LoginView displays validation errors when using mock services.

src/Mobile/eShopOnContainers/eShopOnContainers.Core/App.xaml

@@ -17,6 +17,7 @@
             <Color x:Key="GreenColor">#00A69C</Color>
             <Color x:Key="DarkGreenColor">#00857D</Color>
             <Color x:Key="GrayColor">#e2e2e2</Color>
+            <Color x:Key="ErrorColor">#ff5252</Color>
             
             <!-- FONTS -->
             <OnPlatform 
@@ -100,6 +101,7 @@
             <!-- CONVERTERS -->
             <converters:CountToBoolConverter x:Key="CountToBoolConverter" />
             <converters:DatetimeConverter x:Key="DatetimeConverter" />
+			<converters:FirstValidationErrorConverter x:Key="FirstValidationErrorConverter" />
             <converters:ImageConverter x:Key="ImageConverter" />
             <converters:ItemTappedEventArgsConverter x:Key="ItemTappedEventArgsConverter" />
             <converters:InverseCountToBoolConverter x:Key="InverseCountToBoolConverter" />
@@ -109,6 +111,14 @@
 			<converters:WebNavigatingEventArgsConverter x:Key="WebNavigatingEventArgsConverter" />
             
             <!-- STYLES -->
+            <Style x:Key="ValidationErrorLabelStyle"
+				   TargetType="{x:Type Label}">
+				   <Setter Property="TextColor"
+						   Value="{StaticResource ErrorColor}" />
+				   <Setter Property="FontSize"
+					       Value="{StaticResource LittleSize}" />
+ 			</Style>
+			
             <Style x:Key="EntryStyle"
                    TargetType="{x:Type Entry}">
                 <Setter Property="FontFamily"

src/Mobile/eShopOnContainers/eShopOnContainers.Core/Converters/FirstValidationErrorConverter.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using Xamarin.Forms;
+
+namespace eShopOnContainers.Core.Converters
+{
+	public class FirstValidationErrorConverter : IValueConverter
+	{
+		public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+		{
+			ICollection<string> errors = value as ICollection<string>;
+			return errors != null && errors.Count > 0 ? errors.ElementAt(0) : null;
+		}
+
+		public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+		{
+			throw new NotImplementedException();
+		}
+	}
+}

src/Mobile/eShopOnContainers/eShopOnContainers.Core/Validations/ValidatableObject.cs

@@ -1,6 +1,5 @@
 using eShopOnContainers.Core.ViewModels.Base;
 using System.Collections.Generic;
-using System.Collections.ObjectModel;
 using System.Linq;
 
 namespace eShopOnContainers.Core.Validations
@@ -8,13 +7,24 @@ namespace eShopOnContainers.Core.Validations
     public class ValidatableObject<T> : ExtendedBindableObject, IValidity
     {
         private readonly List<IValidationRule<T>> _validations;
-        private readonly ObservableCollection<string> _errors;
+		private List<string> _errors;
         private T _value;
         private bool _isValid;
 
         public List<IValidationRule<T>> Validations => _validations;
 
-        public ObservableCollection<string> Errors => _errors;
+		public List<string> Errors
+		{
+			get
+			{
+				return _errors;
+			}
+			set
+			{
+				_errors = value;
+				RaisePropertyChanged(() => Errors);
+			}
+		}
 
         public T Value
         {
@@ -22,7 +32,6 @@ namespace eShopOnContainers.Core.Validations
             {
                 return _value;
             }
-
             set
             {
                 _value = value;
@@ -36,11 +45,9 @@ namespace eShopOnContainers.Core.Validations
             {
                 return _isValid;
             }
-
             set
             {
                 _isValid = value;
-                _errors.Clear();
                 RaisePropertyChanged(() => IsValid);
             }
         }
@@ -48,7 +55,7 @@ namespace eShopOnContainers.Core.Validations
         public ValidatableObject()
         {
             _isValid = true;
-            _errors = new ObservableCollection<string>();
+            _errors = new List<string>();
             _validations = new List<IValidationRule<T>>();
         }
 
@@ -59,11 +66,7 @@ namespace eShopOnContainers.Core.Validations
             IEnumerable<string> errors = _validations.Where(v => !v.Check(Value))                             
                 .Select(v => v.ValidationMessage);
 
-            foreach (var error in errors)
-            {
-                Errors.Add(error);
-            }
-
+			Errors = errors.ToList();
             IsValid = !Errors.Any();
 
             return this.IsValid;

src/Mobile/eShopOnContainers/eShopOnContainers.Core/ViewModels/LoginViewModel.cs

@@ -2,7 +2,6 @@
 using eShopOnContainers.Core.Models.User;
 using eShopOnContainers.Core.Services.Identity;
 using eShopOnContainers.Core.Services.OpenUrl;
-using eShopOnContainers.Core.Services.User;
 using eShopOnContainers.Core.Validations;
 using eShopOnContainers.Core.ViewModels.Base;
 using IdentityModel.Client;
@@ -128,6 +127,10 @@ namespace eShopOnContainers.Core.ViewModels
 
         public ICommand SettingsCommand => new Command(async () => await SettingsAsync());
 
