为什么当onAppear()放在swiftUI中的NavigationView内的元素上时，会执行两次？(Xcode 12.0)

struct ContentView: View {
    @State private var isFirstAppear = true
    
    var body: some View {
        NavigationView {
            VStack {
                Text("Hello, World!")
                    .onAppear {
                        if isFirstAppear {
                            // 只在首次加载时执行
                            print("View appeared for the first time")
                            isFirstAppear = false
                        }
                    }
                
                NavigationLink(destination: DetailView()) {
                    Text("Go to Detail")
                }
            }
        }
    }
}

struct DetailView: View {
    var body: some View {
        Text("Detail View")
    }
}