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类，它具有像putInt（）这样的方法，就像这样。怎么可以在swift中完成？

我想我可以解决这个问题如下：

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))
}

这是非常冗长的，任何更简单的方法？

Answer 1

你的循环可以更紧凑地写成

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

或者，创建一个UnsafeBufferPointer并转换它到一个数组：

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)

Answer 2

改进了@Martin R的回答。使用UInt16，UInt32和UInt64：

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)
    }
}

Answer 3

我也有类似的需求，并试图学习一些有关手动内存管理的知识。我是否缺少某些东西，或者使用较新版本的Swift变得更容易了吗？（我正在使用Swift 5）

从[UInt8]创建字节数组（UInt32）

let example: UInt32 = 1

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

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

或者如果要附加到现有数组，则可以在闭包中完成：

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]

Answer 4

我对Xcode 8 Beta 6有类似的问题：写这一行

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 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)
}

Answer 6

另一种选择是直接扩展FixedWidthInteger协议，因此任何UnsignedInteger都会自动免费获得该功能。这就是@Benson答案的样子：

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])
}

将UInt32拆分为swift

6 个答案: