Michael on Swift and Dynamic Dispatch

Michael Tsai, “It’s a Coup”:

The costs for not using message passing, on the other hand, can be high because they make the code more rigid. You cannot retroactively make compiled code more dynamic. And yet, since dynamic is not the default, the odds are that a lot more methods will be static than need to be. Most of the time, objc_msgSend is not why your code is slow, yet Swift acts like it needs to protect you from this.

I’m no fan of swizzling and don’t care what happens to it. I throw swizzling in the same bucket as SIMBL and APE and haxies, and I’m glad they’re not screwing up my apps these days. (Newer Cocoa developers have no idea what I’m talking about, I realize.)

But I do care about KVO. Very much. (Warts and all.)

This die-hard speed freak has never been concerned with the speed of objc_msgSend. I’ve noticed it in Shark and in Instruments, but the real performance issues were elsewhere in my code.

That said, it hasn’t escaped my notice that most of the time static dispatch would be fine. And most of my classes could be marked as final. So I’m not actually outraged or anything — I keep an open mind that I would actually get noticeably better performance from Swift.

Especially given that now we do have the dynamic modifier.

Objective-C is supple by default. Amazingly so. But I shouldn’t pretend I use that suppleness more than I do. And if Swift isn’t so supple — well, that’s probably closer to what we actually need than what we think we need.

Still, though, there is code in Vesper that can’t be ported to pure Swift. (Model code.) The point that you can use something like Core Data but couldn’t write it in Swift remains important. But, then, Swift is young, not even 1.0, and part of the deal is that it’s up to us developers to communicate our needs.

18 Aug 2014

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