How to animate collapsing for a Text in SwiftUI

Question

I need help with SwiftUI animation/transition behavior. I have to implement collapsing/expanding Text during changing lineLimit from nil to 5 and vice versa.

At the moment I have a working code, you can see it below.

import SwiftUI

public struct ContentView: View {
    private let description: String
    
    @State private var isCollapsed: Bool = true
    @State private var isExpandeButtonShow: Bool = false
    
    public init(description: String) {
        self.description = description
    }

    // MARK: - Body

    public var body: some View {
        VStack(alignment: .leading, spacing: 0) {
            text.padding(.top, 26)
                .font(Font.system(size: 12))
            if isExpandeButtonShow {
                collapseButton.padding(.top, 9)
            }
            
            Spacer()
        }
        .padding(.horizontal, 16)
    }

    // MARK: - Private

    private var text: some View {
        Text(description)
            .lineLimit(isCollapsed ? 5 : nil)
            .background(
                GeometryReader { geometry in
                    Color.clear.onAppear {
                        truncateIfNeeded(withGeometry: geometry)
                    }
                }
            )
            .animation(.linear(duration: 4))
            .transition(.opacity)
    }

    private func truncateIfNeeded(withGeometry geometry: GeometryProxy) {
        let total = description.boundingRect(
            with: CGSize(
                width: geometry.size.width,
                height: .greatestFiniteMagnitude
            ),
            options: .usesLineFragmentOrigin,
            attributes: [.font: UIFont.systemFont(ofSize: 12)],
            context: nil
        )

        if total.size.height > geometry.size.height {
            isExpandeButtonShow = true
        }
    }

    private var collapseButton: some View {
        button(title: collapseButtonTitle()) {
            isCollapsed.toggle()
        }
    }
    
    private func button(title: String, handler: @escaping () -> Void) -> some View {
        Button(action: handler) {
            Text(title)
                .foregroundColor(Color.blue)
                .contentShape(Rectangle())
        }
        .buttonStyle(PlainButtonStyle())
    }

    private func collapseButtonTitle() -> String {
        isCollapsed ? "Collapse" : "Expand"
    }
}

It almost does what I want. But there are two points of my pain from this behavior. First, when I try to collapse text, no animation/transition starts. It just collapses immediately. Secondly, I want to have a line appearing by line with animating each one from 0 opacity to 1. How can I do it?

Any thoughts can be helpful. Regards.

Answer 1

Is something like this what you're trying to do?

struct RandomView: View {
    
    @State var isCollapsed: Bool = false
    @State var lineCount: Int = 1
    @State var isAnimating: Bool = false
    
    var body: some View {
        Text("Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.")
            .font(.largeTitle)
            .lineLimit(lineCount)
            .animation(.linear(duration: 1.0))
            .onTapGesture {
                animateLines()
            }
    }
    
    func animateLines() {
        
        guard !isAnimating else { return } // Check not currently animating
        isCollapsed.toggle()
        isAnimating = true

        let timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { (timer) in
            
            if isCollapsed {
                //Expand
                lineCount += 1
                if lineCount >= 10 { // max lines
                    timer.invalidate()
                    isAnimating = false
                }
            } else {
                //Collapse
                lineCount -= 1
                if lineCount <= 1 { // max lines
                    timer.invalidate()
                    isAnimating = false
                }
            }
            
        }
        timer.fire()
    }
}

EDIT: If you want to animate opacity line by line, you need to break the String into different Text items, which each have individual opacities. Like this:

var body: some View {
        VStack {
            Text("Line 1")
                .opacity(lineCount >= 1 ? 1 : 0)
            Text("Line 2")
                .opacity(lineCount >= 2 ? 1 : 0)
            Text("Line 3")
                .opacity(lineCount >= 3 ? 1 : 0)
            Text("Line 4")
                .opacity(lineCount >= 4 ? 1 : 0)
            Text("Line 5")
                .opacity(lineCount >= 5 ? 1 : 0)
        }
        .font(.largeTitle)
        .animation(Animation.linear(duration: 1.0))
        .onTapGesture {
            animateLines()
        }

    }