class Throws extends BaseMatcher {
  final Matcher _matcher;

  const Throws([Matcher matcher]) :
    this._matcher = matcher;

  bool matches(item, MatchState matchState) {
    if (item is Future) {
      // Queue up an asynchronous expectation that validates when the future
      // completes.
      item.onComplete(expectAsync1((future) {
        if (future.hasValue) {
          expect(false, isTrue,
              "Expected future to fail, but succeeded with '${future.value}'.");
        } else if (_matcher != null) {
          var reason;
          if (future.stackTrace != null) {
            var stackTrace = future.stackTrace.toString();
            stackTrace = "  ${stackTrace.replaceAll("\n", "\n  ")}";
            reason = "Actual exception trace:\n$stackTrace";
          }
          expect(future.exception, _matcher, reason);
        }
      }));

      // It hasn't failed yet.
      return true;
    }

    try {
      item();
      return false;
    } catch (e, s) {
      if (_matcher == null ||_matcher.matches(e, matchState)) {
        return true;
      } else {
        matchState.state = {
            'exception' :e,
            'stack': s
        };
        return false;
      }
    }
  }

  Description describe(Description description) {
    if (_matcher == null) {
      return description.add("throws an exception");
    } else {
      return description.add('throws an exception which matches ').
          addDescriptionOf(_matcher);
    }
  }

  Description describeMismatch(item, Description mismatchDescription,
                               MatchState matchState,
                               bool verbose) {
    if (_matcher == null ||  matchState.state == null) {
      return mismatchDescription.add(' no exception');
    } else {
      mismatchDescription.
          add(' exception ').addDescriptionOf(matchState.state['exception']);
      if (verbose) {
          mismatchDescription.add(' at ').
          add(matchState.state['stack'].toString());
      }
       mismatchDescription.add(' does not match ').addDescriptionOf(_matcher);
       return mismatchDescription;
    }
  }
}