swift willSet didSet and get set methods in a property

问题

What is the difference between willSet - didSet, and get - set, when working with this inside a property?

From my point of view, both of them can set a value for a property. When, and why, should I use willSet - didSet, and when get - set?

I know that for willSet and didSet, the structure looks like this:

var variable1 : Int = 0 {
    didSet {
        println (variable1)
    }
    willSet(newValue) {
    ..
    }
}

var variable2: Int {
    get {
        return variable2
    }
    set (newValue){
    }
}

回答1:

When and why should I use willSet/didSet

willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.

Consider your example with outputs:

var variable1 : Int = 0 {
        didSet{
            print("didSet called")
        }
        willSet(newValue){
            print("willSet called")
        }
    }

    print("we are going to add 3")

     variable1 = 3

    print("we added 3")

Output:

we are going to add 3
willSet called
didSet called
we added 3

it works like pre/post -condition

On the other hand, you can use get if you want to add, for example, a read-only property:

var value : Int {
 get {
    return 34
 }
}

print(value)

value = 2 // error: cannot assign to a get-only property 'value'

回答2:

@Maxim's answer is for the 1st part of your question.

As for when to use get and set: when you want a computed property. This:

var x: Int

creates a stored property, which is automatically backed up by a variable (not directly accessible though). Setting a value to that property is translated in setting the value in the property, and similarly for getting.

Instead:

var y = {
    get { return x + 5 }
    set { x = newValue - 5}
}

will create a computed property, which is not backed up by a variable - instead you have to provide the implementation of the getter and/or setter, usually reading and writing values from another property and more generally as a result of a computation (hence the computed property name)

回答3:

get set:

get set are 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

Additionally you can define Read-Only Computed Properties. A read-only computed property always returns a value, and can be accessed through dot syntax, but cannot be set to a different value

Example get only property-

 var number: Double {
        return .pi*2
    }

willSet didSet:

willSet didSet are Property Observers

Property observers observe and respond to changes in a property’s value. Property observers are called every time a property’s value is set, even if the new value is the same as the property’s current value.

willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.

Example -

var score: Int = 0 {
    willSet(newScore) {
        print("willSet  score to \(newScore)")
    }
    didSet {
        print("didSet score to \(oldValue) new score is: \(score)")
    }
}
score = 10
//Output 
//willSet  score to 10
//didSet score to 0 new score is: 10

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html

回答4:

var variable1 : Int = 0 { //It's a store property
    didSet {
        print (variable1)
    }
    willSet(newValue) {
    ..
    }
}

var variable2: Int { //It's a Computed Proprties
    get {
        return variable2
    }
    set (newValue){
    }
}

For detail info of Store property and computed property
So when you are trying to assign the value into variable at that assigning time come the concept of 'didSet' & 'willSet'. As @Maxim says

willSet is called just before the value is stored.
didSet is called immediately after the new value is stored.

Example of 'willSet' & 'didSet' :

class Number {
   var variable1 : Int = 0 {
        didSet{
            print("didSet called")
        }
        willSet(newValue){
            print("willSet called")
        }

    }
}
print("we are going to add 3")
Number().variable1 = 3
print("we added 3")

//o/p:
we are going to add 3
willSet called
didSet called
we added 3

And generally when two property are dependent at that time 'get' & 'set' used. (It's also used in protocol this is different concept.)

Example of 'get' & 'set':

class EquilateralTriangle{
    var sideLength: Double = 0.0
    init(sideLength: Double){
        self.sideLength = sideLength
    }
    var perimeter: Double {
        get {
            return 3.0 * sideLength
        }
        set {
            sideLength = newValue / 3.0
        }
    }

}

var triangle = EquilateralTriangle(sideLength: 3.0)
print(triangle.perimeter) //o/p: 9.0
triangle.perimeter = 12.0
print(triangle.sideLength) //o/p: 4.0

来源：https://stackoverflow.com/questions/25398753/swift-willset-didset-and-get-set-methods-in-a-property

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