Xamarin Forms Conditional statements - asp.net-web-api

Question

Hello guys: Am stuck with my code here. There is a point in my app where I need to notify users to register and sign in order to save their preferences. I have been trying to add a display alert inside a ViewModel but it's not working. Please help I am stuck.
ApproveViewModel
namespace MyApp.ViewModels
{
public class ApproveViewModel
{
private DataService dataService = new DataService();
public Oppotunity SelectedOppotunity { get; set; }
public ICommand SaveCommand => new Command(async () =>
{
await dataService.PostSaveOppotunity(SelectedOppotunity, Settings.AccessToken);
});
public ApproveViewModel()
{
SelectedOppotunity = new Oppotunity();
}
}
}
ApprovePage.xaml
<ScrollView>
<StackLayout Padding ="15">
<Label Text ="{Binding SelectedOppotunity.Title}"/>
<Label Text ="{Binding SelectedOppotunity.Description }"/>
<Label Text ="{Binding SelectedOppotunity.Organisation}"/>
<Label Text ="{Binding SelectedOppotunity.Venue }"/>
<Label Text ="{Binding SelectedOppotunity.Eligibility}"/>
<Label Text ="{Binding SelectedOppotunity.Benefits}"/>
<Label Text ="{Binding SelectedOppotunity.Province}"/>
<Label Text ="{Binding SelectedOppotunity.Country}"/>
<Label Text ="{Binding SelectedOppotunity.OppotunityLink}"/>
<Label Text ="{Binding SelectedOppotunity.Category}"/>
<Label Text ="{Binding SelectedOppotunity.Deadline}"/>
<!--
<Switch IsToggled ="{Binding SelectedOppotunity.IsApproved}"></Switch>
-->
<Button Text ="Apply" BackgroundColor ="#A91717" TextColor ="White"
Command ="{Binding SaveCommand }"/>
</StackLayout>
The code i wish to invoke on saving:
if (!string.IsNullOrEmpty(Settings.AccessToken))
{
// Implement the SaveCommand from the ViewModel;
}
// Go to Login form to get an access token
else if (!string.IsNullOrEmpty(Settings.Username) &&
!string.IsNullOrEmpty(Settings.Password))
{
MainPage = new NavigationPage(new Login());
}
else
{
//Register first
MainPage = new NavigationPage(new NewRegisterPage());
}

Answer 1

Solution:
You can use MessageCenter subscriptions to work through this problem.
Xamarin.Forms MessagingCenter enables view models and other components
to communicate with without having to know anything about each other
besides a simple Message contract.
Subscribe - Listen for messages with a certain signature and perform
some action when they are received. Multiple subscribers can be
listening for the same message.
Send - Publish a message for listeners to act upon. If no
listeners have subscribed then the message is ignored.
Code in the View Model:
if (!string.IsNullOrEmpty(Settings.AccessToken))
{
// Implement the SaveCommand from the ViewModel;
}
// Go to Login form to get an access token
else if (!string.IsNullOrEmpty(Settings.Username) &&
!string.IsNullOrEmpty(Settings.Password))
{
MainPage = new NavigationPage(new Login());
}
else
{
//Register first
//if you want to notify users to register here, use MessageCenter.Send
MessageCenter.Send(this, "displayAlert")
MainPage = new NavigationPage(new NewRegisterPage());
}
Code in the View:
MessagingCenter.Subscribe<ViewModelName>(this, "displayAlert", (sender) => {
// do something whenever the "displayAlert" message is sent
DisplayAlert("notification", "you should register first", "ok");
});
Send the message where you want to display an alert in the viewModel.
For more information about MessageCenter, you can refer to https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center

Answer 2

If you wan't to display an alert from the ViewModel you can:
Application.Current.MainPage.DisplayAlert();