pkg/scheduled_test/lib/src/schedule.dart - sdk.git - Git at Google

 // 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 'mock_clock.dart' as mock_clock;
 import 'schedule_error.dart';
 import 'substitute_future.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 in [errors]. 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 in [errors]. Note that expectation failures count as
   /// errors.
   ///
   /// This queue runs after [onException]. If an error occurs while running
   /// [onException], that error will be available in [errors] after 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;

   /// The current state of the schedule.
   ScheduleState get state => _state;
   ScheduleState _state = ScheduleState.SET_UP;

   // TODO(nweiz): make this a read-only view once issue 8321 is fixed.

   /// Errors thrown by the task queues.
   ///
   /// When running tasks in [tasks], this will always be empty. If an error
   /// occurs in [tasks], it will be added to this list and then [onException]
   /// will be run. If an error occurs there as well, it will be added to this
   /// list and [onComplete] will be run. Errors thrown during [onComplete] will
   /// also be added to this list, although no scheduled tasks will be run
   /// afterwards.
   ///
   /// Any out-of-band callbacks that throw errors will also have those errors
   /// added to this list.
   final errors = <ScheduleError>[];

   /// The task queue that's currently being run. One of [tasks], [onException],
   /// or [onComplete]. This starts as [tasks], and can only be `null` after the
   /// schedule is done.
   TaskQueue get currentQueue =>
     _state == ScheduleState.DONE ? null : _currentQueue;
   TaskQueue _currentQueue;

   /// The time to wait before terminating a task queue for inactivity. Defaults
   /// to 30 seconds. This can be set to `null` to disable timeouts entirely.
   ///
   /// If a task queue times out, an error will be raised that can be handled as
   /// usual in the [onException] and [onComplete] queues. If [onException] times
   /// out, that can only be handled in [onComplete]; if [onComplete] times out,
   /// that cannot be handled.
   ///
   /// If a task times out and then later completes with an error, that error
   /// cannot be handled. The user will still be notified of it.
   Duration get timeout => _timeout;
   Duration _timeout = new Duration(seconds: 30);
   set timeout(Duration duration) {
     _timeout = duration;
     heartbeat();
   }

   /// 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;

   /// The timer for keeping track of task timeouts. This may be null.
   Timer _timeoutTimer;

   /// Creates a new schedule with empty task queues.
   Schedule() {
     _tasks = new TaskQueue._("tasks", this);
     _onComplete = new TaskQueue._("onComplete", this);
     _onException = new TaskQueue._("onException", this);
     _currentQueue = _tasks;

     heartbeat();
   }

   /// 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) {
         // Even though the scheduling failed, we need to run the onException and
         // onComplete queues, so we set the schedule state to RUNNING.
         _state = ScheduleState.RUNNING;
         throw new ScheduleError.from(this, e, stackTrace: stackTrace);
       }

       _state = ScheduleState.RUNNING;
       return tasks._run();
     }).catchError((e) {
       _addError(e);
       return onException._run().catchError((innerError) {
         // If an error occurs in a task in the onException queue, make sure it's
         // registered in the error list and re-throw it. We could also re-throw
         // `e`; ultimately, all the errors will be shown to the user if any
         // ScheduleError is thrown.
         _addError(innerError);
         throw innerError;
       }).then((_) {
         // If there are no errors in the onException queue, re-throw the
         // original error that caused it to run.
         throw e;
       });
     }).whenComplete(() {
       return onComplete._run().catchError((e) {
         // If an error occurs in a task in the onComplete queue, make sure it's
         // registered in the error list and re-throw it.
         _addError(e);
         throw e;
       });
     }).whenComplete(() {
       if (_timeoutTimer != null) _timeoutTimer.cancel();
       _state = ScheduleState.DONE;
     });
   }

   /// 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]) {
     heartbeat();

     var scheduleError = new ScheduleError.from(this, error,
         stackTrace: stackTrace);
     if (_state == ScheduleState.DONE) {
       throw new StateError(
           "An out-of-band error was signaled outside of wrapAsync after the "
               "schedule finished running.\n"
           "${errorString()}");
     } else if (state == ScheduleState.SET_UP) {
       // If we're setting up, throwing the error will pipe it into the main
       // error-handling code.
       throw scheduleError;
     } else {
       _currentQueue._signalError(scheduleError);
     }
   }

