DynamicViewProperty

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1

I’m trying to figure out how DynamicViewProperty in SwiftUI works, that mechanism is something I want as well :-> Does that use a custom reflection to make it work?

struct MyOwnCoolViewProperty : DynamicViewProperty {
  var gotCalled = 0
  mutating func update() { gotCalled += 1 }
}
struct MyView: View {
  var prop = MyOwnCoolViewProperty()
  var body ....
}

This is calling update before the body property is accessed. But how? 🙂

It is easy enough to get the dynamic properties using something like:

let dynamicProps = Mirror(reflecting: view).children.compactMap {
  $0.value as? DynamicViewProperty
}

But that doesn't allow you to call a mutating function. Are they using custom reflection to achieve that, or doesn't this involve reflection at all?

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2

On a high level it seems to be using much more than reflection, its actually directly calling into Swift runtime. Logic for this is implemented in private AttributeGraph framework written in C++, that weakly imports a lot of Swift runtime symbols.
This has interesting side effects, you can make the property ‘let’ and it will still be updated :upside_down_face:

I’ve looked at it briefly, so maybe someone will be able to provide more details.

3

Yes, SwiftUI uses Swift runtime metadata to reflect over views.

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4

Calling into the Swift runtime is reflection :-) It just seems that there is no Swift API for that.

For now I solved it in a different way, which funny enough also has the same let sideeffect ;-)

P.S.: I think the C++ part is actually unrelated, that "just" seems to be the structure for the rendering tree.

5

Right, just looking at the symbols it references, they seem to be doing more than just iterating over properties and calling Swift functions

6

Taking this opportunity to suggest that Swift.Mirror should get some love

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7

So this is my implementation: SwiftWebUI/DynamicViewProperty.swift at master · SwiftWebUI/SwiftWebUI · GitHub
I'm using Reflection (the lib) to find the offset positions of the properties in the View.

To make the View's value "mutable" one just casts the pointer to a mutable one. Which explains why this works even if the properties are defined as let. Not sure whether that is really sound (optimiser might layout the properties differently?), but I guess the layout should be stable as part of the "ABI".

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