如何设置在按钮和标题之间移动的矩形的动画?

您错过了动画:

 struct ContentView: View {
    
    @Namespace private var pagePickerAnimation
    @State private var selectedPage: PageItem = .page1

    var body: some View {
        
        HStack(spacing: 16) {
            
            ForEach(PageItem.allCases) { page in
                
                Button { selectedPage = page } label: {
                    
                    VStack {
                        
                        if page == selectedPage {
                            
                            Rectangle()
                                .fill(Color(.label))
                                .frame(maxWidth: .infinity)
                                .frame(height: 1)
                                .matchedGeometryEffect(id: "pageIndicator", in: pagePickerAnimation)
                            
                        }
                        
                        
                        Text(page.title)
                            .padding(.vertical, 8)
                            .padding(.horizontal, 12)
                        
                        
                        
                    }
                    .contentShape(Rectangle())
                    
                }
            }
        }
        .padding()
        .animation(.default, value: selectedPage) // <<: here
    }
}



enum PageItem: String, CaseIterable, Identifiable {
    case page1
    case page2

    var id: String { rawValue }

    var title: LocalizedStringKey {
        switch self {
        case .page1:
            return "Page 1"
        case .page2:
            return "Page 2"
        }
    }
}