My App does not load list of items

Question

I have a App Xamarin.Formsthat consumes two services rest. The first service, consumes normally. However, the second, that I need to pass a parameter, this is not being consumed. I’ve tried several ways and it doesn’t work. I know my code is bugged, but I don’t know where, I don’t know how to consume the service. The variable _data that will fill the Grid with the information, is not come filled and yes null. Below my codes: My data service that should load the URL entries:

public async Task<List<ItensLib>> GetItensLibAsync(int idorcamento)
        {
            try
            {
                string url = "http://www.inetglobal.com.br/autorizador/api/itens/{idorcamento}";
                var response = await client.GetStringAsync(url);
                var itenslib = JsonConvert.DeserializeObject<List<ItensLib>>(response);
                return itenslib.ToList();
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message);
            }
        } 

My call on the constructor of the xaml.Cs class of my App

public MainPageItens(int idorcamento)
        {
            InitializeComponent();
            this.IdOrcamento = idorcamento;

            CarregaDados(idorcamento);
....

async void CarregaDados(int idorcamento)
        {
            _data = await dataService.GetItensLibAsync(idorcamento);
        }

My complete builder, with the assembly of the Grid

public MainPageItens(int idorcamento)
        {
            InitializeComponent();
            this.IdOrcamento = idorcamento;

            CarregaDados(idorcamento);

            // Crete a grid for "title"
            Grid grid = CreateGrid();
            grid.Children.Add(new Label { Text = "Produto", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 0, 1, 0, 1);
            grid.Children.Add(SeparatorV(), 1, 2, 0, 1);
            grid.Children.Add(new Label { Text = "Qtde", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 2, 3, 0, 1);
            grid.Children.Add(SeparatorV(), 3, 4, 0, 1);
            grid.Children.Add(new Label { Text = "Unitário", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 4, 5, 0, 1);
            grid.Children.Add(SeparatorV(), 5, 6, 0, 1);
            grid.Children.Add(new Label { Text = "Custo", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 6, 7, 0, 1);
            grid.Children.Add(SeparatorV(), 7, 8, 0, 1);
            grid.Children.Add(new Label { Text = "Custo Dia", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 8, 9, 0, 1);
            grid.Children.Add(SeparatorV(), 9, 10, 0, 1);
            grid.Children.Add(new Label { Text = "Ult. Vencto", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 10, 11, 0, 1);
            grid.Children.Add(SeparatorV(), 11, 12, 0, 1);
            grid.Children.Add(new Label { Text = "Total", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 12, 13, 0, 1);
            grid.Children.Add(SeparatorV(), 13, 14, 0, 1);

            grid.Children.Add(SeparatorH(), 0, 14, 1, 2);

            // Create the ListView to visualize my data
            ListView lv = new ListView() { HasUnevenRows = true, SeparatorVisibility = SeparatorVisibility.None };
            lv.ItemsSource = _data;
            lv.ItemTemplate = new DataTemplate(typeof(ListViewTemplateGrid));

            StackLayout sl = new StackLayout() { Children = { grid, lv }, Spacing = 0 };

            this.Content = sl;
        }

This is the statement of the _date Property at the beginning of the class:

List<ItensLib> _data { get; set; }
List<ItensLib> _itens ;
DataService dataService;

Here is the screenshot of the error(says nothing), as I know that _data is null, I think that’s the problem:

EDIT1 I overwrote the Onappering method and removed the Constructor’s call and construction of the Grid and switched to this method. And in the Charged Method, I passed him to async Task CarregaDados(int idorcamento);. What passes is the call in the Onappering of this method is giving this error:

Wait operator can only be used in an asynchronous method.

Look how it turned out:

protected override void OnAppearing()
        {
            base.OnAppearing();

            await CarregaDados(IdOrcamento);**Aqui dá erro**

            // Crete a grid for "title"
            Grid grid = CreateGrid();
            grid.Children.Add(new Label { Text = "Produto", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 0, 1, 0, 1);
            grid.Children.Add(SeparatorV(), 1, 2, 0, 1);
            grid.Children.Add(new Label { Text = "Qtde", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 2, 3, 0, 1);
            grid.Children.Add(SeparatorV(), 3, 4, 0, 1);
            grid.Children.Add(new Label { Text = "Unitário", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 4, 5, 0, 1);
            grid.Children.Add(SeparatorV(), 5, 6, 0, 1);
            grid.Children.Add(new Label { Text = "Custo", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 6, 7, 0, 1);
            grid.Children.Add(SeparatorV(), 7, 8, 0, 1);
            grid.Children.Add(new Label { Text = "Custo Dia", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 8, 9, 0, 1);
            grid.Children.Add(SeparatorV(), 9, 10, 0, 1);
            grid.Children.Add(new Label { Text = "Ult. Vencto", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 10, 11, 0, 1);
            grid.Children.Add(SeparatorV(), 11, 12, 0, 1);
            grid.Children.Add(new Label { Text = "Total", TextColor = Color.Red, VerticalOptions = LayoutOptions.Center, HorizontalOptions = LayoutOptions.Center }, 12, 13, 0, 1);
            grid.Children.Add(SeparatorV(), 13, 14, 0, 1);

            grid.Children.Add(SeparatorH(), 0, 14, 1, 2);

            // Create the ListView to visualize my data
            ListView lv = new ListView() { HasUnevenRows = true, SeparatorVisibility = SeparatorVisibility.None };
            lv.ItemsSource = _data;
            lv.ItemTemplate = new DataTemplate(typeof(ListViewTemplateGrid));

            StackLayout sl = new StackLayout() { Children = { grid, lv }, Spacing = 0 };

            this.Content = sl;
        }

And the method Loaded:

async Task CarregaDados(int idorcamento)
        {
            _data = await dataService.GetItensLibAsync(idorcamento);
        }

Answer 1

As the reply of the comment follows possible solution:

On android check the following:

1. You must have internet permission in the manifest;

2. Every internet call has to be made in an Asynctask or any other background task. In your case, by code, it’s being done directly on the screen. Then I recommend creating an Asynctask (not the Xamarin task, but the Asynctask class from android) and then making the call inside it. Follow an example of class.

public class BgTask : AsyncTask<String, Java.Lang.Void, String> { /// <summary> /// Método que vai fazer a chamada em background /// </summary> protected override String RunInBackground(params string[] @params) { } /// <summary> /// Executado depois que o RunInBackground terminar, geralmente usado para retornar o resultado. /// </summary> protected override void OnPostExecute(string result) { } }

The call for this task

new BgTask().execute("param");

Answer 2

The problem is when your code gets here:

lv.ItemsSource = _data;

...the method CarregaDados(idorcamento) which has been called some lines above has not yet executed, nor populated the variable _data. You need to wait for this method to finish its execution, for example:

Task.Run(async () => await CarregaDados(idorcamento)).GetAwaiter().GetResult();

This is one way to execute synchronously an asynchronous method, which is the case of CarregaDados.