问使用SwiftUI，我们在列表中安装了一个按钮。为什么当我点击按钮以显示模式，然后再次关闭它时，模式会消失？

EN

struct ContentView: View {
  @State var show_modal = false
  var body: some View {
    List {
      Button(action: {
        print("Button Pushed")
        self.show_modal = true
      }) {
        Text("Show Modal")
      }.sheet(isPresented: self.$show_modal, onDismiss: {
        print("dismiss")
      }) {
        ModalView()
      }
    }
  }
}

struct ContentView: View {
  @State var show_modal = false
  var body: some View {
      Button(action: {
        print("Button Pushed")
        self.show_modal = true
      }) {
        Text("Show Modal")
      }.sheet(isPresented: self.$show_modal, onDismiss: {
        print("dismiss")
      }) {
        ModalView()
      }
  }
}

struct ModalView: View {
    // 1. Add the environment variable
    @Environment(\.presentationMode) var presentationMode

    var body: some View {
        // 2. Embed Text in a VStack
        VStack {
            // 3. Add a button with the following action
            Button(action: {
                print("dismisses form")
                self.presentationMode.wrappedValue.dismiss()
            }) {
                Text("Dismiss")
            }.padding(.bottom, 50)
            Text("This is a modal")
        }
    }
}