Add a scheduled test library.

pkg/scheduled_test/lib/src/schedule.dart

Index: pkg/scheduled_test/lib/src/schedule.dart
diff --git a/pkg/scheduled_test/lib/src/schedule.dart b/pkg/scheduled_test/lib/src/schedule.dart
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+++ b/pkg/scheduled_test/lib/src/schedule.dart
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+// Copyright (c) 2013, 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 schedule;
+
+import 'dart:async';
+import 'dart:collection';
+
+import 'package:unittest/unittest.dart' as unittest;
+
+import 'schedule_error.dart';
+import 'task.dart';
+
+/// The schedule of tasks to run for a single test. This has three separate task
+/// queues: [tasks], [onComplete], and [onException]. It also provides
+/// visibility into the current state of the schedule.
+class Schedule {
+  /// The main task queue for the schedule. These tasks are run before the other
+  /// queues and generally constitute the main test body.
+  TaskQueue get tasks => _tasks;
+  TaskQueue _tasks;
+
+  /// The queue of tasks to run if an error is caught while running [tasks]. The
+  /// error will be available via [error]. These tasks won't be run if no error
+  /// occurs. Note that expectation failures count as errors.
+  ///
+  /// This queue runs before [onComplete], and errors in [onComplete] will not
+  /// cause this queue to be run.
+  ///
+  /// If an error occurs in a task in this queue, all further tasks will be
+  /// skipped.
+  TaskQueue get onException => _onException;
+  TaskQueue _onException;
+
+  /// The queue of tasks to run after [tasks] and possibly [onException] have
+  /// run. This queue will run whether or not an error occurred. If one did, it
+  /// will be available via [error]. Note that expectation failures count as
+  /// errors.
+  ///
+  /// This queue runs after [onException]. If an error occurs while running
+  /// [onException], that error will be available via [error] in place of the
+  /// original error.
+  ///
+  /// If an error occurs in a task in this queue, all further tasks will be
+  /// skipped.
+  TaskQueue get onComplete => _onComplete;
+  TaskQueue _onComplete;
+
+  /// Returns the [Task] that's currently executing, or `null` if there is no
+  /// such task. This will be `null` both before the schedule starts running and
+  /// after it's finished.
+  Task get currentTask => _currentTask;
+  Task _currentTask;
+
+  /// Whether the schedule has finished running. This is only set once
+  /// [onComplete] has finished running. It will be set whether or not an
+  /// exception has occurred.
+  bool get done => _done;
+  bool _done = false;
+
+  /// The error thrown by the task queue. This will only be set while running
+  /// [onException] and [onComplete], since an error in [tasks] will cause it to
+  /// terminate immediately.
+  ScheduleError get error => _error;
+  ScheduleError _error;
+
+  /// The task queue that's currently being run, or `null` if there is no such
+  /// queue. One of [tasks], [onException], or [onComplete]. This will be `null`
+  /// before the schedule starts running.
+  TaskQueue get currentQueue => _done ? null : _currentQueue;
+  TaskQueue _currentQueue;
+
+  /// The number of out-of-band callbacks that have been registered with
+  /// [wrapAsync] but have yet to be called.
+  int _pendingCallbacks = 0;
+
+  /// A completer that will be completed once [_pendingCallbacks] reaches zero.
+  /// This will only be non-`null` if [_awaitPendingCallbacks] has been called
+  /// while [_pendingCallbacks] is non-zero.
+  Completer _noPendingCallbacks;
+
+  /// Creates a new schedule with empty task queues.
+  Schedule() {
+    _tasks = new TaskQueue._("tasks", this);
+    _onComplete = new TaskQueue._("onComplete", this);
+    _onException = new TaskQueue._("onException", this);
+  }
+
+  /// Sets up this schedule by running [setUp], then runs all the task queues in
+  /// order. Any errors in [setUp] will cause [onException] to run.
+  Future run(void setUp()) {
+    return new Future.immediate(null).then((_) {
+      try {
+        setUp();
+      } catch (e, stackTrace) {
+        throw new ScheduleError.from(this, e, stackTrace: stackTrace);
+      }
+
+      return tasks._run();
+    }).catchError((e) {
+      _error = e;
+      return onException._run().then((_) {
+        throw e;
+      });
+    }).whenComplete(() => onComplete._run()).whenComplete(() {
+      _done = true;
+    });
+  }
+
+  /// Signals that an out-of-band error has occurred. Using [wrapAsync] along
+  /// with `throw` is usually preferable to calling this directly.
+  ///
+  /// The metadata in [AsyncError]s and [ScheduleError]s will be preserved.
+  void signalError(error, [stackTrace]) {
+    var scheduleError = new ScheduleError.from(this, error,
+        stackTrace: stackTrace, task: currentTask);
+    if (_done) {
+      unittest.registerException(new ExpectException(scheduleError.toString()));

[Bob Nystrom, 2013/02/08 16:15:37]

Are we ever supposed to get here? I think this may do the wrong thing. If you've
already advanced to the next test, this will tell unittest to advance from that
one.

Maybe just throw a StateError or something here?

