What would you do if you received two identical states when submitting data?

Identify the source of repeated calls. Probable causes:

• Error in the UI layer: Double click, bug in event handling logic (e.g., button not deactivated after the first press).
• Error in the network layer: Repeated requests due to unsuccessful attempts, but without proper handling or cancellation of the previous one.
• Error in the business logic layer: Incorrect state management leading to re-running the operation.

Actions to resolve:

1. Deduplication/Cancellation of requests at the network layer:

   • Use of cancellation tokens (URLSessionTask.cancel()) to cancel the previous pending request when sending a new identical one.
   • Implementation of a mechanism to track active requests to prevent starting a new one if the same is already in progress.

   // Example with URLSessionTask.cancel()
   var dataTask: URLSessionDataTask?
   
   func sendData(data: Data) {
       dataTask?.cancel() // Cancel the previous request if any
       let url = URL(string: "YOUR_ENDPOINT")!
       var request = URLRequest(url: url)
       request.httpMethod = "POST"
       request.httpBody = data
   
       dataTask = URLSession.shared.dataTask(with: request) { data, response, error in
           // Handle response
       }
       dataTask?.resume()
   }
   

2. Lock UI elements: Disable the button or another control element after the first action until a response is received.

   // Example with UIButton
   @IBOutlet weak var sendButton: UIButton!
   
   @IBAction func sendButtonTapped(_ sender: UIButton) {
       sender.isEnabled = false // Disable the button
       sendData(data: someData) { success in
           DispatchQueue.main.async {
               sender.isEnabled = true // Enable the button after receiving the response
               // Update UI
           }
       }
   }
   

3. Manage state in business logic: Use flags or state machines to track the current state of the operation (started, in progress, completed).

   // Example with isProcessing flag
   private var isProcessing = false
   
   func processData() {
       guard !isProcessing else { return } // Exit if already in progress
       isProcessing = true
   
       // Data processing and request sending logic
       sendData() { success in
           self.isProcessing = false // Reset the flag after completion
           // Further actions
       }
   }
   

4. Deduplication on the server side (if possible): Although this context refers to the client side, it is worth mentioning that the server can also have mechanisms to handle repeated requests (e.g., idempotency).

The specific approach depends on the context and where the duplication occurs. Usually, a combination of methods is used.