unittest: refactor ahead of matcher changes

lib/src/prints_matcher.dart

+// Copyright (c) 2014, 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 matcher.prints_matcher;
+
+import 'dart:async';
+
+import 'package:matcher/matcher.dart' hide completes, wrapAsync, expect;
+
+import '../unittest.dart';
+
+/// Matches a [Function] that prints text that matches [matcher].
+///
+/// [matcher] may be a String or a [Matcher].
+///
+/// If the function this runs against returns a [Future], all text printed by
+/// the function (using [Zone] scoping) until that Future completes is matched.
+///
+/// This only tracks text printed using the [print] function.
+Matcher prints(matcher) => new _Prints(wrapMatcher(matcher));
+
+class _Prints extends Matcher {
+  final Matcher _matcher;
+
+  _Prints(this._matcher);
+
+  bool matches(item, Map matchState) {
+    if (item is! Function) return false;
+
+    var buffer = new StringBuffer();
+    var result = runZoned(item,
+        zoneSpecification: new ZoneSpecification(print: (_, __, ____, line) {
+      buffer.writeln(line);
+    }));
+
+    if (result is! Future) {
+      var actual = buffer.toString();
+      matchState['prints.actual'] = actual;
+      return _matcher.matches(actual, matchState);
+    }
+
+    return completes.matches(result.then(wrapAsync((_) {
+      expect(buffer.toString(), _matcher);
+    }, 'prints')), matchState);
+  }
+
+  Description describe(Description description) =>
+      description.add('prints ').addDescriptionOf(_matcher);
+
+  Description describeMismatch(
+      item, Description description, Map matchState, bool verbose) {
+    var actual = matchState.remove('prints.actual');
+    if (actual == null) return description;
+    if (actual.isEmpty) return description.add("printed nothing.");
+
+    description.add('printed ').addDescriptionOf(actual);
+
+    // Create a new description for the matcher because at least
+    // [_StringEqualsMatcher] replaces the previous contents of the description.
+    var innerMismatch = _matcher
+        .describeMismatch(actual, new StringDescription(), matchState, verbose)
+        .toString();
+
+    if (innerMismatch.isNotEmpty) {
+      description.add('\n   Which: ').add(innerMismatch.toString());
+    }
+
+    return description;
+  }
+}