Why isn't onPreferenceChange being called if it's inside a ScrollView in SwiftUI?

Question

I\'ve been seeing some strange behavior for preference keys with ScrollView. If I put the onPreferenceChange inside the ScrollView it won\'t be cal

Answer 1

The problem here is actually not in ScrollView but in usage - this mechanism allow to transfer data up in viewTree:

A view with multiple children automatically combines its values for a given preference into a single value visible to its ancestors.

source

The keywords here - with multiple children. This mean that u can pass it in viewTree from child to parent.

Let's review u'r code:

struct ContentView: View {
    var body: some View {
        ScrollView {
            Text("Hello")
                .preference(key: WidthPreferenceKey.self, value: 20)
                .onPreferenceChange(WidthPreferenceKey.self) {
                    print($0) // Not being called, we're in a scroll view.
                }
        }
    }
}

As u can see now - child pass value to itself, and not to parent - so this don't want to work, as per design.

And working case:

struct ContentView: View {
    var body: some View {
        ScrollView {
            Text("Hello")
                .preference(key: WidthPreferenceKey.self, value: 20)
        }
        .onPreferenceChange(WidthPreferenceKey.self) {
            print($0)
        }
    }
}

Here, ScrollView is parent and Text is child, and child talk to parent - everything works as expected.

So, as I sad in the beginning the problem here not in ScrollView but in usage and in Apple documentation (u need to read it few times as always).

---

And regarding this:

Bound preference WidthPreferenceKey tried to update multiple times per frame.

This is because u may change multiply values in same time and View can't be rendered, try to .receive(on:) or DispatchQueue.main.async as workaround (I guess this may be a bug)