Wrong year and video.
It was during WWDC 2016, "What's New in Swift" session.
https://developer.apple.com/videos/play/wwdc2016/402/
From the transcript. More on the video.
"I want to talk about a particular case study, and that's the Dock. And the Dock is interesting for two reasons.
The first reason is it's a lot more than people think. It's not just the bar at the bottom of your desktop. It's actually a large amount of the macOS Windows, you know, management experience. So things like Mission Control, LaunchPad, Spaces, this is all what we consider part of the Dock.
The second reason why Dock is interesting is it's actually been a Swift adopter for two releases. It started adopting Swift in El Capitan.
So we have two releases to look at and to compare in how it was using Swift. So a lot of changes happened to the Dock in Sierra.
First, almost all of Mission Control was rewritten in Swift, and the accessibility engine was rewritten in Swift. Why did we do this? We just didn't rewrite code just for the sake of rewriting code. The code had kind of grown organically over time as features had been added and it was time to refactor it.
But also some new functionality needed to be implemented, and so it was the appropriate time to make this, you know, to do this rewrite.
Now, what does this look like at the project level? So to give you an idea, Dock is about 200,000 lines of code with tens of thousands of lines written in Swift.
And between El Capitan and Sierra, the amount of Swift code in the project more than doubled. And this actually, because this is part of a rewrite, this replaced a significant amount of C and Objective-C code. And the effects here are somewhat non-linear, because there's some refactoring and some new features being added, but we saw there was about 15 percent less code to replace the existing C and Objective-C code in Swift, and this is with new features being added at the same time. And the net result is this project is actually slightly smaller in Sierra than it was in El Capitan. Now, there's a lot you can read into statistics. I think the most important thing here is the engineers working on the Dock love using Swift. They are not mandated to use it. They volunteered."
So one would expect that during the last two years, the amount of Swift code has only increased, specially with the tiny spot Objective-C now gets at WWDC.