Neste artigo eu vou mostrar como criar serviços backend usando ASP .NET Core MVC com suporte a aplicações móveis nativas.(Android, iOS e Windows Phone)

namespace XF_Tarefas.Models
{
    public class TarefaItem
    {
        public string ID { get; set; }
        public string Nome { get; set; }
        public string Notas { get; set; }
        public bool Concluido { get; set; }
    }
}

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using XF_Tarefas.Models;

namespace XF_Tarefas.Service
{
    public class DataService
    {
        HttpClient client = new HttpClient();

        public async Task<List<TarefaItem>> GetTarefasAsync()
        {
            try
            {
                string url = "http://192.168.1.18:5000/api/tarefa";
                var response = await client.GetStringAsync(url);
                var tarefas = JsonConvert.DeserializeObject<List<TarefaItem>>(response);
                return tarefas;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        public async Task AddTarefaAsync(TarefaItem tarefa)
        {
            try
            {
                string url = "http://192.168.1.18:5000/api/tarefa/{0}";
                var uri = new Uri(string.Format(url, tarefa.ID));
                var data = JsonConvert.SerializeObject(tarefa);
                var content = new StringContent(data, Encoding.UTF8, "application/json");
                HttpResponseMessage response = null;
                response = await client.PostAsync(uri, content);

                if (!response.IsSuccessStatusCode)
                {
                    throw new Exception("Erro ao incluir tarefa");
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        public async Task UpdateTarefaAsync(TarefaItem tarefa)
        {
            string url = "http://192.168.1.18:5000/api/tarefa/{0}";
            var uri = new Uri(string.Format(url, tarefa.ID));
            var data = JsonConvert.SerializeObject(tarefa);
            var content = new StringContent(data, Encoding.UTF8, "application/json");
            HttpResponseMessage response = null;
            response = await client.PutAsync(uri, content);

            if (!response.IsSuccessStatusCode)
            {
                throw new Exception("Erro ao atualizar tarefa");
            }
        }
        public async Task DeletaTarefaAsync(TarefaItem tarefa)
        {
            try
            { 
              string url = "http://192.168.1.18:5000/api/tarefa/{0}";
              var uri = new Uri(string.Format(url, tarefa.ID));
              await client.DeleteAsync(uri);
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
    }
}

    <StackLayout Orientation="Vertical">
        <StackLayout Padding="5,5,0,0">
            <Label Text="Adicionar uma Tarefa" TextColor="Green" />
        </StackLayout>
        <StackLayout Padding="10,0,10,0">
            <Entry x:Name="txtNome" Placeholder="Nome da Tarefa" HorizontalOptions="Start"
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="400" FontSize="Small"/>
            <Entry x:Name="txtNotas" Placeholder="Observações" HorizontalOptions="Start"
                    VerticalOptions="StartAndExpand" HeightRequest="30" WidthRequest="400" FontSize="Small" />
            <StackLayout Orientation="Horizontal">
                <Label Text="A Tarefa foi Concluida ?" HorizontalOptions="Start"/>
                <Switch x:Name="opConcluido" HorizontalOptions="End" VerticalOptions="StartAndExpand" IsToggled="{Binding Concluido}" />
            </StackLayout>
            <Button HorizontalOptions="FillAndExpand" VerticalOptions="StartAndExpand" HeightRequest="40" Text="Adicionar/Atualizar Tarefa"
                    Clicked="btnAdicionar_Clicked" FontSize="Small"/>
        </StackLayout>
        <StackLayout Orientation="Vertical" Padding="10,5,10,0">
            <ListView x:Name="listaTarefas" ItemSelected="listaTarefas_ItemSelected"
                      BackgroundColor="Aqua" SeparatorColor="Blue" HasUnevenRows="True">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <ViewCell.ContextActions>
                                <MenuItem Clicked="OnAtualizar" CommandParameter="{Binding .}" Text="Atualizar" />
                                <MenuItem Clicked="OnDeletar" CommandParameter="{Binding .}" Text="Deletar" IsDestructive="True" />
                            </ViewCell.ContextActions>
                            <StackLayout Padding="5,5">
                                <Label Text="{Binding Nome}" HorizontalOptions="StartAndExpand"/>
                                <Label Text="{Binding Notas}" TextColor="Blue" HorizontalOptions="StartAndExpand"/>
                                <Label Text="{Binding Concluido}" HorizontalOptions="StartAndExpand"/>
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>
    </StackLayout>

</ContentPage>

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