{"id":50122,"date":"2025-11-20T16:17:12","date_gmt":"2025-11-20T21:17:12","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=50122"},"modified":"2025-12-01T16:13:18","modified_gmt":"2025-12-01T21:13:18","slug":"no-mainactor-by-default","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2025\/11\/20\/no-mainactor-by-default\/","title":{"rendered":"(No) MainActor by Default"},"content":{"rendered":"

Matt Massicotte<\/a> (Mastodon<\/a>):<\/p>\n

Swift 6.2 gives you the ability make MainActor<\/code> the default isolation. Unlike the rest of the features introduced as part of “Approachable Concurrency<\/a>”, this is a long-term mode. It is optional and will remain so. However, this mode is enabled for new app targets in Xcode 26. And many people take this as a very strong signal from Apple, myself included.<\/p>

[…]<\/p>

Well I have made up my mind, at least for now. We should not. I’m not sure we should even have the ability to do so.<\/p>

[…]<\/p>

But when you finally encounter concurrency, and you almost certainly will<\/strong>, a default of MainActor<\/code> can make those encounters much more difficult to understand and address.<\/p>

[…]<\/p>

(I also did a whole talk<\/a> on this if that’s interesting.)<\/p><\/blockquote>\n\n

I still find this all rather confusing, but I’m inclined to agree with his argument. I understood the original motivation as sort of progressive disclosure. You shouldn’t have to deal with concurrency at all if you don’t need your code to be concurrent. But for most real apps you will have to understand and use the full Swift Concurrency. If opting in to MainActor<\/code> (the keyword, not the default isolation) is easier than opting out, is that really helping in the end? Why start down a garden path that leads to a cliff? And is having the simple mode worth the confusion of having two modes, and having to keep straight which module is using which? (This is on top of having separate modes for Swift 5 and 6.)<\/p>\n\n

Ryan Booker<\/a>:<\/p>\n

I’ve seen quite a few people saying something similar. What worries me is that not only is MainActor the default now, the WWDC videos introducing it are quite clear that Apple think you should make your App target and UI based modules all MainActor by default, with non UI modules nonisolated by default.<\/p>

So I’m worried about fighting the tide, but also the idea that you have two inverted concurrency systems to reason about in one code base if you follow Apple’s suggestion!<\/p><\/blockquote>\n\n

Jonathan Wight<\/a>:<\/p>\n

\n

The most vocal Swift Concurrency expert outside of Apple is recommending to avoid MainActor default isolation and you’re left wondering what is even the point.<\/p>\n<\/blockquote>\n\n

Previously:<\/p>\n