WPF MVVM IDataErrorInfo - wpf

Question

i have an app in MVVM pattern ; it contains one textbox and one button;
i add a validation that if textbox is empty , textbox color change to red;
and at this point i want that the button enable be false till the user
enter character in textbox and then button enable change to true;
my XAML Code is:
<Window.Resources>
<ControlTemplate x:Key="ErrorTemplate">
<DockPanel LastChildFill="True">
<Border BorderBrush="Pink" BorderThickness="1">
<AdornedElementPlaceholder />
</Border>
</DockPanel>
</ControlTemplate>
<Style TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip"
Value="{Binding RelativeSource={x:Static RelativeSource.Self},Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<TextBox Margin="10" Text="{Binding ValidateInputText,UpdateSourceTrigger=PropertyChanged, Mode=TwoWay, ValidatesOnDataErrors=True, NotifyOnValidationError=True, ValidatesOnExceptions=True}"
Validation.ErrorTemplate="{StaticResource ErrorTemplate}">
</TextBox>
<Button Margin="10" Grid.Row="2" Command="{Binding ValidateInputCommand}"/>
and my command class is:
public class RelayCommand : ICommand
{
private Action WhatToExcute;
private Func<bool> WhenToExecute;
public RelayCommand(Action what,Func<bool>when )
{
WhatToExcute = what;
WhenToExecute = when;
}
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
return WhenToExecute();
}
public void Execute(object parameter)
{
WhatToExcute();
}
}
and my viewmodel is:
public class ViewModel : IDataErrorInfo , INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
Product product = new Product();
public ViewModel()
{
ValidateInputCommand = new RelayCommand(action, valid);
}
public void action()
{
}
public bool valid()
{
if (product.Name == null)
return false;
return true;
}
public string ValidateInputText
{
get { return product.Name; }
set {
product.Name = value;
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(ValidateInputText));
}
}
}
public RelayCommand ValidateInputCommand { get; set; }
public string this[string columnName]
{
get
{
if ("ValidateInputText" == columnName)
{
if (String.IsNullOrEmpty(ValidateInputText))
{
return "Please enter a Name";
}
}
return "";
}
}
public string Error
{
get
{
throw new NotImplementedException();
}
}
}
now when i run my app , the color of text box is red and when i enter a
character it change to normal that is ok but the
button enable is false and does not change
so i could not click button.
what should i do?

Answer 1

The CanExecuteChanged event on your RelayCommand never gets fired. Therefore the button to which the command has been bound will never re-evaluate its IsEnabled status. Your view model could notify the command on changes in its valid status and the command should in turn raise its CanExecuteChanged event.