I need to have a picker (or some other control) offering the user options inside a MVVM implementation (based on Xamarin.Forms MVVM: How to Work with SQLite DB(C# — Xaml)) but cant see how to load the async options into the binding.
I have a detail edit screen for a record type called viewings, with straight forward entry fields bound to a ViewingDetailViewingModel.
...BindingContext = new ViewingDetailViewModel(viewModel ?? new ViewingViewModel(), viewingStore, pageService, viewingsRestStore);
in the XAML's codebehind.
In addition to the relatively straight forward entry cells binding to properties of the Viewing's model (forgive me if I don't use the correct nomenclature, new to this), I want to have one of the fields use a picker to save a Client's id based on the user selecting a client.
So I'm guessing, as the SQLite clientstore's get return is async, I should have a command that is triggered from somewhere, I imagine ideally from the codebehind's onAppearing method that updates an IList of Clients that the picker binds to - though I encounter two problems here:
- I can't figure out how to trigger the command from there so I'm trying to execute it from the end of the viewmodel's constructor for now.
- Doing that (although I don't think because of that), the app crashes once it tries to add the first client to the list while looping through the get results with "...System.NullReferenceException: Object reference not set to an instance of an object at AgentApp.ViewingDetailViewModel.LoadClients (System.Collections.Generic.IList`1[T] Clients)..."
so in my XAML I have
<EntryCell Label="Agt_description" Text="{Binding Path=Viewing.Agt_description}" />
<ViewCell >
<Picker Title="Select a client" ItemsSource="{Binding Clients}" ItemDisplayBinding ="{Binding Agt_FirstName}" />
</ViewCell>
In the view's code-behind:
public ViewingDetailPage(ViewingViewModel viewModel)
{
Console.WriteLine("ViewingDetailPage()");
InitializeComponent();
var viewingStore = new SQLiteViewingStore(DependencyService.Get<ISQLiteDb>());
var viewingsRestStore = new RESTViewingStore();
var pageService = new PageService();
bool isNewViewing = (viewModel.Agt_name == " ");
Title = (isNewViewing) ? "New Viewing" : "Edit Viewing";
if (isNewViewing)
{
var app = Application.Current as App;
viewModel.Agt_ac = app.AgencyIdInt;
viewModel.Agt_b = app.BranchIDInt;
viewModel.Agt_at = app.AccountIdInt;
}
BindingContext = new ViewingDetailViewModel(viewModel ?? new ViewingViewModel(), viewingStore, pageService, viewingsRestStore);
}
and then the ViewingDetailViewModel:
class ViewingDetailViewModel : BaseViewModel
{
private readonly IViewingStore _viewingStore;
private readonly IViewingStore _viewingRestStore;
private readonly IPageService _pageService;
public Viewing Viewing { get; private set; }
public ICommand SaveCommand { get; private set; }
//trying
public ICommand LoadClientsCommand { get; private set; }
private IClientStore _clientStore;
private IList<ClientViewModel> _clients;
public IList<ClientViewModel> Clients
{
get { return _clients; }
set { SetProperty(ref _clients, value); } // from BaseViewModel, implements OnPropertyChanged()
}
//
public ViewingDetailViewModel(ViewingViewModel viewModel, IViewingStore viewingStore, IPageService pageService, IViewingStore viewingRestStore)
{
var app = Application.Current as App;
if (viewModel == null)
throw new ArgumentNullException(nameof(viewModel));
_pageService = pageService;
_viewingStore = viewingStore;
_viewingRestStore = viewingRestStore;
LoadClientsCommand = new Command(async () => await LoadClients());
SaveCommand = new Command(async () =>
{
var SaveDBTask = SaveDB("primary");
var SaveRESTTask = SaveRest();
var saveTasks = new List<Task> { SaveDBTask, SaveRESTTask };
while (saveTasks.Count > 0)
{
Task finishedTask = await Task.WhenAny(saveTasks);
if (finishedTask == SaveDBTask)
{
Console.WriteLine("SaveDBTask finished:" + SaveDBTask);
}
else if (finishedTask == SaveRESTTask)
{
Console.WriteLine("SaveRESTTask finished, do db update:" + SaveRESTTask);
await SaveDB("secondary update");
}
saveTasks.Remove(finishedTask);
}
Console.WriteLine("Save tasks complete.");
});
Viewing = new Viewing
{
Id = viewModel.Id,
Agt_description = viewModel.Agt_description,
Agt_name = viewModel.Agt_name,
RemoteId = viewModel.RemoteId,
Agt_ac = viewModel.Agt_ac > 0 ? viewModel.Agt_ac : app.AgencyIdInt,
Agt_b = viewModel.Agt_b > 0 ? viewModel.Agt_b : app.BranchIDInt,
Agt_at = viewModel.Agt_at > 0 ? viewModel.Agt_at : app.AccountIdInt,
Agt_pr = viewModel.Agt_pr,
Agt_datetime_scheduled = viewModel.Agt_datetime_scheduled,
Agt_datetime_start = viewModel.Agt_datetime_start,
Agt_datetime_end = viewModel.Agt_datetime_end,
StatusLocal = viewModel.StatusLocal,
CreatedLocal = viewModel.CreatedLocal,
ModifiedLocal = viewModel.ModifiedLocal,
CreatedRemote = viewModel.CreatedRemote,
ModifiedRemote = viewModel.ModifiedRemote,
LastSync = viewModel.LastSync,
};
LoadClientsCommand.Execute(null);//doubt this is a good idea
}
async Task LoadClients()
{
_clientStore = new SQLiteClientStore(DependencyService.Get<ISQLiteDb>());
var clients = await _clientStore.GetClientsAsync();
foreach (var client in clients)
{
Console.WriteLine("for client:" + client.Agt_FirstName);
Clients.Add(new ClientViewModel(client)); //crashes around here
}
}
async Task SaveDB(string savetype)
{
if (String.IsNullOrWhiteSpace(Viewing.Agt_name))
{
await _pageService.DisplayAlert("Error", "Please enter the name.", "OK");
return;
}
if (Viewing.Id == 0)
{
await _viewingStore.AddViewing(Viewing);
MessagingCenter.Send(this, Events.ViewingAdded, Viewing);
}
else
{
await _viewingStore.UpdateViewing(Viewing);
MessagingCenter.Send(this, Events.ViewingUpdated, Viewing);
}
if (savetype == "primary")
await _pageService.PopAsync();
}
async Task SaveRest()
{
if (String.IsNullOrWhiteSpace(Viewing.Agt_name))
{
Console.WriteLine("REST save could not, invalid name.");
return;
}
if (Viewing.RemoteId == 0)
{
await _viewingRestStore.AddViewing(Viewing);
}
else
{
await _viewingRestStore.UpdateViewing(Viewing);
MessagingCenter.Send(this, Events.ViewingUpdated, Viewing);
}
}
}
I'm a complete newb coming from a simple php background and all this OOP complexity is confusing me somewhat; I'm probably making a whole lot of really stupid mistakes - and maybe I'm overcomplicating things - I hope someone can point me at a good simple way of loading the picker's async data.
Thanks in advance!