   /// Notifies the schedule of an error that occurred in a task or out-of-band
   /// callback after the appropriate queue has timed out. If this schedule is
   /// still running, the error will be added to the errors list to be shown
   /// along with the timeout error; otherwise, a top-level error will be thrown.
   void _signalPostTimeoutError(error, [stackTrace]) {
     var scheduleError = new ScheduleError.from(this, error,
         stackTrace: stackTrace);
     _addError(scheduleError);
     if (_state == ScheduleState.DONE) {
       throw new StateError(
         "An out-of-band error was caught after the test timed out.\n"
         "${errorString()}");
     }
   }

   /// 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 in test code.
   Function wrapAsync(fn(arg)) {
     if (_state == ScheduleState.DONE) {
       throw new StateError("wrapAsync called after the schedule has finished "
           "running.");
     }
     heartbeat();

     var queue = currentQueue;
     // It's possible that the queue timed out before this.
     bool _timedOut() => queue != currentQueue || _pendingCallbacks == 0;

     _pendingCallbacks++;
     return (arg) {
       try {
         return fn(arg);
       } catch (e, stackTrace) {
         if (_timedOut()) {
           _signalPostTimeoutError(e, stackTrace);
         } else {
           signalError(e, stackTrace);
         }
       } finally {
         if (_timedOut()) return;

         _pendingCallbacks--;
         if (_pendingCallbacks == 0 && _noPendingCallbacks != null) {
           _noPendingCallbacks.complete();
           _noPendingCallbacks = null;
         }
       }
     };
   }

   /// Like [wrapAsync], this ensures that the current task queue waits for
   /// out-of-band asynchronous code, and that errors raised in that code are
   /// handled correctly. However, [wrapFuture] wraps a [Future] chain rather
   /// than a single callback.
   ///
   /// The returned [Future] completes to the same value or error as [future].
   ///
   /// The top-level `wrapFuture` function should usually be used in preference
   /// to this in test code.
   Future wrapFuture(Future future) {
     var doneCallback = wrapAsync((_) => null);
     done() => new Future.immediate(null).then(doneCallback);

     future = future.then((result) {
       done();
       return result;
     }).catchError((e) {
       signalError(e);
       done();
       throw e;
     });

     // Don't top-level the error, since it's already been signaled to the
     // schedule.
     future.catchError((_) => null);

     return future;
   }

   /// Returns a string representation of all errors registered on this schedule.
   String errorString() {
     if (errors.isEmpty) return "The schedule had no errors.";
     if (errors.length == 1) return errors.first.toString();
     var errorStrings = errors.map((e) => e.toString()).join("\n================"
         "================================================================\n");
     return "The schedule had ${errors.length} errors:\n$errorStrings";
   }

   /// Notifies the schedule that progress is being made on an asynchronous task.
   /// This resets the timeout timer, and can be used in long-running tasks to
   /// keep them from timing out.
   void heartbeat() {
     if (_timeoutTimer != null) _timeoutTimer.cancel();
     if (_timeout == null) {
       _timeoutTimer = null;
     } else {
       _timeoutTimer = mock_clock.newTimer(_timeout, _signalTimeout);
     }
   }

   /// The callback to run when the timeout timer fires. Notifies the current
   /// queue that a timeout has occurred.
   void _signalTimeout(_) {
     // Reset the timer so that we can detect timeouts in the onException and
     // onComplete queues.
     _timeoutTimer = null;

     var error = new ScheduleError.from(this, "The schedule timed out after "
         "$_timeout of inactivity.");

     _pendingCallbacks = 0;
     if (_noPendingCallbacks != null) {
       var noPendingCallbacks = _noPendingCallbacks;
       _noPendingCallbacks = null;
       noPendingCallbacks.completeError(error);
     } else {
       currentQueue._signalTimeout(error);
     }
   }

   /// 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);
     if (_noPendingCallbacks == null) _noPendingCallbacks = new Completer();
     return _noPendingCallbacks.future;
   }

