Open Radar

 
Summary:
I ran into an unexpected (for me) behaviour of property observers today, where a property observer is triggered whenever you access a computed property of the property for which the observer is added.

Say you have a protocol A which defines a get property x. You then create a class AImpl, in which you conform to the protocol by means of a computed property. Now, in yet another class B, you define a property p of type A and give it a didSet property observer.

Observed behaviour:
Whenever you access p inside a member function of A, the property observer will be triggered.

Expected behaviour:
Whenever you access p inside a member function of A, the property observer will NOT be triggered.

If instead we define a property q of type Aimpl, the observed behaviour is as expected: the property observer will NOT be triggered whenever you access q inside a member function of A.

See the attached playground for a reproducible case.

Steps to Reproduce:
1. Open the Playground
2. Run the Playground

Expected Results:
The text “Observed property observer called” is printed only once: when we set the property/

Actual Results:
The text “Observed property observer called” is printed twice: when we set the property as well as when we access it.

Notes:

Example playground can also be found here:
https://github.com/larslockefeer/Radars/blob/master/UnexpectedPropertyObserverBehaviour.playground/Contents.swift