The initial button click does not display data

Question

The initial button click does not display data

I am currently facing an issue with my app when trying to display JSON data retrieved from a PHP web service. The problem occurs when I click the fetch button for the first time and the data does not show up on the labels.

After debugging, I noticed that the data is successfully passed to the array but it fails to populate the labels in the view controller.

Can anyone provide assistance with resolving this issue?

Thank you in advance.

Button click

@IBAction func btn_ler(_ sender: Any)
    {
        
        //txt_matricula.text = ""
        //txt_marca.text = ""
        //txt_modelo.text = ""
        
        retrieve_vehicle_data(licensePlate: input_licensePlate.text!)
        
        for vehicle in all_vehicles
        {
            txt_matricula.text = vehicle.LICENSE_PLATE
            txt_modelo.text = vehicle.MODEL
            txt_marca.text = vehicle.BRAND
        }
        
        //lbl_num_servicos.text = String( all_vehicles.count)
    }

Function to retrieve the JSON

func retrieve_vehicle_data (licensePlate : String)
    {
        //Variable containing the URL
        let v_url = "URL"
        
        let o_url = URL(string: v_url)
        
        URLSession.shared.dataTask(with: o_url!)
        {
            (data, response, error) in
            
            do
            {
                let vehiclesData = try JSONDecoder().decode([Vehicle].self, from: data!)
                

                //for vehicleData in vehiclesData
                //{
                    //print(vehicleData.LICENSE_PLATE)
                all_vehicles = []
                    all_vehicles.append(contentsOf: vehiclesData)
                //}
            }
            catch
            {
                print("We have an error!")
                //self.txt_matricula.text = "We have an error!"
            }
            print( all_vehicles.count)
            }.resume()
    }

ios json swift

Answer 1

Answer №1

To successfully retrieve data, ensure you include a completion closure in your obter_dados_veiculo method as shown below:

func obter_dados_veiculo (matricula : String, completion:@escaping(Bool,Error?)->())
{
    //Variable to store the URL
    let v_url = "URL"
    
    let o_url = URL(string: v_url)
    
    URLSession.shared.dataTask(with: o_url!)
    {
        (data, response, error) in
        
        do
        {
            let matriculas = try JSONDecoder().decode([s_veiculo].self, from: data!)
            
            
            //for matricula in matriculas
            //{
            //print(matricula.MATRICULA)
            a_veiculos = []
            a_veiculos.append(contentsOf: matriculas)
            completion(true,nil)
            //}
        }
        catch let error as NSError
        {
            print("We have an error!")
            completion(false,error)
            //self.txt_matricula.text = "We have an error!"
        }
        print( a_veiculos.count)
        }.resume()
}

Then, make the following call:

@IBAction func btn_ler(_ sender: Any)
{
    
    //txt_matricula.text = ""
    //txt_marca.text = ""
    //txt_modelo.text = ""
    
    obter_dados_veiculo(matricula: input_matricula.text!) { (success, error) in
        for veiculo in self.a_veiculos
        {
            txt_matricula.text = veiculo.MATRICULA
            txt_modelo.text = veiculo.MODELO
            txt_marca.text = veiculo.MARCA
        }
    }
    
    //lbl_num_servicos.text = String( a_veiculos.count)
}

Answer 2

To successfully retrieve data, ensure you include a completion closure in your obter_dados_veiculo method as shown below:

func obter_dados_veiculo (matricula : String, completion:@escaping(Bool,Error?)->())
{
    //Variable to store the URL
    let v_url = "URL"
    
    let o_url = URL(string: v_url)
    
    URLSession.shared.dataTask(with: o_url!)
    {
        (data, response, error) in
        
        do
        {
            let matriculas = try JSONDecoder().decode([s_veiculo].self, from: data!)
            
            
            //for matricula in matriculas
            //{
            //print(matricula.MATRICULA)
            a_veiculos = []
            a_veiculos.append(contentsOf: matriculas)
            completion(true,nil)
            //}
        }
        catch let error as NSError
        {
            print("We have an error!")
            completion(false,error)
            //self.txt_matricula.text = "We have an error!"
        }
        print( a_veiculos.count)
        }.resume()
}

Then, make the following call:

@IBAction func btn_ler(_ sender: Any)
{
    
    //txt_matricula.text = ""
    //txt_marca.text = ""
    //txt_modelo.text = ""
    
    obter_dados_veiculo(matricula: input_matricula.text!) { (success, error) in
        for veiculo in self.a_veiculos
        {
            txt_matricula.text = veiculo.MATRICULA
            txt_modelo.text = veiculo.MODELO
            txt_marca.text = veiculo.MARCA
        }
    }
    
    //lbl_num_servicos.text = String( a_veiculos.count)
}

Answer 3

Answer №2

The issue you are facing stems from the fact that you are making asynchronous network requests, which means there is no guarantee that the response will be received before you try to assign text to labels.

    retrieve_vehicle_data(licensePlate: input_licensePlate.text!)

    //RESPONSE MAY NOT BE RECEIVED

    for vehicle in a_vehicles
    {
        txt_licensePlate.text = vehicle.LICENSE_PLATE
        txt_model.text = vehicle.MODEL
        txt_brand.text = vehicle.BRAND
    }

One approach you can take is to use property observers on a_vehicles, like this:

var a_vehicles: [s_vehicle] = [] {
    didSet {
        for vehicle in a_vehicles
          {
             txt_licensePlate.text = vehicle.LICENSE_PLATE
             txt_model.text = vehicle.MODEL
             txt_brand.text = vehicle.BRAND
          }
    }
}

Answer 4

The issue you are facing stems from the fact that you are making asynchronous network requests, which means there is no guarantee that the response will be received before you try to assign text to labels.

    retrieve_vehicle_data(licensePlate: input_licensePlate.text!)

