问@State更新视图，但@State没有

EN

struct Form: View {
  @ObservedObject var model = FormInput()

  var body: some View {
    Form {
      TextArea("Details", text: $model.details.value)
        .validation(message: model.details.message)
    }
  }
}

final class FormInput: ObservableObject {
  @Published var details: TextInput

  // ... other code
}

 struct MyForm: View {
      @State var details = TextInput()

      var body: some View {
        Form {
          TextArea("Details", text: $details.value)
            .validation(message: details.message)
        }
      }
    }

final class FormInput: ObservableObject {
  @Published var details: TextInput

  init(details: String = "") {
    self.details = TextInput(value: details, isValid: false, validations: [.length(12)])
  }

  // other code
}

struct TextInput {
  var value: String {
    didSet {
      self.validate()
    }
  }
  var validations: [TextValidation] // this is just an enum of different types of validations
  var isValid: Bool
  var message: String


  mutating func validate() {
    for validation in validations {
      // run validation
    }
  }
}

struct TextArea: View {
  @Binding var text: String
  @State var height: CGFloat = 12

  init(text: Binding<String>) {
    self._text = text
  }

  var body: some View {
    TextAreaField(text: $text)
  }
}

 struct TextAreaField: UIViewRepresentable {
   @Binding var text: String
   @Binding var height: CGFloat


   func makeUIView(context: Context) -> UITextView {
     let textField = UITextView()

     textField.isEditable = true
     textField.isSelectable = true
     textField.isUserInteractionEnabled = true
     textField.isScrollEnabled = false
     // ..other initializers removed for brevity

      textField.delegate = context.coordinator
      return textField
    }

   func updateUIView(_ uiView: UITextView, context: Context) {
     calculateHeight(uiView)
   }

   func calculateHeight(_ uiView: UIView) {
        let newSize = uiView.sizeThatFits(CGSize(width: uiView.frame.size.width, height: CGFloat.greatestFiniteMagnitude))
        if self.height != newSize.height {
          DispatchQueue.main.async  {
            self.height = newSize.height
          }
        }
      }


   func makeCoordinator() -> Coordinator {
     return Coordinator(self)
   }


   final class Coordinator: NSObject, UITextViewDelegate {
     var parent: TextAreaField


     init(_ parent: TextAreaField) {
       self.parent = parent
     }

     func textViewDidChange(_ uiView: UITextView) {
       self.parent.text = uiView.text
       self.parent.calculateHeight(uiView)
      }

    }
}

struct Validation: ViewModifier {
  let message: String

  func body(content: Content) -> some View {
    let isValid = message == ""
    print(message)

    return VStack(alignment: .leading) {
      content

      if isValid == false {
        Text(message)
          .font(.footnote)
          .italic()
          .foregroundColor(Color.red)
          .frame(height: 24)
      }

    }
    .padding(.bottom, isValid ? 24 : 0)
  }
}


extension View {
  func validation(message: String) -> some View {
    self.modifier(Validation(message: message))
  }
}