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Jag Reehal on Agile Development, ASP.NET MVC, Silverlight and all manner of good stuff

Enabling buttons in Silverlight and WPF using MVVM and ValidatesOnExceptions

Posted on | July 6, 2010 | No Comments

In a previous post we saw how exceptions could be used for Silverlight validation.

While could validate the users input using exceptions, it wasn’t possible to disable the calculate button if the input values were invalid (because they were either blank or non-numeric).

The code used in this post can be downloaded here.

So how are we going to solve the problem?

Throughout the Silverlight refactoring series I’ve tried to illustrate how important SOLID design principles are for having testable applications.

So if we think about the ViewModel, we need to ask ourselves if it’s the right place or should be responsible for validation?

In this case I would say no.

When should the button be enabled?

For a calculate button to be enabled, both text boxes must contain numeric values

This means we have to know if both text boxes are valid at the same time. Taking a step back here, let’s think about the bigger picture.

What if there are three or four text boxes?

What we really after is a class that will be responsible for knowing if any text boxes are invalid.

This can be done by using a validation base class.

In the code below notice how the ValidationBase class doesn’t know anything about enabling or disabling the calculate button.

public class ValidationBase
{
    public readonly Dictionary<string, string> Errors;

    public ValidationBase()
    {
        Errors = new Dictionary<string, string>();
    }

    public void AddError(string propertyName, string message)
    {
        if (!Errors.ContainsKey(propertyName))
        {
            Errors[propertyName] = message;
        }
    }

    public void RemoveErrors(string propertyName)
    {
        Errors.Remove(propertyName);
    }

    public bool IsPropertyValid(string propertyName)
    {
        return !Errors.ContainsKey(propertyName);
    }

    public string GetErrorMessageForProperty(string propertyName)
    {
        string message;
        Errors.TryGetValue(propertyName, out message);
        return message;
    }

    public bool IsValid()
    {
        return Errors.Count == 0;
    }
}

The code below shows the unit tests for the ValidationBase class.

[TestFixture]
public class When_using_the_ValidatiorBase
{
    private ValidationBase _validationBase;

    [SetUp]
    public void SetUp()
    {
        _validationBase = new ValidationBase();
    }

    [Test]
    public void IsValid_should_return_false_when_errors_exist()
    {
        // Arrange
        _validationBase.AddError("propertyName", "message");

        // Act
        var result = _validationBase.IsValid();

        // Assert
        result.ShouldBeFalse();
    }

    [Test]
    public void IsValid_should_return_true_when_no_errors_exist()
    {
        // Arrange
        // collection will be empty at this point

        // Act
        var result = _validationBase.IsValid();

        // Assert
        result.ShouldBeTrue();
    }

    [Test]
    public void IsPropertyValid_should_return_false_if_error_exists()
    {
        // Arrange
        _validationBase.AddError("propertyName", "message");

        // Act
        var result = _validationBase.IsPropertyValid("propertyName");

        // Assert
        result.ShouldBeFalse();
    }

    [Test]
    public void IsPropertyValid_should_return_true_if_error_does_not_exist()
    {
        // Arrange
        // collection will be empty at this point

        // Act
        var result = _validationBase.IsPropertyValid("X");

        // Assert
        result.ShouldBeTrue();
    }

    [Test]
    public void Should_be_able_to_return_message_for_error()
    {
        // Arrange
        _validationBase.AddError("propertyName", "message");

        // Act
        var result = _validationBase.GetErrorMessageForProperty("propertyName");

        // Assert
        result.ShouldEqual("message");
    }

    [Test]
    public void Should_return_null_if_message_does_not_exist_for_error()
    {
        // Arrange
        // collection will be empty at this point

        // Act
        var result = _validationBase.GetErrorMessageForProperty("propertyName");

        // Assert
        result.ShouldBeNull();
    }
}

Validating the users input

There are three outcomes when validating what the user has entered:

  • The value is blank
  • The value is not a number
  • The value is a number

All the ViewModel wants to know is if the users input is valid, choosing the appropriate error message isn’t its concern. This means we need a class that will be responsible for validation and returning the relevant message.

For this we will use the CalculatorValidator class we created in the ‘Applying the Open Closed Principle in Silverlight and WPF using MEF‘ post.

[Export(typeof(ICalculatorValidator))]
public class CalculatorValidator : ValidationBase, ICalculatorValidator
{
    [ImportMany]
    public IEnumerable<ICalculatorValidationRule> CalculatorValidationRules { get; set; }

    public void ValidateNumber(string propertyName, string value)
    {
        RemoveErrors(propertyName);

        foreach (var calculatorValidationRule in CalculatorValidationRules)
        {
            if (!calculatorValidationRule.IsValid(value))
            {
                AddError(propertyName, calculatorValidationRule.ErrorMessage);
                return;
            }
        }
    }
}

This is useful because if we decide to change how to validate the user’s input neither the CalculatorValidator or the ViewModel classes need to be modified.

