Apple says:
To make for a future release of macOS in which 32-bit software will no prolonged run without compromise, beginning in macOS High Sierra 10.13.4, a user is informed on the launch of an app that depends on 32-bit software. The alert arrives only once per app.
When users try to launch a 32-bit app in 10.13.4, it will still begin, but it will do so with a notification message notifying the user that the app will ultimately not be compatible with the operating system unless it is updated. This serves the same approach that Apple took with iOS, which developed its sunset of 32-bit app support with iOS 11 last fall.
Developers and users inquiring about how this will play out will be able to look at the related process in iOS for context. On January 1 of this year, Apple suspended accepting 32-bit app submissions in the Mac App Store. This June, the business will also stop accepting updates for existing 32-bit applications. iOS observed a similar progression, with 32-bit app submissions ending in February of 2015 and taking of app updates for 32-bit apps ending in June of 2015.
And before you say, “Well, that’s just the Mac App Store,” note that Apple also said the audience this week:
If you distribute your apps outside the Mac App Store, we highly recommend distributing 64-bit binaries to make sure your users can continue to run your apps on future versions of macOS.
Developers have a some of tools already open to them to help with this transition. First off, there is now a Terminal App which will force an app to run in 64-bit mode, providing its developer to see if it meets any issues in what will eventually be a 64-bit-only environment. Apple has also made new demonstrative tools available for testing 64-bit compatibility in the newly released Xcode 9.3 beta.
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