本文主要从内存和汇编去分析枚举的关联值和原始值
枚举成员值
用法
枚举的声明如下：
enum Direction {
case East
case South
case West
case North
}
使用：
let direction1 = Direction.east
let direction2: Direction = .east
print(direction1 == direction2)
if direction1 == .east {
...
}
switch direction1 {
case .east:
...
case .south:
...
case .west:
...
case .north:
...
}
内存分析
通过MemoryLayout可以获取到枚举大小，内存对齐后大小，对齐字节
print(MemoryLayout<Direction>.size) // 1
print(MemoryLayout<Direction>.stride) // 1
print(MemoryLayout<Direction>.alignment) // 1
Direction只需要1个字节的内存存储成员值，用于区分不同的类型，可以通过LLDB查看其内存值，也可以通过下面的方法打印
var d = Direction.east
var ptr = withUnsafePointer(to: d, { UnsafeRawPointer($0) })
print(String(format: "0x%02X", ptr.load(as: UInt8.self))) // 0x00
d = .south
ptr = withUnsafePointer(to: d, { UnsafeRawPointer($0) })
print(String(format: "0x%02X", ptr.load(as: UInt8.self))) // 0x01
d = .west
ptr = withUnsafePointer(to: d, { UnsafeRawPointer($0) })
print(String(format: "0x%02X", ptr.load(as: UInt8.self))) // 0x02
d = .north
ptr = withUnsafePointer(to: d, { UnsafeRawPointer($0) })
print(String(format: "0x%02X", ptr.load(as: UInt8.self))) // 0x03
Swift的枚举是通过其成员值来区分不同类型的
关联值 (Associated Value)
用法
在Swift中，可以将枚举的成员值跟其他类型的值关联存储在一起
打个比方：在网络请求中，基本只有成功和失败2种情况，而且结果只能是这两种中的其中一个，这种情况下用枚举是要比结构体和类更好的，应该请求结果只能是成功或失败，不可能既成功又失败。请求结果和数据可以通过枚举中的关联值来实现。
enum Response {
case success([String:Any], String)
case failure(Int, String)
}
var resp = Response.success(["name": "Swift"], "请求成功")
// resp = .failure(-1, "请求失败")
switch resp {
case let .success(data, msg):
// Server response success: data=["name": "Swift"] msg=请求成功
print("Server response success: data=\(data)\tmsg=\(msg)")
case let .failure(errorCode, msg):
// Server response failure: errorCode=-1 msg=请求失败
print("Server response failure: errorCode=\(errorCode)\tmsg=\(msg)")
}
成功地通过了枚举类型来传值了
内存分析
为了方便观察内存布局，将关联值类型改为Int和Bool
enum TestEnum {
case test1(Int)
case test2(Int, Int)
case test3(Int, Bool)
}
print(MemoryLayout<TestEnum>.size) // 17
print(MemoryLayout<TestEnum>.stride) // 24
print(MemoryLayout<TestEnum>.alignment) // 8
可以看到，TestEnum的size不再是1了，而是17，为什么是17呢？
关联值的枚举，有点像C/C++中的共用体union，不同的case用同一块内存，因此分配的内存的大小，肯定选最大的case所需的内存。现在基本都是64位系统，因此Int占4个字节，Bool占1个字节，所以TestEnum中所需内存最大的case是test2，需要16字节，那为什么打印出来的是17字节呢？因为在上面讲到了，枚举需要1个字节的内存去存储成员值，所以TestEnum真正用到的内存大小为17字节，由于内存对齐方式，真正分配到的内存是24字节，以alignment值对齐内存。
var e1 = TestEnum.test1(1)
print(Mems.memStr(ofVal: &e1))
// 0x0000000000000001 0x0000000000000000 0x0000000000000000
e1 = .test2(2, 3)
print(Mems.memStr(ofVal: &e1))
// 0x0000000000000002 0x0000000000000003 0x0000000000000001
e1 = .test3(4, true)
print(Mems.memStr(ofVal: &e1))
// 0x0000000000000004 0x0000000000000001 0x0000000000000002
可以很直观的看到，前2*8个字节用于存放关联值，第2*8+1个字节用于存放成员值
原始值 (Raw Value)
用法
枚举的原始值类型不止可以是Int，UInt，还可以是String，Float，Bool类型，声明了原始值之后，可以通过ra wValue计算属性获取原始值，还可以通过init?(rawValue: String)构造器创建枚举
