struct & class

Both struct and class have:

1. var

var body: some View {
    Text("Hellow World!")
}

2. let

let defaultColor = Color.orange

3. functions

func multiply(operand: Int, by: Int) -> Int {
    return operand * by
}
multiply(operand: 5, by: 6)

• 注意：每个parameter可以有2个label：

func multiply(_ operand: Int, by otherOperand: Int) -> Int {
   return operand * otherOperand
}
multiply(5, by: 6)

第一个label给外部输入用；第二个label给func内部用。

• _ means "unused" / "leave it out".

4. initializers

• Special functions that are called when creating a struct or class.

struct MemoryGame {
    init (numberOfParisOfCards: Int) {
        // create a game with that many paris of cards.
    }
}

• 一个struct中可以有很多个init，每个init携带不同参数。

Difference between struct and class

• struct is a Value type, but class is a Reference type.

  struct	class
  Value type	Reference type
  copied when passed or assigned	passed around via pointers in heap
  copy on write	automatically reference counted
  functional programming	object-oriented programming
  no inheritance	inheritance (single)
  "free" init initializes ALL vars	"free" init initializes NO vars
  Mutability explicitly stated	Always mutable

• Most things you see are struct. Arrays, Bools, Dictionaries...
• struct is your "go-to" data structure, whereas class is only used in specific circumstances.
• ViewModel in MVVM is always a class.
• UIKit (old-style iOS) is class-based.

protocol

• View is a protocol.

type parameter (don't care)

• Sometimes we may want to manipulate data structures that we are "type agnostic" about. In other words, we do not know what type it is and we do not care.

• Swift is a strongly-typed language, so we cannot have variables that are "untyped". Therefore, we use type parameter.

• An awesome example of generics: Array.

How Array uses a "don't care" type

struct Array<Element> {
...
func append(_ element; Element) {...}
}

• The type of the argument to append is Element: a "don't care" type.
• The Element is determined when someone uses Array.

var a = Array<Int>()
a.append(5)

enum

functions

• Functions are also types.

var operation: (Double) -> Double

This is a var called operation. It is of type "function that takes a Double and returns a Double".

func square(operand: Double) -> Double {
    return operand * operand
}
operation = square
let result = operation(4) // result would equal 16
operation = sqrt //sqrt is a built-in function

• We do not use argument labels (e.g. operand:) when executing function types.

Closures

When passing functions around that we are very often inlining them, we call such an inlined function a closure

SwiftUI is mostly about functional programming, therefore "functions as types" is a very important concept in Swift.