How to validate the number in Xamarin.Android with MVVMCross

Question

I have the Xamarin.Android project with using MVVMCross. I work with Visual Studio.

Windows 10 64 Pro
Visual Studio 2017

I need to create the text area for the number input and integrate basic validation (is this a number?) In case of error user should see the error message.

I have created the layout of this elements. Area for the number and for the error message.

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:local="http://schemas.android.com/apk/res-auto"

<EditText
        android:id="@+id/userNumber"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:maxLength="10"
        android:phoneNumber="true"
        local:MvxBind="Text UserNumber" />

<TextView
        android:id="@+id/incorrectNumber"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="12sp"
        android:textColor="#F44336"
        local:MvxBind="Text IncorrectNumber" />

And here is NumberViewModel.cs :

using MvvmCross.Platform.Converters;
using System;
using System.Collections.Generic;
using System.Globalization;
using My.project.Core.Models;

namespace My.project.Core.ViewModels
{
    class NumberViewModel : BaseViewModel
    {
        private string _userNumber;
        private string _incorrectNumber;

        public string UserNumber
        {
            get
            {
                return _userNumber;
            }
            set
            {
                _userNumber = value;
                this.RaisePropertyChanged(() => this.UserNumber);
                this.RaisePropertyChanged(() => this.IncorrectNumber);
            }
        }

        public string IncorrectNumber
        {
            get
            {
                if (UserNumber is Int16 || UserNumber is Int32) 
                {
                    return null;
                }
                else
                {
                    return "Incorrect phone number";
                }
            }

        }

    }
 }

But for some reason, I can't see the error message if I type non-numeric characters in this area. It should appear under the number.

I'm the very newbie in MVVMCross and a little bit confused. Please, help

UPD. I have added changes from fmaccaroni advice. Now the error message is always here. Even if the area contains only digits.

Answer 1

You can try to achieve it like this:

int numb;
if (!int.TryParse(UserNumber, out numb) && UserNumber.Length != 0) 
{
    return "Incorrect phone number";
}

Answer 2

Your bindings are wrong, they should be local:MvxBind="Text UserNumber" and local:MvxBind="Text IncorrectNumber" check the docs .

And unless you are using Fody.PropertyChanged package (which I'd recommend you to use) you should call this.RaisePropertyChanged(() => this.IncorrectNumber) inside the setter of UserNumber, ie after _userNumber = value; in order to inform the View that some property ( IncorrectNumber ) has changed from the ViewModel

Update

You should also change your validation in IncorrectNumber setter as @mrisek said. It should be like

public string IncorrectNumber => !string.IsNullOrEmpty(UserNumber) && int.TryParse(UserNumber, out int n) ? null : "Incorrect phone number" ;

If you use the is operator, it compares if the object is of that Type which is not clearly. By the way remember the length of an Int32 if you want a higher number you should use Int64 or make the validation with Regex: Regex.IsMatch(UserNumber, @"^\\d+$")