#16 Provide lazy evaluation of values

[Daniel Cazzulino (kzu)]

When setting expectations on object values, it's quite useful to evaluate the expected values in a lazy way, so that the test code has a chance to set the value.

i.e. consider the case where I'm setting an expectation on a view interface that will check that an argument passed-in matches a value existing on the correponding presenter, which is what's under test:

Expect.Once.On(view).
Method("SetDatabaseFilename").With(DatabaseFilename);
Expect.Once.On(view).
Method("SetDatabase").With(new LazyEqualsToMatcher(delegate() { return presenter.Database; }));

presenter.DatabaseFileChanged(Path.GetFullPath(DatabaseFilename));

Note that it's the presenter action DatabaseFileChanged the one that actually sets the presenter.Database value that we need in order to verify the expectation on the view. Without a lazy evaluation of that property at the time the method is invoked on the view, we have no way of actually setting the expectation. If we had used:

Expect.Once.On(view).
Method("SetDatabase").With(presenter.Database);

The database would have been null at that point, and the expectation would be wrong.

The LazyEqualsToMatcher is pretty simple:

public delegate object LazyValueHandler();

public class LazyEqualsToMatcher : Matcher
{
LazyValueHandler valueHandler;

public LazyEqualsToMatcher(LazyValueHandler valueHandler)
{
this.valueHandler = valueHandler;
}

public override void DescribeTo(TextWriter writer)
{
Is.EqualTo(valueHandler()).DescribeTo(writer);
}

public override bool Matches(object o)
{
return Is.EqualTo(valueHandler()).Matches(o);
}
}

It just wraps the Is.EqualsTo matcher but evaluates the property at matching time instead of construction time.

Thanks.