Question

What is 'property observer' in swift language ?

Answer 1

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.

Answer 2

Being able to observe changes in various values is essential for many different kinds of programming styles and techniques. Whether we’re using delegates, functions, or reactive programming — much of our logic is often driven by changes in states and values.

There are two very appropriately named property observers in Swift: willSet, and didSet. They are called exactly when you think they would be, right before and right after setting, respectively. On top of this, they are also implicitly given constants describing the newValue and oldValue respectively.