SwiftUI - Get size of child?

Question

Is there any way to get the size of a child view in SwiftUI?

I'm basically looking to do the UIKit equivalent of:

self.child.frame.origin.x -= self.child.intrinsicContentSize.width/2.0

I don't think a GeometryReader would work since that returns the available size in the parent.

[Edit] I've found it's possible to get and save the dimensions using .alignmentGuide(_, computeValue:) though that's definitely a hack.

LessonSliderText(text: self.textForProgress(self.progress), color: self.completedColor)
    .alignmentGuide(HorizontalAlignment.leading) { (dimensions) -> Length in
        self.textSize = CGSize(width: dimensions.width, height: dimensions.height)
        return 0
    }
    .offset(x: self.width*self.currentPercentage - self.textSize.width / 2.0)
    .offset(y: -self.textSize.height/2.0)
    .animation(nil)
    .opacity(self.isDragging ? 1.0 : 0.0)
    .animation(.basic())

Answer 1

You can use AnchorPreferences to bubble information about a child view's geometry up to a parent view. See: https://swiftui-lab.com/communicating-with-the-view-tree-part-2/

Answer 2

Basically, the answer at this point is to use a GeometryReader inside of the child's background(...) modifier.

// This won't be valid until the first layout pass is complete
@State var childSize: CGSize = .zero

var body: some View {
  ZStack {
    Text("Hello World!")
      .background(
        GeometryReader { proxy in
          Color.clear.
            .preference(key: SizePreferenceKey.self, value: proxy.size)
         }
       )
  }
  .onPreferenceChange(SizePreferenceKey.self) { preferences in
    self.childSize = preferences
  }
}

struct SizePreferenceKey: PreferenceKey {
    typealias Value = CGSize
    static var defaultValue: Value = .zero

    static func reduce(value: inout Value, nextValue: () -> Value) {
        return nextValue()
    }
}

Answer 3

Updated and generalized @arsenius code. Now you can easily bind a parent view's state variable.

struct ChildSizeReader<Content: View>: View {
    @Binding var size: CGSize
    let content: () -> Content
    var body: some View {
        ZStack {
            content()
                .background(
                    GeometryReader { proxy in
                        Color.clear
                            .preference(key: SizePreferenceKey.self, value: proxy.size)
                    }
                )
        }
        .onPreferenceChange(SizePreferenceKey.self) { preferences in
            self.size = preferences
        }
    }
}

struct SizePreferenceKey: PreferenceKey {
    typealias Value = CGSize
    static var defaultValue: Value = .zero

    static func reduce(value _: inout Value, nextValue: () -> Value) {
        _ = nextValue()
    }
}

Usage:

struct ChildSizeReaderExample: View {
    @State var textSize: CGSize = .zero
    var body: some View {
        VStack {
            ChildSizeReader(size: $textSize) {
                Text("Hello I am some arbitrary text.")
            }
            Text("My size is \(textSize.debugDescription)!")
        }
    }
}