Does setting the same value for a property still call willSet and didSet?

Question

If I set a property value to the same value as it is currently set to, will willSet and didSet get called? This is important to know when you have side effects occurring in these functions.

Answer 1

Yes, willSet and didSet get called even when setting to the same value. I tested it in a playground:

class Class1 {
    var willSetCount = 0
    var didSetCount = 0
    var var1: String = "A" {
        willSet {
            willSetCount++
        }
        didSet {
            didSetCount++
        }
    }
}

let aClass1 = Class1()  // {0 0 "A"}
aClass1.var1 = "A"      // {1 1 "A"}
aClass1.var1 = "A"      // {2 2 "A"}

If you want to prevent side effects from happening when the same value is being set, you can compare the value to newValue/oldValue:

class Class2 {
    var willSetCount = 0
    var didSetCount = 0
    var var1: String = "A" {
        willSet {
            if newValue != var1 {
                willSetCount++
            }
        }
        didSet {
            if oldValue != var1 {
                didSetCount++
            }
        }
    }
}

let aClass2 = Class2()  // {0 0 "A"}
aClass2.var1 = "A"      // {0 0 "A"}
aClass2.var1 = "B"      // {1 1 "B"}

Answer 2

Yes, it does. You could have a property of type that does not conform to Equatable protocol and then "same value" would not make sense. willSet and didSet isn't called only when the value is set inside initializer.