enum Direction : String {
case east = "东"
case south = "南"
case west = "西"
case north = "北"
}
let direction1 = Direction.east
print("\(direction1) rawValue is: \(direction1.rawValue)") // east rawValue is: 东
let d1 = Direction(rawValue: "东")
print("\(d1!) rawValue is: \(d1!.rawValue)") // east rawValue is: 东
需要注意的是，通过init?(rawValue: String)构造器生成的枚举是可选类型的，因为不确保用户传入的原始值一定是正确
如果我们不主动声明原始值的话，编译器会自动分配原始值：
enum Direction : String {
case east
case south
case west
case north
}
let direction1 = Direction.east
print("\(direction1) rawValue is: \(direction1.rawValue)") // east rawValue is: east
let d1 = Direction(rawValue: "east")
print("\(d1!) rawValue is: \(d1!.rawValue)") // east rawValue is: east
还可以实现RawRepresentable协议：
enum Direction : RawRepresentable {
typealias RawValue = String
case east
case south
case west
case north
init?(rawValue: Self.RawValue) {
switch rawValue {
case "东":
self = .east
case "南":
self = .south
case "西":
self = .west
case "北":
self = .north
default:
return nil
}
}
var rawValue: String {
switch self {
case .east:
return "东"
case .south:
return "南"
case .west:
return "西"
case .north:
return "北"
}
}
}
let direction1 = Direction.west
print("\(direction1) rawValue is: \(direction1.rawValue)") // west rawValue is: 西
let d1 = Direction(rawValue: "西")
print("\(d1!) rawValue is: \(d1!.rawValue)") // west rawValue is: 西
一旦声明实现RawRepresentable协议，编译器就不会自动帮我们生成init?(rawValue:)方法和rawValue计算属性。
分析
枚举的原始值不占用存储空间
enum Direction : String {
case east = "东"
case south = "南"
case west = "西"
case north = "北"
}
print(MemoryLayout<Direction>.size) // 1
print(MemoryLayout<Direction>.stride) // 1
print(MemoryLayout<Direction>.alignment) // 1
那枚举的原始值“东”，“南”，“西”，“北”放在哪里呢？
其实是编译器自动为我们实现了rawValue计算属性，在内部通过switch、case返回原始值的
let d = Direction.east
print(d.rawValue)
通过LLDB，我们进入到了Direction枚举的rawValue计算属性的get方法里面
Swift-CommandLine`Direction.rawValue.getter:
0x1000020a0 <+0>: pushq %rbp
0x1000020a1 <+1>: movq %rsp, %rbp
0x1000020a4 <+4>: subq $0x30, %rsp
0x1000020a8 <+8>: movb %dil, %al
0x1000020ab <+11>: movb $0x0, -0x8(%rbp)
0x1000020af <+15>: movb %al, -0x8(%rbp)
0x1000020b2 <+18>: movzbl %al, %edi
0x1000020b5 <+21>: movl %edi, %ecx
0x1000020b7 <+23>: subb $0x3, %al
0x1000020b9 <+25>: movq %rcx, -0x10(%rbp)
0x1000020bd <+29>: movb %al, -0x11(%rbp)
0x1000020c0 <+32>: ja 0x1000020d6 ; <+54> at <compiler-generated>
0x1000020c2 <+34>: leaq 0x9b(%rip), %rax ; Swift_CommandLine.Direction.rawValue.getter : Swift.String + 196
0x1000020c9 <+41>: movq -0x10(%rbp), %rcx
0x1000020cd <+45>: movslq (%rax,%rcx,4), %rdx
0x1000020d1 <+49>: addq %rax, %rdx
0x1000020d4 <+52>: jmpq *%rdx
0x1000020d6 <+54>: ud2
0x1000020d8 <+56>: xorl %edx, %edx
0x1000020da <+58>: leaq 0x51f0(%rip), %rdi ; "\xe4\xb8\x9c"
0x1000020e1 <+65>: movl $0x3, %esi
