学习 MacOS 开发 (Part 10: 继续学习Swift语法)

[nonocast]

Snippets 1

struct User {
  var name: String?
}

let user = User(name: "nonocast")
print(user.name!)

• let和var的区别:

  
  12> var foo = 1
  foo: Int = 1
  13> foo = 2
  14>  
  15> foo
  $R0: Int = 2
  16>  
  17> let bar = 1
  bar: Int = 1
  18> bar = 2
  expression failed to parse:
  repl.swift:17:1: note: change 'let' to 'var' to make it mutable
  let bar = 1
  ^~~
  var

error: repl.swift:18:1: error: cannot assign to value: 'bar' is a 'let' constant bar = 2 ^~~

repl.swift:17:1: note: change 'let' to 'var' to make it mutable let bar = 1 ^~~ var

- 定义变量时可以通过推测`var name = "hello"`，如果没有推导依据就需要通过冒号后显示表示类型

```swift
let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0
let emptyMessage: String
let message: String = "hello world"

• String?表示optionals

let x: Int? = 7
print(x!)

• 如果不需要考虑继承优先考虑效率更高的struct

struct Resolution {
    var width = 0
    var height = 0
}

let vga = Resolution(width: 640, height: 480)
print(vga.width)

struct为值拷贝，class为引用拷贝。

• struct FrameView : View，这里不是继承，View是protocol, 即protocol View

protocol Namable {
  var name: String? { get set }
}

struct User : Namable {
  var name: String?
}

• User(name: "nonocast")，可以在new的过程中直接设置任意属性的值，不用写N个构造函数。class不支持。