JuniorTheoryCommon

What does the program print?

SWIFT

struct Score {
    var points = 0 {
        didSet {
            print(points - oldValue, terminator: " ")
        }
    }
}

var s = Score()
s.points = 5
s.points += 3

5 3

Expecting 5 8: thinking oldValue is the property's first value. It is the value just before the current write, so the second number is 8 - 5.
Expecting 0 0: thinking the property still holds the old value inside didSet. didSet runs after the write, so points already holds the new value.

What does the program print if the observer is willSet with print(newValue - points)?
Why does var s = Score() print nothing?

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JuniorTheoryCommon

What does the program print?

SWIFT

struct Score {
    var points = 0 {
        didSet {
            print(points - oldValue, terminator: " ")
        }
    }
}

var s = Score()
s.points = 5
s.points += 3

5 3

Expecting 5 8: thinking oldValue is the property's first value. It is the value just before the current write, so the second number is 8 - 5.
Expecting 0 0: thinking the property still holds the old value inside didSet. didSet runs after the write, so points already holds the new value.

What does the program print if the observer is willSet with print(newValue - points)?
Why does var s = Score() print nothing?

Back to questions Open topic in roadmap →Answer it yourself — Quiz