Where to call async GetDataFromServer in Xamarin.Forms? - sql

Question

Where would be the best place to call GetDataFromServer method?
My gut feeling and reason say it belongs in the repository, but I've no clue where to call it. I've tried to call it in the constructor, but that didn't work out too well. It had issues with it being an async method.
public class SQLiteRepository : ISQLiteRepository
{
private HttpClient _httpClient = new HttpClient();
private readonly SQLiteAsyncConnection _efContext;
public SQLiteRepository()
{
_efContext = DependencyService.Get<ISQLiteDb>().GetAsyncConnection();
_efContext.CreateTableAsync<EfPartner>();
}
public async Task<IEnumerable<EfPartner>> GetAllPartnersAsync()
{
try
{
var partners = await _efContext.Table<EfPartner>().ToListAsync();
return partners;
}
catch (Exception ex)
{
throw ex;
}
}
public async Task GetDataFromServerAsync()
{
try
{
var partners = await GetPartnersFromServerAsync();
var companies = await GetCompaniesFromServerAsync();
await _efContext.InsertAllAsync(partners);
}
catch (Exception ex)
{
throw ex;
}
}
private async Task<IEnumerable<EfPartner>> GetPartnersFromServerAsync()
{
try
{
var jsonObject = await _httpClient.GetStringAsync(Constants.PartnersUrl);
var dotNetObject = JsonConvert.DeserializeObject<List<EfPartner>>(jsonObject);
return new List<EfPartner>(dotNetObject);
}
catch (Exception ex)
{
throw ex;
}
}
private async Task<IEnumerable<EfCompany>> GetCompaniesFromServerAsync()
{
try
{
var jsonObject = await _httpClient.GetStringAsync(Constants.CompaniesUrl);
var dotNetObject = JsonConvert.DeserializeObject<List<EfCompany>>(jsonObject);
return new List<EfCompany>(dotNetObject);
}
catch (Exception ex)
{
throw ex;
}
}
}
I called the GetDataFromServerAsync from PartnersListPage.xaml.cs -> which feels wrong.
I'd appreciate any help.
Thank you.
============================ UPDATE ============================
The app I'm working on creates the pages in a MasterDetailPage like so:
private void MenuListView_ItemSelected(object sender, SelectedItemChangedEventArgs e)
{
if (e.SelectedItem == null)
return;
var menuItem = e.SelectedItem as EfMenuItem;
if (menuItem == null)
return;
var page = (Page)Activator.CreateInstance(menuItem.TargetPage);
page.Title = menuItem.Title;
Detail = new NavigationPage(page);
IsPresented = false;
MdpMasterPage.MenuListView.SelectedItem = null;
}
And here is the PartnersListPage.xaml.cs, from where is GetDataFromServer called at the moment:
public partial class PartnersListPage : ContentPage
{
private readonly SQLiteRepository _repo;
public PartnersListPage()
{
InitializeComponent();
_repo = new SQLiteRepository();
}
protected override async void OnAppearing()
{
await _repo.GetDataFromServerAsync();
var partners = await _repo.GetAllPartnersAsync();
InitializeGrid(partners);
base.OnAppearing();
}

Answer 1

This is an important topic related to MVVM, what you are doing is "correct" but it may have issues later on once your application grow bigger, let's imaging this scenario:
The user is a desperate one and he/she wants to Navigate on your app real fast, he/she open this specific page 5 or 10 times, what will happen then?
Your OnAppearing() method will be called every time the user lands on this page and a new thread will be created for instance your application will start to behave poorly on the performance part. (Specially on Android).
So my suggestion will be to use an MVVM framework to handle those cases based on the pattern, here is a small tutorial using Prism:
https://xamgirl.com/prism-in-xamarin-forms-step-by-step-part-1/
And in regards of your question, your UI ListView specifically your ItemSource (If you are using a ListView) property should be binded to your GetAllPartnersAsync() return type.
If that is not the case and based on the context I suppose InitializeGrid(partners); is creating cells for a Grid in your XAML class so I suppose you can debug step by step if the UI is being created correctly. A Grid does not have an ItemSource property
If you want to have a Grid with an ItemSource property I suggest you sue this ones:
https://github.com/Manne990/XamTest
https://github.com/daniel-luberda/DLToolkit.Forms.Controls/tree/master/FlowListView