Nsryjdtyk

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());

        }

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://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center

If you wan't to display an alert from the ViewModel you can:

Application.Current.MainPage.DisplayAlert();