0x1000020e6 <+70>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x1000020eb <+75>: movq %rax, -0x20(%rbp)
0x1000020ef <+79>: movq %rdx, -0x28(%rbp)
0x1000020f3 <+83>: jmp 0x10000214a ; <+170> at main.swift
0x1000020f5 <+85>: xorl %edx, %edx
0x1000020f7 <+87>: leaq 0x51d7(%rip), %rdi ; "\xe5\x8d\x97"
0x1000020fe <+94>: movl $0x3, %esi
0x100002103 <+99>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002108 <+104>: movq %rax, -0x20(%rbp)
0x10000210c <+108>: movq %rdx, -0x28(%rbp)
0x100002110 <+112>: jmp 0x10000214a ; <+170> at main.swift
0x100002112 <+114>: xorl %edx, %edx
0x100002114 <+116>: leaq 0x51be(%rip), %rdi ; "\xe8\xa5\xbf"
0x10000211b <+123>: movl $0x3, %esi
0x100002120 <+128>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002125 <+133>: movq %rax, -0x20(%rbp)
0x100002129 <+137>: movq %rdx, -0x28(%rbp)
0x10000212d <+141>: jmp 0x10000214a ; <+170> at main.swift
0x10000212f <+143>: xorl %edx, %edx
0x100002131 <+145>: leaq 0x51a5(%rip), %rdi ; "\xe5\x8c\x97"
0x100002138 <+152>: movl $0x3, %esi
0x10000213d <+157>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002142 <+162>: movq %rax, -0x20(%rbp)
0x100002146 <+166>: movq %rdx, -0x28(%rbp)
0x10000214a <+170>: movq -0x28(%rbp), %rax
0x10000214e <+174>: movq -0x20(%rbp), %rcx
0x100002152 <+178>: movq %rax, -0x30(%rbp)
0x100002156 <+182>: movq %rcx, %rax
0x100002159 <+185>: movq -0x30(%rbp), %rdx
0x10000215d <+189>: addq $0x30, %rsp
0x100002161 <+193>: popq %rbp
0x100002162 <+194>: retq
第一眼看到第一行的位置Swift-CommandLine`Direction.rawValue.getter: 明确了这段汇编代码所在的位置
接下来看不懂汇编没关系，我自己也不是很会，但是懂得找关键指令就行了
0x1000020da <+58>: leaq 0x51f0(%rip), %rdi ; "\xe4\xb8\x9c"
0x1000020f7 <+87>: leaq 0x51d7(%rip), %rdi ; "\xe5\x8d\x97"
0x100002114 <+116>: leaq 0x51be(%rip), %rdi ; "\xe8\xa5\xbf"
0x100002131 <+145>: leaq 0x51a5(%rip), %rdi ; "\xe5\x8c\x97"
"\xe4\xb8\x9c"，"\xe5\x8d\x97"，"\xe8\xa5\xbf"，"\xe5\x8c\x97"分别对应"东"，"南"，"西"，"北"这四个字的UTF-8编码。
为了进一步验证猜想，我们自己实现RawRepresentable协议中的rawValue计算属性看看
enum Direction : RawRepresentable {
typealias RawValue = String
case east
case south
case west
case north
init?(rawValue: Self.RawValue) {
switch rawValue {
case "东":
self = .east
case "南":
self = .south
case "西":
self = .west
case "北":
self = .north
default:
return nil
}
}
var rawValue: String {
switch self {
case .east:
return "东1"
case .south:
return "南"
case .west:
return "西"
case .north:
return "北"
}
}
}
这里为了有点区别，在ravValue计算属性的get方法里面我把“东”改成了“东1”，对应的汇编为：
Swift-CommandLine`Direction.rawValue.getter:
0x1000020a0 <+0>: pushq %rbp
0x1000020a1 <+1>: movq %rsp, %rbp
0x1000020a4 <+4>: subq $0x30, %rsp
0x1000020a8 <+8>: movb %dil, %al
0x1000020ab <+11>: movb $0x0, -0x8(%rbp)
0x1000020af <+15>: movb %al, -0x8(%rbp)
0x1000020b2 <+18>: movzbl %al, %edi
0x1000020b5 <+21>: movl %edi, %ecx
0x1000020b7 <+23>: subb $0x3, %al
0x1000020b9 <+25>: movq %rcx, -0x10(%rbp)
