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When should you use property observers?

Paul Hudson    @twostraws   

Updated for Xcode 12.0

Swift’s property observers let us attach functionality to be run before or after a property is changed, using willSet and didSet respectively. Most of the time property observers aren’t required, just nice to have – we could just update a property normally then call a function ourselves in code. So why bother? When would you actually use property observers?

The most important reason is convenience: using a property observer means your functionality will be executed whenever the property changes. Sure, you could use a function to do that, but would you remember? Always? In every place you change the property?

This is where the function approach goes sour: it’s on you to remember to call that function whenever the property changes, and if you forget then you’ll have mysterious bugs in your code. On the other hand, if you attach your functionality directly to the property using didSet, it will always happen, and you’re transferring the work of ensuring that to Swift so your brain can focus on more interesting problems.

There is one place where using a property observer is a bad idea, and that’s if you put slow work in there. If you had a User struct with an age integer, you would expect that changing age would take place virtually instantly – it’s just a number, after all. If you attach a didSet property observer that does all sorts of slow work, then suddenly changing a single integer could take way longer than you expected, and it could cause all sorts of problems for you.

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