For me it was design patterns. I read the book in the late 90s. I was working mostly on Mac development using PowerPlant. But despite the fact that it used seve

For me it was design patterns. I read the book in the late 90s. I was working mostly on Mac development using PowerPlant. But despite the fact that it used several of the design patterns described in the Gang of Four book, many of them still didn’t seem very clear to me. Then when MacOS became OS X, and I started using Cocoa, it just clicked. Having delegates finally made sense, for example. Previously, it was so abstract, but once I saw it in action within Cocoa, it made a lot more sense for some reason.