0x1000020bd <+29>: movb %al, -0x11(%rbp)
0x1000020c0 <+32>: ja 0x1000020d6 ; <+54> at main.swift:356:14
0x1000020c2 <+34>: leaq 0x9b(%rip), %rax ; Swift_CommandLine.Direction.rawValue.getter : Swift.String + 196
0x1000020c9 <+41>: movq -0x10(%rbp), %rcx
0x1000020cd <+45>: movslq (%rax,%rcx,4), %rdx
0x1000020d1 <+49>: addq %rax, %rdx
0x1000020d4 <+52>: jmpq *%rdx
0x1000020d6 <+54>: ud2
0x1000020d8 <+56>: xorl %edx, %edx
0x1000020da <+58>: leaq 0x5200(%rip), %rdi ; "\xe4\xb8\x9c1"
0x1000020e1 <+65>: movl $0x4, %esi
0x1000020e6 <+70>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x1000020eb <+75>: movq %rax, -0x20(%rbp)
0x1000020ef <+79>: movq %rdx, -0x28(%rbp)
0x1000020f3 <+83>: jmp 0x10000214a ; <+170> at main.swift
0x1000020f5 <+85>: xorl %edx, %edx
0x1000020f7 <+87>: leaq 0x51d7(%rip), %rdi ; "\xe5\x8d\x97"
0x1000020fe <+94>: movl $0x3, %esi
0x100002103 <+99>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002108 <+104>: movq %rax, -0x20(%rbp)
0x10000210c <+108>: movq %rdx, -0x28(%rbp)
0x100002110 <+112>: jmp 0x10000214a ; <+170> at main.swift
0x100002112 <+114>: xorl %edx, %edx
0x100002114 <+116>: leaq 0x51be(%rip), %rdi ; "\xe8\xa5\xbf"
0x10000211b <+123>: movl $0x3, %esi
0x100002120 <+128>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002125 <+133>: movq %rax, -0x20(%rbp)
0x100002129 <+137>: movq %rdx, -0x28(%rbp)
0x10000212d <+141>: jmp 0x10000214a ; <+170> at main.swift
0x10000212f <+143>: xorl %edx, %edx
0x100002131 <+145>: leaq 0x51a5(%rip), %rdi ; "\xe5\x8c\x97"
0x100002138 <+152>: movl $0x3, %esi
0x10000213d <+157>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
0x100002142 <+162>: movq %rax, -0x20(%rbp)
0x100002146 <+166>: movq %rdx, -0x28(%rbp)
0x10000214a <+170>: movq -0x28(%rbp), %rax
0x10000214e <+174>: movq -0x20(%rbp), %rcx
0x100002152 <+178>: movq %rax, -0x30(%rbp)
0x100002156 <+182>: movq %rcx, %rax
0x100002159 <+185>: movq -0x30(%rbp), %rdx
0x10000215d <+189>: addq $0x30, %rsp
0x100002161 <+193>: popq %rbp
0x100002162 <+194>: retq
看起来和编译器自动生成的汇编没什么区别，可能有人说我直接copy编译器生成的汇编了。仔细对比两段汇编中0x1000020da地址的汇编指令：
// 编译器生成
0x1000020da <+58>: leaq 0x51f0(%rip), %rdi ; "\xe4\xb8\x9c"
0x1000020e1 <+65>: movl $0x3, %esi
0x1000020e6 <+70>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
// 自己实现的
0x1000020da <+58>: leaq 0x5200(%rip), %rdi ; "\xe4\xb8\x9c1"
0x1000020e1 <+65>: movl $0x4, %esi
0x1000020e6 <+70>: callq 0x100006e1c ; symbol stub for: Swift.String.init(_builtinStringLiteral: Builtin.RawPointer, utf8CodeUnitCount: Builtin.Word, isASCII: Builtin.Int1) -> Swift.String
编译器生成的，0x51f0(%rip)地址存放的是"\xe4\xb8\x9c"="东"，UTF-8编码的长度为0x3；
自己实现rawValue计算属性，0x5200(%rip)地址存放的是"\xe4\xb8\x9c1"="东1"，UTF-8编码的长度为0x4
最后可以确定了，原始值并不是说枚举会真的存储对应的原始值，而是编译器会自动生成rawValue计算属性，根据枚举值，switch，case返回对应的原始值
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