To make the new comment appear immediately after posting, you don’t need to fetch the entire product details again. You can modify the comments list directly after successfully posting a new comment and update the @Published property to trigger the UI update.

class ProductDetailsViewModel: ObservableObject {
    @Published var productDetails: ProductDetails? = nil
    @Published var commentText: String = ""
    @Published var message: String = ""
    @Published var isLoading: Bool = false
    private var cancellables = Set<AnyCancellable>()

    // Fetch Product Details
    func fetchProductDetails(id: Int) {
        isLoading = true
        service.fetchProductDetails(id: id)
            .receive(on: DispatchQueue.main)
            .sink { [weak self] completion in
                self?.isLoading = false
                switch completion {
                case .finished:
                    break
                case .failure(let error):
                    // Handle error here
                    print(error.localizedDescription)
                }
            } receiveValue: { [weak self] response in
                self?.productDetails = response.data
            }
            .store(in: &cancellables)
    }

    // Post Comment and Update Comments List
    func postComment(id: Int) {
        isLoading = true
        let parameters: [String: Any] = ["comment": commentText]
        service.postRequest(url: Config.baseURL + "item/(id)/comment", parameters: parameters, type: CommentResponse.self)
            .receive(on: DispatchQueue.main)
            .sink { [weak self] completion in
                self?.isLoading = false
                switch completion {
                case .finished:
                    break
                case .failure(let error):
                    // Handle error here
                    print(error.localizedDescription)
                }
            } receiveValue: { [weak self] response in
                guard let self = self else { return }
                if let message = response.message {
                    self.message = message
                }

                // Create a new comment object
                let newComment = Comment(id: UUID().uuidString, text: self.commentText)
                
                // Append the new comment to the product's comments list
                self.productDetails?.comments.append(newComment)

                // Clear the comment text after posting
                self.commentText = ""
            }
            .store(in: &cancellables)
    }
}
`