.net Maui, viewModel alternative to OnAppearing

Question

I notice that in .net MAUI OnAppearing doesn;t work. I need to find a way to open up a "DisplayAlert" as soon as the app appears and it must be triggered from the ViewModel.

Thank you in advanced.

Answer 1

The OnAppearing is an event you can use in the "codebehind" file.

So, if you have a MainPage.xaml, you can override the OnAppearing in MainPage.xaml.cs file.

If you want to use OnAppearing inside the ViewModel file, you can use the Maui Community Toolkit

    <ContentPage.Behaviors>
        <mct:EventToCommandBehavior Command="{Binding AppearingCommand}" EventName="Appearing" />
        <mct:EventToCommandBehavior Command="{Binding DisappearingCommand}" EventName="Disappearing" />
    </ContentPage.Behaviors>

and in your ViewModel

        [RelayCommand]
        void Appearing()
        {
            try
            {

                // DoSomething
           
            }
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine(ex.ToString());
            }
        }

        [RelayCommand]
        void Disappearing()
        {
            try
            {

                // DoSomething
           
            }
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine(ex.ToString());
            }
        }


      

Answer 2

Alternatively, if you are using a BaseViewModel with MVVM and a BasePage you can declare an abstract method in your BaseViewModel and define the method in your inheriting ViewModels. You then call your OnAppearing method only in the BasePage class, so its technically not 100% MVVM but relatively close.

in BasePageViewModel.cs

public abstract partial class BasePageViewModel : ObservableObject 
{        
    public abstract Task OnAppearing();
}

in BasePage.xaml.cs

private async void BasePage_Appearing(object sender, EventArgs e)    
{            
    await _vm.OnAppearing();   
}

in BasePage.xaml

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"                    x:Class="myNameSpace.Views.BasePage"                 
xmlns:viewmodels="clr-namespace:myNameSpace.ViewModels"             
x:DataType="viewmodels:BasePageViewModel"             
Appearing="BasePage_Appearing"             
Disappearing="ContentPage_Disappearing"                 
ControlTemplate="{DynamicResource BasePage}" />

That way you dont have to use EventToCommandBehaviour and you force all your ViewModels to do the page initialisation in the same method. If not all pages have to implement a loading event you can also make the method virtual instead abstract to make it optional.