如何在 SwiftUI 中为文本的折叠设置动画

Question

我需要有关 SwiftUI 动画/过渡行为的帮助。在将 lineLimit 从 nil 更改为 5 期间，我必须实现折叠/扩展 Text，反之亦然。

目前我有一个工作代码，你可以在下面看到它。

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"
    }
}

它几乎可以满足我的要求。但是这种行为让我有两点痛苦。 首先，当我尝试折叠文本时，没有动画/过渡开始。它只是立即崩溃。其次，我想让一条线逐行出现，每条线的不透明度从 0 变为 1。我该怎么做？

任何想法都会有所帮助。问候。

Answer 1

这是你想要做的事情吗？

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()
    }
}

编辑：如果要逐行设置不透明度动画，则需要将字符串分解为不同的文本项目，每个项目都有单独的不透明度。像这样：

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()
        }

    }