(start: $0, count: count)
    }
}
let byteArray = Array(bytePtr)

- Martin R

---

谢谢！乍一看，这个循环似乎更难读，但它确实更紧凑。 - Kelo

Answer 2

我有类似的需求，尝试学习一些手动内存管理知识。我是否漏掉了什么，或者在Swift的新版本中这变得更容易了？（我正在使用Swift 5）

从UInt32创建一个字节数组（[UInt8]）

let example: UInt32 = 1

let byteArray = withUnsafeBytes(of: example.bigEndian) {
    Array($0)
}

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

如果你想要将值追加到现有的数组中，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: $0)
}

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

- user1325158

Answer 3

改进了@Martin R的回答。对UInt16、UInt32和UInt64进行了修改：

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) {
            $0.withMemoryRebound(to: Byte.self, capacity: count) {
                UnsafeBufferPointer(start: $0, 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)
    }
}

- Benson

Answer 4

您可以将一个UnsafeMutablePointer类型转换为另一个：

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

- matt

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) {
    $0.withMemoryRebound(to: UInt8.self, capacity: count) {
        UnsafeBufferPointer(start: $0, count: count)
    }
  }

  return Array(bytePtr)
}

- Alfy

Answer 6

另一个选项是直接扩展FixedWidthInteger协议，这样任何UnsignedInteger都可以自动获得该函数。以下是在@Benson的答案基础上构建的示例：

extension FixedWidthInteger where Self: UnsignedInteger {

    var bytes: [UInt8] {
        var _endian = littleEndian
        let bytePtr = withUnsafePointer(to: &_endian) {
            $0.withMemoryRebound(to: UInt8.self, capacity: MemoryLayout<Self>.size) {
                UnsafeBufferPointer(start: $0, 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])
}

- Kane Cheshire

Answer 7

我在 Xcode 8 Beta 6 中遇到了类似的问题：

写下这行代码

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

   to


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

并解决问题

- pradip kikani