Hooking it all up

To use the CalculatorValidator in the ViewModel it has to be injected/imported.

[ImportingConstructor]
public CalculatorViewModel(ICalculator calculator, ICalculatorValidator calculatorValidator)
{
    _calculator = calculator;
    _calculatorValidator = calculatorValidator;
    ...
}

Each time a user enters a value in the text boxes the CheckIfNumberIsValid method checks if the calculate button should be enabled and throws an exception if the users value is not valid.

public string FirstValue
{
    get { return _firstValue; }
    set
    {
        CheckIfNumberIsValid("FirstValue", out _firstValue, value);
    }
}

public string SecondValue
{
    get { return _secondValue; }
    set
    {
        CheckIfNumberIsValid("SecondValue", out _secondValue, value);
    }
}

public void CheckIfNumberIsValid(string propertyName, out string propertyValue, string value)
{
    _calculatorValidator.ValidateNumber(propertyName, value);

    CheckIfCalculteButtonShouldBeEnabled();

    if (_calculatorValidator.IsPropertyValid(propertyName))
    {
        propertyValue = value;
        OnPropertyChanged(propertyName);
    }
    else
    {
        throw new Exception(_calculatorValidator.GetErrorMessageForProperty(propertyName));
    }
}

public void CheckIfCalculteButtonShouldBeEnabled()
{
    _calculateCommand.IsEnabled = _calculatorValidator.IsValid();
}

Unit testing the ViewModel in MVVM

By using Moq we can unit test the ViewModel to ensure the button is enabled when there are no validation errors and not enabled when there are validation errors.

[TestFixture]
public class When_using_the_CalculatorViewModel
{
    private Mock<ICalculator> _calculator;
    private Mock<ICalculatorValidator> _calculatorValidator;
    private CalculatorViewModel _calculatorViewModel;

    [SetUp]
    public void SetUp()
    {
        _calculator = new Mock<ICalculator>();
        _calculatorValidator = new Mock<ICalculatorValidator>();
        _calculatorViewModel = new CalculatorViewModel(_calculator.Object, _calculatorValidator.Object);
    }

    [Test]
    public void Initial_value_of_first_number_is_0()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.FirstValue;

        // Assert
        result.ShouldEqual("0");
    }

    [Test]
    public void Initial_value_of_second_number_is_0()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.SecondValue;

        // Assert
        result.ShouldEqual("0");
    }

    [Test]
    public void Initial_value_of_calculate_button_is_enabled()
    {
        // Arrange
        // checking initial value

        // Act
        var result = _calculatorViewModel.CalculateCommand.IsEnabled;

        // Assert
        result.ShouldBeTrue();
    }

    [Test]
    [ExpectedException(typeof(Exception))]
    public void Will_throw_exception_if_input_is_invalid()
    {
        // Arrange
        string propertyValue;
        _calculatorValidator.Setup(c => c.IsPropertyValid("X")).Throws(new Exception());

        // Act
        _calculatorViewModel.CheckIfNumberIsValid("X", out propertyValue, "X");

        // Assert
        // should throw exception
    }

    [Test]
    public void Will_set_property_value_if_input_is_valid()
    {
        // Arrange
        string propertyValue;
        _calculatorValidator.Setup(c => c.IsPropertyValid("X")).Returns(true);

        // Act
        _calculatorViewModel.CheckIfNumberIsValid("X", out propertyValue, "11");

        // Assert
        propertyValue.ShouldEqual("11");

    }

    [Test]
    public void Calculate_command_should_not_be_enabled_if_ViewModel_is_not_valid()
    {
        // Arrange
        _calculatorValidator.Setup(c => c.IsValid()).Returns(false);

        // Act
        _calculatorViewModel.CheckIfCalculteButtonShouldBeEnabled();

        // Assert
        _calculatorViewModel.CalculateCommand.IsEnabled.ShouldBeFalse();
    }

    [Test]
    public void Calculate_command_should_be_enabled_if_ViewModel_is_valid()
    {
        // Arrange
        _calculatorValidator.Setup(c => c.IsValid()).Returns(true);

        // Act
        _calculatorViewModel.CheckIfCalculteButtonShouldBeEnabled();

        // Assert
        _calculatorViewModel.CalculateCommand.IsEnabled.ShouldBeTrue();
    }
}

So what have we achieved?

By using a validation base class we are able to store validation errors and can determine if the controls on the page are all valid.

The ViewModel can take advantage of this functionality and use it to enable and disable the calculate button.

Using exceptions for validation isn’t every developers cup of tea, so be sure to keep an eye on the Silverlight refactoring series to see other approaches we can take to do validation in Silverlight 4.

Category: MVVM, Moq, Silverlight, Unit tests, WPF

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