unittest: refactor ahead of matcher changes

lib/src/throws_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 unittest.throws_matcher;
+
+import 'dart:async';
+
+import 'package:matcher/matcher.dart' hide fail, expect;
+
+import '../unittest.dart';
+
+/// This can be used to match two kinds of objects:
+///
+/// * A [Function] that throws an exception when called. The function cannot
+///   take any arguments. If you want to test that a function expecting
+///   arguments throws, wrap it in another zero-argument function that calls
+///   the one you want to test.
+///
+/// * A [Future] that completes with an exception. 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.
+const Matcher throws = const Throws();
+
+/// This can be used to match two kinds of objects:
+///
+/// * A [Function] that throws an exception when called. The function cannot
+///   take any arguments. If you want to test that a function expecting
+///   arguments throws, wrap it in another zero-argument function that calls
+///   the one you want to test.
+///
+/// * A [Future] that completes with an exception. 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.
+///
+/// In both cases, when an exception is thrown, this will test that the
+/// exception object matches [matcher]. If [matcher] is not an instance of
+/// [Matcher], it will implicitly be treated as `equals(matcher)`.
+Matcher throwsA(matcher) => new Throws(wrapMatcher(matcher));
+
+class Throws extends Matcher {
+  final Matcher _matcher;
+
+  const Throws([Matcher matcher]) : this._matcher = matcher;
+
+  bool matches(item, Map matchState) {
+    if (item is! Function && item is! Future) return false;
+    if (item is Future) {
+      var done = expectAsync((fn) => fn());
+
+      // Queue up an asynchronous expectation that validates when the future
+      // completes.
+      item.then((value) {
+        done(() {
+          fail("Expected future to fail, but succeeded with '$value'.");
+        });
+      }, onError: (error, trace) {
+        done(() {
+          if (_matcher == null) return;
+          var reason;
+          if (trace != null) {
+            var stackTrace = trace.toString();
+            stackTrace = "  ${stackTrace.replaceAll("\n", "\n  ")}";
+            reason = "Actual exception trace:\n$stackTrace";
+          }
+          expect(error, _matcher, reason: 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 {
+        addStateInfo(matchState, {'exception': e, 'stack': s});
+        return false;
+      }
+    }
+  }
+
+  Description describe(Description description) {
+    if (_matcher == null) {
+      return description.add("throws");
+    } else {
+      return description.add('throws ').addDescriptionOf(_matcher);
+    }
+  }
+
+  Description describeMismatch(
+      item, Description mismatchDescription, Map matchState, bool verbose) {
+    if (item is! Function && item is! Future) {
+      return mismatchDescription.add('is not a Function or Future');
+    } else if (_matcher == null || matchState['exception'] == null) {
+      return mismatchDescription.add('did not throw');
+    } else {
+      mismatchDescription
+          .add('threw ')
+          .addDescriptionOf(matchState['exception']);
+      if (verbose) {
+        mismatchDescription.add(' at ').add(matchState['stack'].toString());
+      }
+      return mismatchDescription;
+    }
+  }
+}