Xamarin DisplayAlert when entry max length reached

Issue

I have an entry which is set to max length of 3. When the user tries to input 4 characters I want to display a simple DisplayAlert message as shown here. I am trying to implement it with MVVM but having hard time with the binding of the await needed for the alert. I know that the max length will always be 3 if that helps.

Xaml:

<Entry Text = "{Binding BoundText}"/>

ViewModel:

string _boundText;
public string BoundText
{
   get => _boundText;
   set
   {
       if(value.Length > 3)
       {
          await DisplayAlert("Alert", "You have been alerted", "OK");
       }
       else
       {
          ...
       }
   }
}

The error I am getting is that the await operator needs to be in an async method, but when I add it I get an error that the modifier async is not valid for the constructor. Any ideas on how to implement this?

Solution

We can't place an async method into Setter method .

And it is not recommended to place DisplayAlert into viewmodel , cause the method belongs to page , it breaks mvvm pattern .

Here are two workarounds .

1. Send MessagingCenter in Setter method , and do something in page .

   
        //viewmodel
        set
            {
                if (value.Length > 3)
                {
                    MessagingCenter.Send<object>(this, "Hi");
                }
            ...
   
   
        //page
        public Page1()
        {
   
            InitializeComponent();
            this.BindingContext = model;
   
            MessagingCenter.Subscribe<object>(this, "Hi", async (obj) =>
            {
                await DisplayAlert("Alert", "You have been alerted", "OK");
            });
        }
   

2. Handle it in TextChanged of Entry .

     <Entry Text = "{Binding BoundText}" TextChanged="Entry_TextChanged"/>
   
   
       private async void Entry_TextChanged(object sender, TextChangedEventArgs e)
       {
           if(e.NewTextValue.Length > 3)
           {
               await DisplayAlert("Alert", "You have been alerted", "OK");
           }
       }
   

Answered By - ColeX - MSFT