How to use GeometryReader in SwiftUI

Issue #735

From my previous post How to use flexible frame in SwiftUI we know that certain views have different frame behaviors. 2 of them are .overlay and GeometryReader that takes up whole size proposed by parent.

By default GeometryReader takes up whole width and height of parent, and align its content as .topLeading

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        VStack {
            Rectangle()
                .fill(Color.gray)
                .overlay(
                    GeometryReader { geo in
                        Text("\(Int(geo.size.width))x\(Int(geo.size.height))")
                            .bold()
                    }
                )
        }
        .frame(width: 300, height: 300)
    }
}

To align content center, we can specify frame with geo information

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        VStack {
            Rectangle()
                .fill(Color.gray)
                .overlay(
                    GeometryReader { geo in
                        Text("\(Int(geo.size.width))x\(Int(geo.size.height))")
                            .bold()
                            .frame(width: geo.size.width, height: geo.size.height, alignment: .center)
                    }
                )
        }
        .frame(width: 300, height: 300)
    }
}

The result is that Text is center aligned

If we were to implement GeometryReader, it would look like this

struct GeometryReader<Content: View>: View {
    let content: (CGSize) -> Content

    func size(proposedSize: CGSize) -> CGSize {
        // Take up whole size proposed by parent
        proposedSize
    }

    func buildBody(calculatedSize: CGSize) -> some View {
        // Pass in the calculated size
        content(calculatedSize)
    }
}