Swift学习笔记之[UInt8]/[Int8]那些事

简介

[UInt8]是Swift中的数组类型，也是作用比较特殊的一种数组类型, 常用于底层交互与内存操作与管理。如: 字符串编解码, 充当字节缓冲区等等

字符串编解码

• [UInt8]（或[Int8]）转 String

String有对应的直接从[UInt8]或[Int8]转换成String的构造函数

/// "这是一个字符串" 的utf8编码
let bytes: [UInt8] = [232, 191, 153, 230, 152, 175, 228, 184, 128, 228, 184, 170, 229, 173, 151, 231, 172, 166, 228, 184, 178]
/// 转换成字符串
if let msg = String(bytes: bytes, encoding: .utf8) {
	print(msg)
}

• String 转 [UInt8](或[Int8]）

String转[UInt8]或[Int8]时需要借助中间角色Data

let msg = "这是一个字符串"

if let data = msg.data(using: .utf8) {
  let bytes = [UInt8](data)
  print(bytes)
}

底层交互

1. 转换到C语言中的const char*, char*, void*,const void*

这里的转换指的是使用同一块内存地址

指针类型对应关系

C语言指针类型	swift语言指针对象类型
char *	UnsafeMutablePointer<Int8>
const char *	UnsafePointer<Int8>
unsigned char *	UnsafeMutablePointer<UInt8>
const unsigned char *	UnsafePointer<UInt8>
void *	UnsafeMutableRawPointer
const void *	UnsafeRawPointer

几种特殊的指针类型

• UnsafeRawBufferPointer
• UnsafeMutableRawBufferPointer
• UnsafeBufferPointer<T>
• UnsafeMutableBufferPointer<T>

这些带有Buffer的指针类型， 可以理解为对应的不带Buffer的指针类型加上了缓冲区大小, 比如:

UnsafePointer<Int> 对应 const int *, 表示仅有地址

那UnsafeBufferPointer<Int>对应 const int * 加 size, 表示该地址与内存大小所对应的一块缓冲区

从[UInt8]或[Int8] 转换到 const unsigned char*或 const char *

[UInt8] -> UnsafeRawBufferPointer -> UnsafeBufferPointer<UInt8> -> UnsafePointer<UInt8> (Int8同理)

/// 0. 原始字节数据 8字节
let buffer = [UInt8](repeating: 0, count: 8) 

/// 1. [UInt8] -> UnsafeRawBufferPointer
let unsafeRawBufferPointer = buffer.withUnsafeBytes { $0 } 

/// 2. UnsafeRawBufferPointer -> UnsafeBufferPointer<UInt8>
let unsafeBufferPointer = unsafeRawBufferPointer.bindMemory(to: UInt8.self)

/// 3. UnsafeBufferPointer<UInt8> -> UnsafePointer<UInt8>
let unsafePointer = unsafeBufferPointer.baseAddress

其中 步骤1和步骤2可以合并

/// 0. 原始字节数据 8字节
let buffer = [UInt8](repeating: 0, count: 8)

/// 1. [UInt8] -> UnsafeBufferPointer<UInt8>
let unsafeBufferPointer = buffer.withUnsafeBufferPointer {$0}

/// 2.. UnsafeBufferPointer<UInt8> -> UnsafePointer<UInt8>
let unsafePointer = unsafeBufferPointer.baseAddress

从[UInt8]或[Int8]转换到 unsigned char *或 char *

[UInt8] -> UnsafeMutableRawBufferPointer -> UnsafeMutableBufferPointer<UInt8> -> UnsafeMutablePointer<UInt8> (Int8同理)

/// 0. 原始字节数据 8字节
var buffer = [UInt8](repeating: 0, count: 8)

/// 1. [UInt8] -> UnsafeMutableRawBufferPointer
let unsafeMutableRawBufferPointer = buffer.withUnsafeMutableBytes { $0 }

/// 2. UnsafeMutableRawBufferPointer -> UnsafeMutableBufferPointer<UInt8>
let unsafeMutableBufferPointer = unsafeMutableRawBufferPointer.bindMemory(to: UInt8.self)

/// 3. UnsafeMutableBufferPointer<UInt8> -> UnsafeMutablePointer<UInt8>
let unsafeMutablePointer = unsafeMutableBufferPointer.baseAddress

其中的步骤1和步骤2也是可以合并的

/// [UInt8] -> UnsafeMutableBufferPointer<UInt8>
let unsafeMutableBufferPointer = buffer.withUnsafeMutableBufferPointer {$0 }

从[UInt8]或[Int8]转换成const void *或者void *

在上述的步骤里, 在步骤1得到的unsafeMutableRawBufferPointer就可以通过该对象的baseAddress字段就能获取到对应的void *

如:

/// 获取 const void *
let unsafeRawPointer = unsafeRawBufferPointer.baseAddress

/// 获取void *
let unsafeMutableRawPointer = unsafeMutableRawBufferPointer.baseAddress

2. 从C语言中的const char * , char *, const unsigned char *, unsigned char *, const void *, void *对应缓冲区的数据创建 Data

/// 这里是示例， 类型为 UnsafeMutablePointer<Int8>， 也就是 char *
/// message可能来自任何C语言的接口
let message = strerror(errno) 

/// 通过指针和大小来构造 UnsafeBufferPointer<Int8>
let unsafeBufferPointer = UnsafeBufferPointer<Int8>(start: message, count: strlen(msg!))

/// 构造Data
let data = Data(buffer: unsafeBufferPointer)

3. 从Data得到 [UInt8] 或者 [Int8]以及String

• Data其实内部就是[UInt8]，在大部分情况下都可以直接当成[UInt8]来使用, 字节的append，remove, insert, find等等都有相同的操作
• 但是如果需要拷贝出来一份 [UInt8]或者[Int8]，可以使用map函数

/// 缓冲区里的数据    
let data = Data(buffer: unsafeBufferPointer)

/// [UInt8]
let u8a = data.map {$0}

/// [Int8]
let s8a = data.map {Int8($0)}

• String也自带从data数据的构造函数

let text = String(data: data, encoding: .utf8)