How to extend an object with a lazy property? Recently, I needed this for Constrictor and, in this post, you’ll learn how you can achieve it.

“How to … ?” is a collection of short posts where I’ll share problems I’ve faced and how I ended up solving them.

tl;dr - Question 👉 Answer

Problem

Let’s start with the obvious solution, which in this case, would be extending UIView with:

Clean and simple, but it doesn’t work… If you give it a try, Xcode will fail to compile with “Extensions must not contain stored properties”.

Solution

While potentially dangerous, Associated Objects can still be very useful as they allow us to add new properties inside an extension of an existing object.

As an example, we are able to create a lazy property in an UIView extension as follows:

In short, we have a lazy property by associating a Constrictor’s instance to a UIView’s instance in runtime. If there is one associated, it will retrieve it. If there isn’t, it will create one and save its “address” in AssociatedKey.constrictor so it can be retrieved in future calls.

Conclusion

As mentioned, Associated Objects can be dangerous if used improperly. To be fully aware of its capabilities and common pitfalls, I recommend reading Mattt’s article from NSHipster.

Last but not least, if you have any subject you would like to see covered and/or discussed, let me know here or on Twitter! 👀

Thanks for reading. ✨