+		public ICommand ValidateUserNameCommand => new Command(() => ValidateUserName());
+
+		public ICommand ValidatePasswordCommand => new Command(() => ValidatePassword());
+
         public override Task InitializeAsync(object navigationData)
         {
             if(navigationData is LogoutParameter)
@@ -250,16 +253,26 @@ namespace eShopOnContainers.Core.ViewModels
 
         private bool Validate()
         {
-            bool isValidUser = _userName.Validate();
-            bool isValidPassword = _password.Validate();
+			bool isValidUser = ValidateUserName();
+            bool isValidPassword = ValidatePassword();
 
             return isValidUser && isValidPassword;
         }
 
+		private bool ValidateUserName()
+		{
+			return _userName.Validate();
+		}
+
+		private bool ValidatePassword()
+		{
+			return _password.Validate();
+		}
+
         private void AddValidations()
         {
-            _userName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "Username should not be empty" });
-            _password.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "Password should not be empty" });
+            _userName.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "A username is required." });
+            _password.Validations.Add(new IsNotNullOrEmptyRule<string> { ValidationMessage = "A password is required" });
         }
 
         public void InvalidateMock()

src/Mobile/eShopOnContainers/eShopOnContainers.Core/Views/LoginView.xaml

@@ -6,6 +6,7 @@
              xmlns:animations="clr-namespace:eShopOnContainers.Core.Animations;assembly=eShopOnContainers.Core"
              xmlns:triggers="clr-namespace:eShopOnContainers.Core.Triggers;assembly=eShopOnContainers.Core"
              xmlns:behaviors="clr-namespace:eShopOnContainers.Core.Behaviors;assembly=eShopOnContainers.Core"
+			 xmlns:effects="clr-namespace:eShopOnContainers.Core.Effects;assembly=eShopOnContainers.Core"
 			 viewModelBase:ViewModelLocator.AutoWireViewModel="true">
     <ContentPage.Title>
         <OnPlatform
@@ -177,30 +178,60 @@
                 Margin="24">
                 <Label
                   Text="User name or email"
-                  Style="{StaticResource HeaderLabelStyle}"/>
-                <Entry
-                    Text="{Binding UserName.Value, Mode=TwoWay}">
-                    <Entry.Style>
+                  Style="{StaticResource HeaderLabelStyle}" />
+                <Entry Text="{Binding UserName.Value, Mode=TwoWay}">
+					<Entry.Style>
                         <OnPlatform x:TypeArguments="Style"
                           iOS="{StaticResource EntryStyle}"
                           Android="{StaticResource EntryStyle}"
                           WinPhone="{StaticResource UwpEntryStyle}"/>
                     </Entry.Style>
+                    <Entry.Behaviors>
+						<behaviors:EventToCommandBehavior
+							EventName="TextChanged"
+							Command="{Binding ValidateUserNameCommand}" />
+					</Entry.Behaviors>
+					<Entry.Triggers>
+						<DataTrigger 
+							TargetType="Entry"
+							Binding="{Binding UserName.IsValid}"
+							Value="False">
+							<Setter Property="effects:LineColorEffect.LineColor" Value="{StaticResource ErrorColor}" />
+						</DataTrigger>
+					</Entry.Triggers>
                 </Entry>
+				<Label 
+					Text="{Binding UserName.Errors, Converter={StaticResource FirstValidationErrorConverter}"
+					Style="{StaticResource ValidationErrorLabelStyle}" />
                 <Label
                   Text="Password"
                   Style="{StaticResource HeaderLabelStyle}"/>
                 <Entry
                   IsPassword="True"
-                  Text="{Binding Password.Value, Mode=TwoWay}"
-                  Style="{StaticResource EntryStyle}">
+                  Text="{Binding Password.Value, Mode=TwoWay}">
                     <Entry.Style>
                         <OnPlatform x:TypeArguments="Style"
                           iOS="{StaticResource EntryStyle}"
                           Android="{StaticResource EntryStyle}"
                           WinPhone="{StaticResource UwpEntryStyle}"/>
                     </Entry.Style>
+					<Entry.Behaviors>
+						<behaviors:EventToCommandBehavior
+							EventName="TextChanged"
+							Command="{Binding ValidatePasswordCommand}" />
+					</Entry.Behaviors>
+					<Entry.Triggers>
+						<DataTrigger 
+							TargetType="Entry"
+							Binding="{Binding Password.IsValid}"
+							Value="False">
+							<Setter Property="effects:LineColorEffect.LineColor" Value="{StaticResource ErrorColor}" />
+						</DataTrigger>
+					</Entry.Triggers>
                 </Entry>
+				<Label 
+					Text="{Binding Password.Errors, Converter={StaticResource FirstValidationErrorConverter}"
+					Style="{StaticResource ValidationErrorLabelStyle}" />
             </StackLayout>
             <!-- LOGIN BUTTON -->
             <Grid

src/Mobile/eShopOnContainers/eShopOnContainers.Core/eShopOnContainers.Core.csproj

@@ -164,6 +164,7 @@
       <DependentUpon>ProductTemplate.xaml</DependentUpon>
     </Compile>
     <Compile Include="Converters\WebNavigatingEventArgsConverter.cs" />
+    <Compile Include="Converters\FirstValidationErrorConverter.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="app.config" />