expect Matchers must implement the Matcher class. The base Matcher class that implements this interface has a generic implementation of describeMismatch so this does not need to be provided unless a more clear description is required. The other two methods (matches and describe) must always be provided as they are highly matcher-specific.

abstract class Matcher {
  /**
   * This does the matching of the actual vs expected values.
   * [item] is the actual value. [matchState] can be supplied
   * and may be used to add details about the mismatch that are too
   * costly to determine in [describeMismatch].
   */
  abstract bool matches(item, MatchState matchState);

  /** This builds a textual description of the matcher. */
  abstract Description describe(Description description);

  /**
   * This builds a textual description of a specific mismatch. [item]
   * is the value that was tested by [matches]; [matchState] is
   * the [MatchState] that was passed to and supplemented by [matches]
   * with additional information about the mismact, and [mismatchDescription]
   * is the [Description] that is being built to decribe the mismatch.
   * A few matchers make use of the [verbose] flag to provide detailed
   * information that is not typically included but can be of help in
   * diagnosing failures, such as stack traces.
   */
  abstract Description describeMismatch(item, Description mismatchDescription,
      MatchState matchState, bool verbose);
}