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What is stored property and computed property?

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1. Stored Property : 

A stored property is a constant or variable that is stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

struct FixedLengthRange {
    var firstValue: Int
    let length: Int

So here, firstValue is mutable (var) storedProperty that can have value and change value,
length is immutable (let) computedProperty that can have value but can not be changed.

We can add property observers to any stored properties you define, except for lazy stored properties.

You have the option to define either or both of these observers on a property:
willSet is called just before the value is stored. (newTotalSteps)
didSet is called immediately after the new value is stored. (oldValue)

2. Computed Property :

Classes, structures, and enumerations can define computed properties, which do not actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

struct Point {
    var x = 0.0, y = 0.0
struct Size {
    var width = 0.0, height = 0.0
struct Rect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        set(newCenter) {
            origin.x = newCenter.x - (size.width / 2)
            origin.y = newCenter.y - (size.height / 2)

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