View showing the problem that occurs when using GeometryReader with a non-greedy view. A companion file to CodeSlicing: CoreConcepts in SwiftUI - GeometryReader

GeometryReaderProblemDemo.swift

import SwiftUI
import PureSwiftUI

private let size: CGFloat = 200
private let gradient = LinearGradient([Color.red, Color.yellow], to: .bottomTrailing)

struct GeometryReaderProblemDemo: View {
    @State private var text = "Hello"
    @State private var textSize: CGSize = .zero
    @State private var appendingText = ""
    var body: some View {
        VStack {
            UnderlinedText(text + " " + appendingText, font: .title)
            TextField("Append something...", text: $appendingText)
                .textFieldStyle(RoundedBorderTextFieldStyle())
                .autocapitalization(.none)
                .padding(25)
        }
        .greedyFrame()
        .background(gradient)
        .edgesIgnoringSafeArea(.all)
    }
}

private struct UnderlinedText: View {
    private let text: String
    private let font: Font
    private let padding: CGFloat
    
    @State private var textSize: CGSize = .zero
    
    init(_ text: String, font: Font = .title, padding: CGFloat = 25) {
        self.text = text
        self.font = font
        self.padding = padding * 2
    }
    
    var body: some View {
        ZStack {
            Text(self.text)
                .font(self.font)
                .fixedSize()
                .geometry(id: text) { (geo: GeometryProxy) in
                    self.textSize = geo.size
            }
            Color.black
                .frame(textSize.width, 2)
                .yOffset(textSize.height / 2)
        }
        .frame(textSize.width + padding, textSize.height + padding)
        .background(Color.white.cornerRadius(10).shadow(5))
        .blendMode(.darken)
    }
}

public extension View {
    
    func geometry(_ geoCallback: @escaping (GeometryProxy) -> ()) -> some View {
        geometry(id: 1, geoCallback)
    }
    
    func geometry<T: Hashable>(id: T, _ geoCallback: @escaping (GeometryProxy) -> ()) -> some View {
        background(GeometryReader { (geo: GeometryProxy) in
            Color.clear.onAppear {
                geoCallback(geo)
            }
            .id(id)
        })
    }
    
}

struct GeometryReaderProblemDemo_Previews: PreviewProvider {
    struct GeometryReaderProblemDemo_Harness: View {
        
        var body: some View {
            GeometryReaderProblemDemo()
        }
    }
    
    static var previews: some View {
        GeometryReaderProblemDemo_Harness()
            .previewDevice(.iPhone_11_Pro_Max)
    }
}