Swift Gotcha: Unexpected observer behaviour for inout arguments

I recently ran into an observer problem: it was being called without updates to the associated variable.

Since I was using the associated variable as an inout argument, that was my first suspect. So I wrote a little test in playground to investigate this:

So, ... indeed. The observer of 'str' is called even though the property is not updated in "testDidSetObserver". Note that even when the if statement is removed from the function the observer is still called. It seems that using a property in an inout argument will always trigger the property observers after the function is finished.

I also ran a test that prints the value of "countDidSet" inside the function, and the count is 0 inside the function. Thus the observer is indeed triggered after the function finishes, whether the property is accessed or not.

So observers and inout arguments don't mix well.

PS: I also ran the test for the willSet observer, same results.

Note: As of Swift 2.2 this behaviour is now documented in the manual with a little explanation as to why.

Happy coding...

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