Behaviors number maximum value, Xamarin Form

Question

good evening. thanks for the help. I need in Xamarin Form, to create a behaivor for an entry that does not allow values greater than 17.

as shown in the picture

for example
17 --------------- 1
28---------------- 2
35---------------- 3

Answer 1

Hello,
Welcome to our Microsoft Q&A platform!
When you use the entry behavior like the following code, TextChanged event loops round, the text will always be "1".

using System;  
using Xamarin.Forms;  
  
namespace WorkingWithBehaviors  
{  
public class NumericValidationBehavior : Behavior<Entry>  
{  
protected override void OnAttachedTo (Entry entry)  
{  
entry.Completed += OnEntryTextCompleted;  
            //entry.TextChanged += OnEntryTextChanged;  
            base.OnAttachedTo (entry);  
}  

        protected override void OnDetachingFrom (Entry entry)  
{  
entry.Completed -= OnEntryTextCompleted;  
            //entry.TextChanged -= OnEntryTextChanged;  
            base.OnDetachingFrom (entry);  
}  
private void OnEntryTextCompleted(object sender, EventArgs e)  
{  
double result = double.Parse(((Entry)sender).Text);  
((Entry)sender).Text = result > 17 ? (result > 28 ? "3" : "2") : "1";  
}  
  
void OnEntryTextChanged (object sender, TextChangedEventArgs args)  
{  
            double result = double.Parse(args.NewTextValue);  
            ((Entry)sender).Text = result >17? (result > 28?"3": "2") : "1";  
        }  
}  
}  

So, you could have a try to use MVVM, ProcessYear method is the key to conver text.

ViewModel  
using System;  
using System.Collections.Generic;  
using System.ComponentModel;  
using System.Runtime.CompilerServices;  
using System.Text;  
  
namespace EntryBehaviorDemo  
{  
    class viewModel: INotifyPropertyChanged  
    {  
        public event PropertyChangedEventHandler PropertyChanged;  
  
        private string year;  
        public string Year { get { return year; } set { if (year != value) { year = ProcessYear(value); OnPropertyChanged(); } } }  
  
        private string ProcessYear(string year_)  
        {  
            if (string.IsNullOrEmpty(year_))  
                return year_;  
  
            double result = double.Parse(year_);  
            year_ = result > 17 ? (result > 28 ? "3" : "2") : "1";  
  
            return year_;  
        }  
        private void OnPropertyChanged([CallerMemberName] string propertyName = null)  
        {  
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));  
        }  
    }  
}  

BindingContext

 public partial class MainPage : ContentPage  
    {  
  
        public MainPage()  
        {  
            InitializeComponent();  
            BindingContext = new viewModel();  
        }  
    }  

XAML

<StackLayout>  
    <Entry Placeholder="enter number"  Text="{Binding Year, Mode=TwoWay}"></Entry>  
</StackLayout>  

Best Regards,
Wenyan Zhang

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Answer 2

Hello, thank you very much for the answer, it looks interesting but the truth is that it did not work, or rather, I have not been able to implement the code on my app, you could adhere to the code please ???

thanks