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Testing ViewModels

One of the benefits of using the MVVM patterns is that the viewmodels are testable. Because the viewmodel doesn't have any dependencies on the user interface components, it is very easy to test.

You can just create an instance of the viewmodel class, set its property values, call a method and verify that the results are correct.

In the following example, you can see how easily the viewmodel can be tested.

[TestMethod]
public void NormalTest()
{
    var viewModel = new SampleViewModel()
    {
        FirstName = "Humphrey",
        LastName = "Appleby"
    };

    var userName = viewModel.GenerateUserName();
    Assert.AreEqual("applebyh", userName);
}

Mocking the Context

DotVVM injects the DotvvmRequestContext object in the viewmodels. If your viewmodel uses some features of this object, you need to mock this object to provide all services to the tested method.

DotVVM contains a prepared mock class TestDotvvmRequestContext. You can set all properties you need (Configuration, Route, Parameters, Query, HttpContext, ModelState, ResourceManager etc., but they are all optional).

In the test, you may need to verify e.g. whether the viewmodel method has redirected to some URL, or whether it returned a file, failed on the model validation etc.

In DotVVM, the methods InterruptRequest, RedirectTo*, RedirectPermanentTo*, FailOnInvalidModelState and ReturnFile throw the DotvvmInterruptRequestExecutionException. This allows to interrupt the execution of the HTTP request and pass it to the following middleware.

The DotvvmInterruptRequestExecutionException exception includes a property indication the reason why the request was interrupted and a CustomData parameter which holds e.g. the URL where the viewmodel action tried to redirect.

Here you can see a test case that verifies the viewmodel has built the proper route:

[TestMethod]
public void NormalTest()
{
    var configuration = DotvvmConfiguration.CreateDefault();
    configuration.Routes.Add("ArticleDetail", "article/{Id}/{Title}", "article.dothtml", null);
    
    var viewModel = new SampleViewModel()
    {
        Context = new TestDotvvmRequestContext()
        {
            Configuration = configuration,
            ApplicationHostPath = "myApp"      // This tells the DotVVM that the application runs in a virtual directory, e.g. on the URL "http://somedomain.com/myApp/"
        }
    };
    
    try
    {
        var article = new ArticleDTO() { Id = 1, Title = "DotVVM is the best framework" };
        viewModel.RedirectToArticle(article);
    }
    catch (DotvvmInterruptRequestException ex)
    {
        Assert.AreEqual(InterruptReason.Redirect, ex.InterruptReason);
        Assert.AreEqual("/myApp/article/1/dotvvm-is-the-best-framework");
        return;
    }
    Assert.Fail("A redirect should have been performed!");
}
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