# Swift 快速参考

*类型、Optional、Protocol、错误处理要点*

> Source: Swift Documentation (swift.org) · MIT

## 基础

### Hello World

```
import Foundation
print("Hello, World!")
```

### 常量与变量

```
let name = "Swift"       // constant (immutable)
var count = 0            // variable (mutable)
count += 1
let pi: Double = 3.14   // explicit type annotation
```

### 注释

```
// single-line comment
/* multi-line
   comment */
/// documentation comment (Markdown supported)
```

## 类型

### 基本类型

| Command | Description |
|---------|-------------|
| `Int` | 平台大小的整数（现代系统为 64 位） |
| `Double` | 64 位浮点数（优先于 Float） |
| `Float` | 32 位浮点数 |
| `Bool` | `true` / `false` |
| `String` | Unicode 字符串，值类型 |
| `Character` | 单个扩展字形簇 |

### 类型推断与转换

```
let score = 95              // inferred as Int
let gpa = 3.8              // inferred as Double
let total = Double(score) + gpa  // explicit conversion
let label = "Score: \(score)"    // string interpolation
```

### 元组

```
let point = (x: 3, y: 5)
print(point.x)            // named access
let (x, y) = point        // decompose
let (first, _) = point    // ignore second value
```

### 类型别名

```
typealias Coordinate = (Double, Double)
let origin: Coordinate = (0.0, 0.0)
```

## 控制流

### If / Else

```
if score > 90 { print("A") }
else if score > 80 { print("B") }
else { print("C") }
```

### Switch

```
switch grade {
case "A": print("excellent")
case "B", "C": print("passing")
default: print("unknown")
}
```

### 循环

```
for i in 0..<5 { }            // half-open range
for name in names { }          // collection
for (i, val) in list.enumerated() { }
while condition { }
repeat { } while condition     // do-while
```

### Guard

```
func process(value: Int?) {
    guard let v = value, v > 0 else { return }
    print(v)   // v is unwrapped and in scope
}
```

## 函数

### 基础函数

```
func greet(name: String) -> String {
    return "Hello, \(name)!"
}
greet(name: "Alice")
```

### 参数标签

```
func move(from start: Int, to end: Int) -> Int {
    return end - start
}
move(from: 0, to: 10)  // external labels
func add(_ a: Int, _ b: Int) -> Int { a + b }
```

### 默认参数与可变参数

```
func join(_ items: String..., separator: String = ", ") -> String {
    items.joined(separator: separator)
}
join("a", "b", "c")
```

### inout 参数

```
func double(_ x: inout Int) { x *= 2 }
var num = 5
double(&num)   // num is now 10
```

## 闭包

### 闭包语法

```
let double = { (x: Int) -> Int in return x * 2 }
let nums = [3, 1, 2]
let sorted = nums.sorted { $0 < $1 }
let mapped = nums.map { $0 * 10 }
```

### 尾随闭包

```
UIView.animate(withDuration: 0.3) {
    view.alpha = 0.0
}
```

### 捕获值

```
func makeCounter() -> () -> Int {
    var count = 0
    return { count += 1; return count }
}
let counter = makeCounter()  // counter() => 1, 2, ...
```

## 类与结构体

### 结构体（值类型）

```
struct Point {
    var x: Double
    var y: Double
}
var p = Point(x: 1, y: 2)  // auto memberwise init
```

### 类（引用类型）

```
class Vehicle {
    var speed: Double = 0
    init(speed: Double) { self.speed = speed }
}
class Car: Vehicle { var gear: Int = 1 }
```

### 结构体 vs 类

| Command | Description |
|---------|-------------|
| `struct` | 值类型，赋值时复制，不支持继承 |
| `class` | 引用类型，按引用共享，支持继承 |
| `mutating` | 修改 `self` 的结构体方法必须标注 |
| `deinit` | 仅类支持的析构器（释放前调用） |

## Protocol

### 定义与遵从

```
protocol Drawable {
    var description: String { get }
    func draw()
}
struct Circle: Drawable { /* implement required members */ }
```

### Protocol 扩展

```
extension Drawable {
    func log() { print("Drawing: \(description)") }
}
// all Drawable conformers get log() for free
```

### 常用 Protocol

| Command | Description |
|---------|-------------|
| `Equatable` | `==` 和 `!=` 比较 |
| `Comparable` | `<`、`>`、`<=`、`>=` 排序 |
| `Hashable` | 可用作字典键或 Set 元素 |
| `Codable` | Encodable + Decodable（JSON、Plist） |
| `CustomStringConvertible` | 自定义 `description` 属性 |
| `Identifiable` | 需要 `id` 属性（SwiftUI） |

## Optional

### 声明 Optional

```
var name: String? = "Alice"  // may contain String or nil
var age: Int? = nil          // currently nil
let count: Int = 5           // non-optional, never nil
```

### 解包

```
if let n = name { print(n) }       // optional binding
guard let n = name else { return }  // guard
let n = name ?? "Unknown"           // nil coalescing
let n = name!                       // force unwrap (crashes if nil)
```

### 可选链

```
let count = user?.address?.zip?.count
// returns nil if any link in the chain is nil
user?.save()   // called only if user is non-nil
```

### Optional map

```
let length = name.map { $0.count }   // Int?
let upper = name.flatMap { $0.isEmpty ? nil : $0.uppercased() }
```

## 枚举

### 基础枚举

```
enum Direction {
    case north, south, east, west
}
var heading = Direction.north
heading = .east   // type inferred
```

### 关联值

```
enum Result {
    case success(data: String)
    case failure(code: Int, message: String)
}
if case .failure(let code, _) = r { print(code) }
```

### 原始值

```
enum Planet: Int {
    case mercury = 1, venus, earth, mars
}
let p = Planet(rawValue: 3)   // Optional(.earth)
print(Planet.earth.rawValue)  // 3
```

### 枚举方法

```
enum Suit: String, CaseIterable {
    case hearts, diamonds, clubs, spades
}
Suit.allCases.forEach { print($0.rawValue) }
```

## 错误处理

### 定义错误

```
enum NetworkError: Error {
    case badURL
    case timeout(seconds: Int)
    case serverError(code: Int)
}
```

### 抛出与捕获

```
func fetch(url: String) throws -> Data {
    guard url.hasPrefix("https") else { throw NetworkError.badURL }
    return Data()
}
do { let data = try fetch(url: "https://example.com") }
catch { print("Error: \(error)") }
```

### try 变体

| Command | Description |
|---------|-------------|
| `try` | 必须在 `do-catch` 内，向上传播错误 |
| `try?` | 返回 Optional，错误时为 `nil` |
| `try!` | 强制 try，出错时崩溃 |
| `throws` | 函数可以抛出错误 |
| `rethrows` | 仅当闭包参数抛出时才抛出 |