unittest: refactor ahead of matcher changes

lib/src/future_matchers.dart

+// Copyright (c) 2012, 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.future_matchers;
+
+import 'dart:async';
+
+import 'package:matcher/matcher.dart' hide throws, throwsA, expect, fail;
+
+import '../unittest.dart';
+
+/// Matches a [Future] that completes successfully with a value.
+///
+/// Note that this creates an asynchronous expectation. The call to `expect()`
+/// that includes this will return immediately and execution will continue.
+/// Later, when the future completes, the actual expectation will run.
+///
+/// To test that a Future completes with an exception, you can use [throws] and
+/// [throwsA].
+final Matcher completes = const _Completes(null, '');
+
+/// Matches a [Future] that completes succesfully with a value that matches
+/// [matcher].
+///
+/// Note that this creates an asynchronous expectation. The call to
+/// `expect()` that includes this will return immediately and execution will
+/// continue. Later, when the future completes, the actual expectation will run.
+///
+/// To test that a Future completes with an exception, you can use [throws] and
+/// [throwsA].
+///
+/// [description] is an optional tag that can be used to identify the completion
+/// matcher in error messages.
+Matcher completion(matcher, [String description = '']) =>
+    new _Completes(wrapMatcher(matcher), description);
+
+class _Completes extends Matcher {
+  final Matcher _matcher;
+  final String _id;
+
+  const _Completes(this._matcher, this._id);
+
+  bool matches(item, Map matchState) {
+    if (item is! Future) return false;
+    var done = expectAsync((fn) => fn(), id: _id);
+
+    item.then((value) {
+      done(() {
+        if (_matcher != null) expect(value, _matcher);
+      });
+    }, onError: (error, trace) {
+      var id = _id == '' ? '' : '${_id} ';
+      var reason = 'Expected future ${id}to complete successfully, '
+          'but it failed with ${error}';
+      if (trace != null) {
+        var stackTrace = trace.toString();
+        stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';
+        reason = '$reason\nStack trace:\n$stackTrace';
+      }
+      done(() => fail(reason));
+    });
+
+    return true;
+  }
+
+  Description describe(Description description) {
+    if (_matcher == null) {
+      description.add('completes successfully');
+    } else {
+      description.add('completes to a value that ').addDescriptionOf(_matcher);
+    }
+    return description;
+  }
+}