Is there a documentation on how SwiftUI works at lower level?

Using Swift
,
1

Hello guys,

I'm curious, are there any docs that refer to how SwiftUI works at lower level? I didn't follow the forums for a long time and I'm guessing there might be some info here as well (?). Reading the official docs doesn't help.
I'm not looking for an in-depth view, I just need to understand it top level so I can debug or better understand things (like @ViewBuilders)

2

SwiftUI isn't open source, so only apple employees know how it works at lower level, but I can link you to some of the newest features of Swift that SwiftUI uses a lot:
omitting return
property wrappers (for example @State)
Identifiable (we saw that it's in SwiftUI, and decided that it would be better if there was a type like that in standard library)
opaque types (it allows you to write some View)
function builders (Not officially part of Swift yet)
There's also a lot of Combine in SwiftUI, but that's another closed source apple framework

3

Hey, thanks for the reply. Some of these did indeed caused some confusion (property wrappers and function buiulders). Also, what I'm having trouble understanding, when you write:

HStack(alignment: .lastTextBaseline, spacing: 0) {

}

It almost looks like you have an init with the last parameter a closure (but there is not _ in, which, last time I was working with Swift was required). Also everything that goes between {} doesn't act as if we'd be in a closure. Are these the function builders? I guessing this should be a compiler feature...

Still, thanks a lot for the info, really helpful!

4

_ in isn't required if your closure uses all parameters using $0 syntax (in reality you just need to use the last one). In your case that closure takes zero parameters, and uses all of them, so _ in isn't required

Yep!

It is. Unofficially right now, but it will be official when it goes through swift evolution process

5

Of, course. My bad.

I feel like I'm on the right path now, most of the confusion was around function builders and property wrappers.

Thanks again

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6

SwiftUI also make use of Dynamic Member Lookup to access member field binding.

Use Cmd-Click/"Go to definition" on any SwiftUI type to see its "source". What it shows is not the real source, but it does let you peek inside a little.

If you Cmd-Click on @ViewBuilder, "Go to definition", you can see it generates TupleView: a tuple of up to 9 View's, this is why you can only have up to 9 views inside any container like VStack. It also can do ForEach, IfElse. You need to know how to use these types if you want to write you own container.

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