What version of Swift is SwiftUI written on?

Using Swift
1

I’m guessing it’s >=5.1 because that’s when property wrappers were introduced.

2

The question is a little unclear, SwiftUI was written with the version of Swift that was current when it was released, and has updated to be written with each subsequent version of Swift since then.

Older apps that haven't been recompiled with a newer version of Swift will still be using the newest released Swift runtime and system libraries when run on the newest version of the OS.

Newer Swift compilers support compatibility modes for older source code via a compiler flag, but that's not the same as actually using an older Swift.

3

SwiftUI was introduced in iOS 13 which bundled a toolchain with Swift 5.1

SwiftUI has progressively been updated to the latest versions like as of recently SwiftUI adopted Swift 6 concurrency.
SwiftUI is updated with each new iOS release to take advantage of features and optimizations meaning that it has been developed on code from Swift 5.1 all the way to 6.0

TLDR
SwiftUI is a System library for Darwin based OSes. These libraries are updated every time there is an update to be maximally compatible with the latest system provided runtime version. SwiftUI has been available since iOS 13 which bundled Swift 5.1

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