   /// Register an error in the schedule's error list. This ensures that there
   /// are no duplicate errors, and that all errors are wrapped in
   /// [ScheduleError].
   void _addError(error) {
     if (errors.contains(error)) return;
     errors.add(new ScheduleError.from(this, error));
   }
 }

 /// An enum of states for a [Schedule].
 class ScheduleState {
   /// The schedule can have tasks added to its queue, but is not yet running
   /// them.
   static const SET_UP = const ScheduleState._("SET_UP");

   /// The schedule is actively running tasks. This includes running tasks in
   /// [Schedule.onException] and [Schedule.onComplete].
   static const RUNNING = const ScheduleState._("RUNNING");

   /// The schedule has finished running all its tasks, either successfully or
   /// with an error.
   static const DONE = const ScheduleState._("DONE");

   /// The name of the state.
   final String name;

   const ScheduleState._(this.name);

   String toString() => name;
 }

 /// 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;

   /// The [SubstituteFuture] for the currently-running task in the queue, or
   /// null if no task is currently running.
   SubstituteFuture _taskFuture;

   TaskQueue._(this.name, this._schedule);

   /// Whether this queue is currently running.
   bool get isRunning => _schedule.state == ScheduleState.RUNNING &&
       _schedule.currentQueue == this;

   /// 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.
   ///
   /// If this is called when this queue is currently running, it will run [fn]
   /// on the next event loop iteration rather than adding it to a queue--this is
   /// known as a "nested task". The current task will not complete until [fn]
   /// (and any [Future] it returns) has finished running. Any errors in [fn]
   /// will automatically be handled. Nested tasks run in parallel, unlike
   /// top-level tasks which run in sequence.
   Future schedule(fn(), [String description]) {
     if (isRunning) {
       var task = _schedule.currentTask;
       var wrappedFn = () => _schedule.wrapFuture(
           new Future.immediate(null).then((_) => fn()));
       if (task == null) return wrappedFn();
       return task.runChild(wrappedFn, description);
     }

     var task = new Task(fn, description, this);
     _contents.add(task);
     return task.result;
   }

   /// Runs all the tasks in this queue in order.
   Future _run() {
     _schedule._currentQueue = this;
     _schedule.heartbeat();
     return Future.forEach(_contents, (task) {
       _schedule._currentTask = task;
       if (_error != null) throw _error;

       _taskFuture = new SubstituteFuture(task.fn());
       return _taskFuture.whenComplete(() {
         _taskFuture = null;
         _schedule.heartbeat();
       }).catchError((e) {
         if (_error != null) _schedule._addError(_error);
         throw new ScheduleError.from(_schedule, e);
       });
     }).whenComplete(() {
       _schedule._currentTask = null;
       return _schedule._awaitNoPendingCallbacks();
     }).then((_) {
       _schedule.heartbeat();
       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) {
     // If multiple errors are detected while a task is running, make sure the
     // earlier ones are recorded in the schedule.
     if (_error != null) _schedule._addError(_error);
     _error = error;
   }

   /// Notifies the queue that it has timed out and it needs to terminate
   /// immediately with a timeout error.
   void _signalTimeout(ScheduleError error) {
     if (_taskFuture != null) {
       // Catch errors coming off the old task future, in case it completes after
       // timing out.
       _taskFuture.substitute(new Future.immediateError(error)).catchError((e) {
         _schedule._signalPostTimeoutError(e);
       });
     } else {
       // This branch probably won't be reached, but it's conceivable that the
       // event loop might get pumped when _taskFuture is null but we haven't yet
       // called _awaitNoPendingCallbacks.
       _signalError(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");
   }
 }
	// 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 'mock_clock.dart' as mock_clock;
	import 'schedule_error.dart';
	import 'substitute_future.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 in [errors]. 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 in [errors]. Note that expectation failures count as
	/// errors.
	///
	/// This queue runs after [onException]. If an error occurs while running
	/// [onException], that error will be available in [errors] after 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;

	/// The current state of the schedule.
	ScheduleState get state => _state;
	ScheduleState _state = ScheduleState.SET_UP;

	// TODO(nweiz): make this a read-only view once issue 8321 is fixed.