    //RESPONSE MAY NOT BE RECEIVED

    for vehicle in a_vehicles
    {
        txt_licensePlate.text = vehicle.LICENSE_PLATE
        txt_model.text = vehicle.MODEL
        txt_brand.text = vehicle.BRAND
    }

One approach you can take is to use property observers on a_vehicles, like this:

var a_vehicles: [s_vehicle] = [] {
    didSet {
        for vehicle in a_vehicles
          {
             txt_licensePlate.text = vehicle.LICENSE_PLATE
             txt_model.text = vehicle.MODEL
             txt_brand.text = vehicle.BRAND
          }
    }
}

Answer 5

Answer №3

Include a method called initData() in your code.

 func initData() {
   for vehicle in vehicleArray
    {
        licensePlate.text = vehicle.LICENSE_PLATE
        model.text = vehicle.MODEL
        brand.text = vehicle.BRAND
    }

 }

Make sure to call this method when initializing your array.

 func get_vehicle_data (license_plate : String)
     {
    //Variable holding the URL
    let url = "URL"

    let urlString = URL(string: url)

    URLSession.shared.dataTask(with: urlString!)
    {
        (data, response, error) in

        do
        {
            let vehicles = try JSONDecoder().decode([Vehicle].self, from: data!)


            //for vehicle in vehicles
            //{
                //print(vehicle.LICENSE_PLATE)
            vehicleArray = []
                vehicleArray.append(contentsOf: vehicles)
          initData()
            //}
        }
        catch
        {
            print("We have an error!")
            //self.licensePlate.text = "We have an error!"
        }
        print( vehicleArray.count)
        }.resume()
}

Answer 6

Include a method called initData() in your code.

 func initData() {
   for vehicle in vehicleArray
    {
        licensePlate.text = vehicle.LICENSE_PLATE
        model.text = vehicle.MODEL
        brand.text = vehicle.BRAND
    }

 }

Make sure to call this method when initializing your array.

 func get_vehicle_data (license_plate : String)
     {
    //Variable holding the URL
    let url = "URL"

    let urlString = URL(string: url)

    URLSession.shared.dataTask(with: urlString!)
    {
        (data, response, error) in

        do
        {
            let vehicles = try JSONDecoder().decode([Vehicle].self, from: data!)


            //for vehicle in vehicles
            //{
                //print(vehicle.LICENSE_PLATE)
            vehicleArray = []
                vehicleArray.append(contentsOf: vehicles)
          initData()
            //}
        }
        catch
        {
            print("We have an error!")
            //self.licensePlate.text = "We have an error!"
        }
        print( vehicleArray.count)
        }.resume()
}

Answer 7

Answer №4

Implement a completion handler to receive updates once your API call is complete and update the UI accordingly. Here's an example of how you can utilize a completion handler:

@IBAction func btn_ler(_ sender: Any)
{
    
    get_vehicle_data(licensePlate: input_licensePlate.text!) {
        DispatchQueue.main.async {
            for vehicle in vehicles
            {
                txt_licensePlate.text = vehicle.LICENSE_PLATE
                txt_model.text = vehicle.MODEL
                txt_brand.text = vehicle.BRAND
            }
        }
    }
}

func get_vehicle_data (licensePlate : String, completionHandler: @escaping () -> Void) {
    //Variable holding the URL
    let url = "URL"
    let requestUrl = URL(string: url)
    URLSession.shared.dataTask(with: requestUrl!) {
        (data, response, error) in
        do {
            let plates = try JSONDecoder().decode([vehicle_details].self, from: data!)
            vehicles = []
            vehicles.append(contentsOf: plates)
        }
        catch {
            print("We have an error!")
            //self.txt_licensePlate.text = "We have an error!"
        }
        print(vehicles.count)
        }.resume()
}

Answer 8

Implement a completion handler to receive updates once your API call is complete and update the UI accordingly. Here's an example of how you can utilize a completion handler:

@IBAction func btn_ler(_ sender: Any)
{
    
    get_vehicle_data(licensePlate: input_licensePlate.text!) {
        DispatchQueue.main.async {
            for vehicle in vehicles
            {
                txt_licensePlate.text = vehicle.LICENSE_PLATE
                txt_model.text = vehicle.MODEL
                txt_brand.text = vehicle.BRAND
            }
        }
    }
}

func get_vehicle_data (licensePlate : String, completionHandler: @escaping () -> Void) {
    //Variable holding the URL
    let url = "URL"
    let requestUrl = URL(string: url)
    URLSession.shared.dataTask(with: requestUrl!) {
        (data, response, error) in
        do {
            let plates = try JSONDecoder().decode([vehicle_details].self, from: data!)
            vehicles = []
            vehicles.append(contentsOf: plates)
        }
        catch {
            print("We have an error!")
            //self.txt_licensePlate.text = "We have an error!"
        }
        print(vehicles.count)
        }.resume()
}

The initial button click does not display data

Answer №1

Answer №2

Answer №3

Answer №4

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