[nweiz, 2013/02/08 22:14:38]

Yeah, a StateError is probably better here. I really hate top-levelling
exceptions in tests, but in this case I don't think there's a better
alternative.

+    } else if (currentQueue == null) {
+      throw scheduleError;
+    } else {
+      _currentQueue._signalError(scheduleError);
+    }
+  }
+
+  /// Returns a function wrapping [fn] that pipes any errors into the schedule
+  /// chain. This will also block the current task queue from completing until
+  /// the returned function has been called. It's used to ensure that
+  /// out-of-band callbacks are properly handled by the scheduled test.
+  ///
+  /// The top-level `wrapAsync` function should usually be used in preference to
+  /// this.

[Bob Nystrom, 2013/02/08 16:15:37]

Make this one private then?

[nweiz, 2013/02/08 22:14:38]

"Usually", not necessarily "always". Also, scheduled_test.dart needs to access
this.

+  Function wrapAsync(fn(arg)) {

[Bob Nystrom, 2013/02/08 16:15:37]

Should check that _noPendingCallbacks isn't already complete. If it is, throw a
StateError.

[nweiz, 2013/02/08 22:14:38]

It's possible to wait for pending callbacks multiple times, although the code
doesn't currently handle that correctly. Fixed.

+    _pendingCallbacks++;
+    return (arg) {
+      try {
+        return fn(arg);
+      } catch (e, stackTrace) {
+        signalError(e, stackTrace);
+      } finally {
+        _pendingCallbacks--;
+        if (_pendingCallbacks == 0 && _noPendingCallbacks != null) {
+          _noPendingCallbacks.complete();
+        }
+      }
+    };
+  }
+
+  /// Returns a [Future] that will complete once there are no pending
+  /// out-of-band callbacks.
+  Future _awaitNoPendingCallbacks() {
+    if (_pendingCallbacks == 0) return new Future.immediate(null);

[Bob Nystrom, 2013/02/08 16:15:37]

What if callbacks are added after this is called?

[nweiz, 2013/02/08 22:14:38]

Then they're not pending :p.

In seriousness, this is called when you're otherwise done with the current
queue, but you want to wait for the existing callbacks to clear out. Future
callbacks belong to the next queue.

+    if (_noPendingCallbacks == null) _noPendingCallbacks = new Completer();
+    return _noPendingCallbacks.future;
+  }
+}
+
+/// A queue of asynchronous tasks to execute in order.
+class TaskQueue {
+  // TODO(nweiz): make this a read-only view when issue 8321 is fixed.
+  /// The tasks in the queue.
+  Collection<Task> get contents => _contents;
+  final _contents = new Queue<Task>();
+
+  /// The name of the queue, for debugging purposes.
+  final String name;
+
+  /// The [Schedule] that created this queue.
+  final Schedule _schedule;
+
+  /// An out-of-band error signaled by [_schedule]. If this is non-null, it
+  /// indicates that the queue should stop as soon as possible and re-throw this
+  /// error.
+  ScheduleError _error;
+
+  TaskQueue._(this.name, this._schedule);
+
+  /// Schedules a task, [fn], to run asynchronously as part of this queue. Tasks
+  /// will be run in the order they're scheduled. In [fn] returns a [Future],
+  /// tasks after it won't be run until that [Future] completes.
+  ///
+  /// The return value will be completed once the scheduled task has finished
+  /// running. Its return value is the same as the return value of [fn], or the
+  /// value it completes to if it's a [Future].
+  ///
+  /// If [description] is passed, it's used to describe the task for debugging
+  /// purposes when an error occurs.
+  Future schedule(fn(), [String description]) {
+    var task = new Task(fn, this, description);
+    _contents.add(task);
+    return task.result;
+  }
+
+  /// Runs all the tasks in this queue in order.
+  Future _run() {
+    _schedule._currentQueue = this;
+    return Future.forEach(_contents, (task) {
+      _schedule._currentTask = task;
+      if (_error != null) throw _error;
+      return task.fn().catchError((e) {
+        throw new ScheduleError.from(_schedule, e, task: task);
+      });
+    }).whenComplete(() {
+      _schedule._currentTask = null;
+      return _schedule._awaitNoPendingCallbacks();
+    }).then((_) {
+      if (_error != null) throw _error;
+    });
+  }
+
+  /// Signals that an out-of-band error has been detected and the queue should
+  /// stop running as soon as possible.
+  void _signalError(ScheduleError error) {
+    _error = error;
+  }
+
+  String toString() => name;
+
+  /// Returns a detailed representation of the queue as a tree of tasks. If
+  /// [highlight] is passed, that task is specially highlighted.
+  ///
+  /// [highlight] must be a task in this queue.
+  String generateTree([Task highlight]) {
+    assert(highlight == null || highlight.queue == this);
+    return _contents.map((task) {
+      var lines = task.toString().split("\n");
+      var firstLine = task == highlight ?
+          "> ${lines.first}" : "* ${lines.first}";
+      lines = new List.from(lines.skip(1).map((line) => "| $line"));
+      lines.insertRange(0, 1, firstLine);
+      return lines.join("\n");
+    }).join("\n");
+  }
+}