	/// Errors thrown by the task queues.
	///
	/// When running tasks in [tasks], this will always be empty. If an error
	/// occurs in [tasks], it will be added to this list and then [onException]
	/// will be run. If an error occurs there as well, it will be added to this
	/// list and [onComplete] will be run. Errors thrown during [onComplete] will
	/// also be added to this list, although no scheduled tasks will be run
	/// afterwards.
	///
	/// Any out-of-band callbacks that throw errors will also have those errors
	/// added to this list.
	final errors = <ScheduleError>[];

	/// The task queue that's currently being run. One of [tasks], [onException],
	/// or [onComplete]. This starts as [tasks], and can only be `null` after the
	/// schedule is done.
	TaskQueue get currentQueue =>
	_state == ScheduleState.DONE ? null : _currentQueue;
	TaskQueue _currentQueue;

	/// The time to wait before terminating a task queue for inactivity. Defaults
	/// to 30 seconds. This can be set to `null` to disable timeouts entirely.
	///
	/// If a task queue times out, an error will be raised that can be handled as
	/// usual in the [onException] and [onComplete] queues. If [onException] times
	/// out, that can only be handled in [onComplete]; if [onComplete] times out,
	/// that cannot be handled.
	///
	/// If a task times out and then later completes with an error, that error
	/// cannot be handled. The user will still be notified of it.
	Duration get timeout => _timeout;
	Duration _timeout = new Duration(seconds: 30);
	set timeout(Duration duration) {
	_timeout = duration;
	heartbeat();
	}

	/// 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;

	/// The timer for keeping track of task timeouts. This may be null.
	Timer _timeoutTimer;

	/// Creates a new schedule with empty task queues.
	Schedule() {
	_tasks = new TaskQueue._("tasks", this);
	_onComplete = new TaskQueue._("onComplete", this);
	_onException = new TaskQueue._("onException", this);
	_currentQueue = _tasks;

	heartbeat();
	}

	/// 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) {
	// Even though the scheduling failed, we need to run the onException and
	// onComplete queues, so we set the schedule state to RUNNING.
	_state = ScheduleState.RUNNING;
	throw new ScheduleError.from(this, e, stackTrace: stackTrace);
	}

	_state = ScheduleState.RUNNING;
	return tasks._run();
	}).catchError((e) {
	_addError(e);
	return onException._run().catchError((innerError) {
	// If an error occurs in a task in the onException queue, make sure it's
	// registered in the error list and re-throw it. We could also re-throw
	// `e`; ultimately, all the errors will be shown to the user if any
	// ScheduleError is thrown.
	_addError(innerError);
	throw innerError;
	}).then((_) {
	// If there are no errors in the onException queue, re-throw the
	// original error that caused it to run.
	throw e;
	});
	}).whenComplete(() {
	return onComplete._run().catchError((e) {
	// If an error occurs in a task in the onComplete queue, make sure it's
	// registered in the error list and re-throw it.
	_addError(e);
	throw e;
	});
	}).whenComplete(() {
	if (_timeoutTimer != null) _timeoutTimer.cancel();
	_state = ScheduleState.DONE;
	});
	}

	/// 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]) {
	heartbeat();

	var scheduleError = new ScheduleError.from(this, error,
	stackTrace: stackTrace);
	if (_state == ScheduleState.DONE) {
	throw new StateError(
	"An out-of-band error was signaled outside of wrapAsync after the "
	"schedule finished running.\n"
	"${errorString()}");
	} else if (state == ScheduleState.SET_UP) {
	// If we're setting up, throwing the error will pipe it into the main
	// error-handling code.
	throw scheduleError;
	} else {
	_currentQueue._signalError(scheduleError);
	}
	}

	/// Notifies the schedule of an error that occurred in a task or out-of-band
	/// callback after the appropriate queue has timed out. If this schedule is
	/// still running, the error will be added to the errors list to be shown
	/// along with the timeout error; otherwise, a top-level error will be thrown.
	void _signalPostTimeoutError(error, [stackTrace]) {
	var scheduleError = new ScheduleError.from(this, error,
	stackTrace: stackTrace);
	_addError(scheduleError);
	if (_state == ScheduleState.DONE) {
	throw new StateError(
	"An out-of-band error was caught after the test timed out.\n"
	"${errorString()}");
	}
	}

