SwiftUI read more component. Requires iOS 16.

ReadMore.swift

import SwiftUI

struct ContentView: View {
    var text = "long text"

    @State private var lines = 3.0

    var body: some View {
        ScrollView {
            ReadMore(lineLimit: Int(lines)) {
                Text(text)
            }
            .padding()
        }
        .safeAreaInset(edge: .bottom, spacing: 0) {
            VStack {
                GroupBox {
                    Slider(value: $lines, in: 1...24, step: 1)
                } label: {
                    LabeledContent {
                        Text(Int(lines), format: .number)
                    } label: {
                        Text("Lines")
                    }

                }
            }
            .padding(.horizontal)
        }
    }
}

struct ReadMore<Content: View>: View {
    let lineLimit: Int

    @ViewBuilder var content: Content

    @State private var isExpanded: Bool = false

    var body: some View {
        ReadMoreLayout {
            ViewThatFits(in: .vertical) {
                if !isExpanded {
                    fullView
                }

                shortView
            }
        }
    }
    
    var fullView: some View {
        content
            .frame(maxWidth: .infinity, alignment: .leading)
            .fixedSize(horizontal: false, vertical: true)
    }

    var shortView: some View {
        VStack(alignment: .leading) {
            ZStack {
                content
                    .lineLimit(isExpanded ? nil : max(1, lineLimit))
                    .contentTransition(.opacity)
                    .frame(maxWidth: .infinity, alignment: .leading)
            }
            .onTapGesture {
                withAnimation {
                    isExpanded.toggle()
                }
            }
            .clipped()

            Button {
                withAnimation {
                    isExpanded.toggle()
                }
            } label: {
                HStack(spacing: 2) {
                    withSymbolTransition {
                        Image(systemName: isExpanded ? "chevron.up" : "chevron.down")
                            .imageScale(.small)
                            .symbolVariant(.circle.fill)
                            .symbolRenderingMode(.hierarchical)
                    }
                    .backportGeometryGroup()

                    Text(isExpanded ? "Less" : "More")
                }
            }
            .backportGeometryGroup()
        }
        .fixedSize(horizontal: false, vertical: true)
    }

    @ViewBuilder func withSymbolTransition(@ViewBuilder image: () -> some View) -> some View {
        if #available(iOS 17.0, *) {
            image()
                .contentTransition(.symbolEffect(.replace.wholeSymbol.offUp))
        } else {
            image()
        }
    }
}

private struct ReadMoreLayout: Layout {
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) -> CGSize {
        guard let view = subviews.first else {
            return proposal.replacingUnspecifiedDimensions()
        }

        let expanded = view.sizeThatFits(
            .init(
                width: proposal.replacingUnspecifiedDimensions().width,
                height: nil
            )
        )
        let collapsed = view.sizeThatFits(
            .init(
                width: proposal.replacingUnspecifiedDimensions().width,
                height: .zero
            )
        )

        if expanded.height <= collapsed.height {
            return expanded
        } else {
            return collapsed
        }
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) {
        let size = sizeThatFits(proposal: proposal, subviews: subviews, cache: &cache)
        subviews.first?.place(
            at: .init(x: bounds.midX, y: bounds.midY),
            anchor: .center,
            proposal: .init(width: size.width, height: size.height)
        )
    }
}

extension View {
    @ViewBuilder public func backportGeometryGroup() -> some View {
        if #available(iOS 17, *) {
            geometryGroup()
        } else {
            transformEffect(.identity)
        }
    }
}

#Preview {
    ContentView()
}