How to make view the size of another view in SwiftUI

Question

I\'m trying to recreate a portion of the Twitter iOS app to learn SwiftUI and am wondering how to dynamically change the width of one view to be the width of another view. I

Answer 1

Let me modestly suggest a slight modification of this bright answer: Version without using preferences:

import SwiftUI

extension HorizontalAlignment {
    private enum UnderlineLeading: AlignmentID {
        static func defaultValue(in d: ViewDimensions) -> CGFloat {
            return d[.leading]
        }
    }

    static let underlineLeading = HorizontalAlignment(UnderlineLeading.self)
}


struct GridViewHeader : View {

    @State private var activeIdx: Int = 0
    @State private var w: [CGFloat] = [0, 0, 0, 0]

    var body: some View {
        return VStack(alignment: .underlineLeading) {
            HStack {
                Text("Tweets").modifier(MagicStuff(activeIdx: $activeIdx, widths: $w, idx: 0))
                Spacer()
                Text("Tweets & Replies").modifier(MagicStuff(activeIdx: $activeIdx, widths: $w, idx: 1))
                Spacer()
                Text("Media").modifier(MagicStuff(activeIdx: $activeIdx, widths: $w, idx: 2))
                Spacer()
                Text("Likes").modifier(MagicStuff(activeIdx: $activeIdx, widths: $w, idx: 3))
                }
                .frame(height: 50)
                .padding(.horizontal, 10)
            Rectangle()
                .alignmentGuide(.underlineLeading) { d in d[.leading]  }
                .frame(width: w[activeIdx],  height: 2)
                .animation(.linear)
        }
    }
}

struct MagicStuff: ViewModifier {
    @Binding var activeIdx: Int
    @Binding var widths: [CGFloat]
    let idx: Int

    func body(content: Content) -> some View {
        var w: CGFloat = 0
        return Group {
            if activeIdx == idx {
                content.alignmentGuide(.underlineLeading) { d in
                    w = d.width
                    return d[.leading]
                }.onTapGesture { self.activeIdx = self.idx }.onAppear(perform: {self.widths[self.idx] = w})

            } else {
                content.onTapGesture { self.activeIdx = self.idx }
            }
        }
    }
}

Version using preferences and GeometryReader:

import SwiftUI

extension HorizontalAlignment {
    private enum UnderlineLeading: AlignmentID {
        static func defaultValue(in d: ViewDimensions) -> CGFloat {
            return d[.leading]
        }
    }

    static let underlineLeading = HorizontalAlignment(UnderlineLeading.self)
}

struct WidthPreferenceKey: PreferenceKey {
    static var defaultValue = CGFloat(0)

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

    typealias Value = CGFloat
}


struct GridViewHeader : View {

    @State private var activeIdx: Int = 0
    @State private var w: [CGFloat] = [0, 0, 0, 0]

    var body: some View {
        return VStack(alignment: .underlineLeading) {
            HStack {
                Text("Tweets")
                    .modifier(MagicStuff(activeIdx: $activeIdx, idx: 0, widthStorage: $w))

                Spacer()

                Text("Tweets & Replies")
                    .modifier(MagicStuff(activeIdx: $activeIdx, idx: 1, widthStorage: $w))

                Spacer()

                Text("Media")
                    .modifier(MagicStuff(activeIdx: $activeIdx, idx: 2, widthStorage: $w))

                Spacer()

                Text("Likes")
                    .modifier(MagicStuff(activeIdx: $activeIdx, idx: 3, widthStorage: $w))

                }
                .frame(height: 50)
                .padding(.horizontal, 10)
            Rectangle()
                .frame(width: w[activeIdx],  height: 2)
                .animation(.linear)
        }
    }
}

struct MagicStuff: ViewModifier {
    @Binding var activeIdx: Int
    let idx: Int
    @Binding var widthStorage: [CGFloat]

    func body(content: Content) -> some View {
        Group {

            if activeIdx == idx {
                content.background(GeometryReader { geometry in
                    return Color.clear.preference(key: WidthPreferenceKey.self, value: geometry.size.width)
                })
                .alignmentGuide(.underlineLeading) { d in
                    return d[.leading]
                }.onTapGesture { self.activeIdx = self.idx }
                    .onPreferenceChange(WidthPreferenceKey.self, perform: { self.widthStorage[self.idx] = $0 })


            } else {
                content.onTapGesture { self.activeIdx = self.idx }.onPreferenceChange(WidthPreferenceKey.self, perform: { self.widthStorage[self.idx] = $0 })
            }
        }
    }
}