I wanted to follow up to a blog post I wrote about Message Dispatch in Swift — https://www.raizlabs.com/dev/2016/12/swift-method-dispatch. I mentioned some changes to NSObject that didn’t result in any objections, so I thought it was time to see what the SE mailing list thought.
I’ve read a few conversations on SE mailing list that have morphed into abstract conversations about dynamic vs static dispatch. I want to focus specifically on how Swift NSObject subclasses behave. I think that there are 2 changes that will result in fewer bugs and will not have a substantial impact on performance: ## Remove Table Dispatch from NSObject NSObject subclasses use table dispatch for the initial class declaration block. I think that using message dispatch for NSObject subclasses everywhere will result in a much more consistent developer experience. ## Block NSObject Visibility Optimizations Swift upgrades method dispatch to final when the compiler can prove that the method is not subclassed. I would like to see Swift be more careful about the impact of these optimizations on message dispatch, and consider message dispatch non-upgradable. I thought it would help to frame this choice as a trade-off between Swift’s goals of safe, fast, and expressive. ## Safe Always using message dispatch for NSObject subclasses will fix a class of runtime errors in framework features that are designed around message passing (e.g. KVO). Arguments against using dynamic features like this are valid, but Cocoa frameworks still use dynamic features and the above behaviors result in actual bugs. As a bonus, this will resolve SR-584, where a table-dispatched method that is overridden by a message dispatch method doesn’t behave correctly. ## Fast The above changes will result in slower dispatch in NSObject subclasses. However, I don't think that these dispatch changes actually have a tangible impact on performance. Most NSObject subclasses sit on top of a lot of `objc_msgSend`, and if there is a specific hot spot, it would still be optimizable via the final keyword. ## Expressive Using table dispatch for NSObject without any source indication or library documentation is not very expressive. I think it’s important to weigh table dispatch behavior against all of the framework documentation and developer experience that assume message dispatch. This will also eliminate the need for a lot of `@objc` and `dynamic` annotations that are often inconsistently applied depending on if they are needed in the scope they are defined in (e.g. class vs extension). If this idea shows promise, I’d be glad to formalize a Swift Evolution Proposal and explore syntactic details. I think being able to flag a class with `dynamic` and applying this flag to `NSObject` may be the only syntactic change needed. However, it would be good to debate the merit of the behavior change before the syntax. Thanks! Brian King _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution