The anticipated cross-platform system allowing Macs to run iOS apps may have its launch delayed for a year from 2018 until as late as 2019, and will also, according to John Gruber, an experienced Apple journalist, function for iOS 13 and macOS 10.15 rather than the initially intended macOS 10.4 and iOS 12.
Gruber stated that despite the confirmation from Gurman and Axios’ Ina Fried that the cross-platform mechanism was intended for development in 2018, the project was extremely likely to be delayed until 2019 when it would incorporate a possible to improvement to the iOS user interface the same year, he also suggested that Apple hoped to develop declarative control APIs for iOS in the hope that the reduced complication would inspire developers to create more modern cross-platform apps.
There’s nothing inherently cross-platform about a declarative control API. But it makes sense that if Apple believes that (a) iOS and MacOS should have declarative control APIs, and (b) they should address the problem of abstracting the API differences between UIKit (iOS) and AppKit (MacOS), that they would tackle them at the same time. Or perhaps the logic is simply that if they’re going to create a cross-platform UI framework, the basis for that framework should be a declarative user interface.
Gruber also suggested on his site, Daring Fireball, that the system was highly unlikely to be explained at WWDC 2018 in June, and that developers should expect its launch in conjunction with MacOS 10.15 and iOS 13.