unittest: refactor ahead of matcher changes

lib/src/expect.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library unittest.expect;
+
+import 'package:matcher/matcher.dart';
+
+/// An exception thrown when a test assertion fails.
+class TestFailure {
+  final String message;
+
+  TestFailure(this.message);
+
+  String toString() => message;
+}
+
+/// The type used for functions that can be used to build up error reports
+/// upon failures in [expect].
+typedef String ErrorFormatter(
+    actual, Matcher matcher, String reason, Map matchState, bool verbose);
+
+/// Assert that [actual] matches [matcher].
+///
+/// This is the main assertion function. [reason] is optional and is typically
+/// not supplied, as a reason is generated from [matcher]; if [reason]
+/// is included it is appended to the reason generated by the matcher.
+///
+/// [matcher] can be a value in which case it will be wrapped in an
+/// [equals] matcher.
+///
+/// If the assertion fails a [TestFailure] is thrown.
+///
+/// In some cases extra diagnostic info can be produced on failure (for
+/// example, stack traces on mismatched exceptions). To enable these,
+/// [verbose] should be specified as `true`.
+void expect(actual, matcher,
+    {String reason, bool verbose: false, ErrorFormatter formatter}) {
+  matcher = wrapMatcher(matcher);
+  var matchState = {};
+  try {
+    if (matcher.matches(actual, matchState)) return;
+  } catch (e, trace) {
+    if (reason == null) {
+      reason = '${(e is String) ? e : e.toString()} at $trace';
+    }
+  }
+  if (formatter == null) formatter = _defaultFailFormatter;
+  fail(formatter(actual, matcher, reason, matchState, verbose));
+}
+
+/// Convenience method for throwing a new [TestFailure] with the provided
+/// [message].
+void fail(String message) => throw new TestFailure(message);
+
+// The default error formatter.
+String _defaultFailFormatter(
+    actual, Matcher matcher, String reason, Map matchState, bool verbose) {
+  var description = new StringDescription();
+  description.add('Expected: ').addDescriptionOf(matcher).add('\n');
+  description.add('  Actual: ').addDescriptionOf(actual).add('\n');
+
+  var mismatchDescription = new StringDescription();
+  matcher.describeMismatch(actual, mismatchDescription, matchState, verbose);
+
+  if (mismatchDescription.length > 0) {
+    description.add('   Which: ${mismatchDescription}\n');
+  }
+  if (reason != null) description.add(reason).add('\n');
+  return description.toString();
+}