CustomMatcher class

A useful utility class for implementing other matchers through inheritance. Derived classes should call the base constructor with a feature name and description, and an instance matcher, and should implement the featureValueOf abstract method.

The feature description will typically describe the item and the feature, while the feature name will just name the feature. For example, we may have a Widget class where each Widget has a price; we could make a CustomMatcher that can make assertions about prices with:

class HasPrice extends CustomMatcher {
  HasPrice(matcher) : super("Widget with price that is", "price", matcher);
  featureValueOf(actual) => (actual as Widget).price;
}

and then use this for example like:

expect(inventoryItem, HasPrice(greaterThan(0)));