	/// 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 in test code.
	Function wrapAsync(fn(arg)) {
	if (_state == ScheduleState.DONE) {
	throw new StateError("wrapAsync called after the schedule has finished "
	"running.");
	}
	heartbeat();

	var queue = currentQueue;
	// It's possible that the queue timed out before this.
	bool _timedOut() => queue != currentQueue \|\| _pendingCallbacks == 0;

	_pendingCallbacks++;
	return (arg) {
	try {
	return fn(arg);
	} catch (e, stackTrace) {
	if (_timedOut()) {
	_signalPostTimeoutError(e, stackTrace);
	} else {
	signalError(e, stackTrace);
	}
	} finally {
	if (_timedOut()) return;

	_pendingCallbacks--;
	if (_pendingCallbacks == 0 && _noPendingCallbacks != null) {
	_noPendingCallbacks.complete();
	_noPendingCallbacks = null;
	}
	}
	};
	}

	/// Like [wrapAsync], this ensures that the current task queue waits for
	/// out-of-band asynchronous code, and that errors raised in that code are
	/// handled correctly. However, [wrapFuture] wraps a [Future] chain rather
	/// than a single callback.
	///
	/// The returned [Future] completes to the same value or error as [future].
	///
	/// The top-level `wrapFuture` function should usually be used in preference
	/// to this in test code.
	Future wrapFuture(Future future) {
	var doneCallback = wrapAsync((_) => null);
	done() => new Future.immediate(null).then(doneCallback);

	future = future.then((result) {
	done();
	return result;
	}).catchError((e) {
	signalError(e);
	done();
	throw e;
	});

	// Don't top-level the error, since it's already been signaled to the
	// schedule.
	future.catchError((_) => null);

	return future;
	}

	/// Returns a string representation of all errors registered on this schedule.
	String errorString() {
	if (errors.isEmpty) return "The schedule had no errors.";
	if (errors.length == 1) return errors.first.toString();
	var errorStrings = errors.map((e) => e.toString()).join("\n================"
	"================================================================\n");
	return "The schedule had ${errors.length} errors:\n$errorStrings";
	}

	/// Notifies the schedule that progress is being made on an asynchronous task.
	/// This resets the timeout timer, and can be used in long-running tasks to
	/// keep them from timing out.
	void heartbeat() {
	if (_timeoutTimer != null) _timeoutTimer.cancel();
	if (_timeout == null) {
	_timeoutTimer = null;
	} else {
	_timeoutTimer = mock_clock.newTimer(_timeout, _signalTimeout);
	}
	}

	/// The callback to run when the timeout timer fires. Notifies the current
	/// queue that a timeout has occurred.
	void _signalTimeout(_) {
	// Reset the timer so that we can detect timeouts in the onException and
	// onComplete queues.
	_timeoutTimer = null;

	var error = new ScheduleError.from(this, "The schedule timed out after "
	"$_timeout of inactivity.");

	_pendingCallbacks = 0;
	if (_noPendingCallbacks != null) {
	var noPendingCallbacks = _noPendingCallbacks;
	_noPendingCallbacks = null;
	noPendingCallbacks.completeError(error);
	} else {
	currentQueue._signalTimeout(error);
	}
	}

	/// 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);
	if (_noPendingCallbacks == null) _noPendingCallbacks = new Completer();
	return _noPendingCallbacks.future;
	}

	/// Register an error in the schedule's error list. This ensures that there
	/// are no duplicate errors, and that all errors are wrapped in
	/// [ScheduleError].
	void _addError(error) {
	if (errors.contains(error)) return;
	errors.add(new ScheduleError.from(this, error));
	}
	}

	/// An enum of states for a [Schedule].
	class ScheduleState {
	/// The schedule can have tasks added to its queue, but is not yet running
	/// them.
	static const SET_UP = const ScheduleState._("SET_UP");

	/// The schedule is actively running tasks. This includes running tasks in
	/// [Schedule.onException] and [Schedule.onComplete].
	static const RUNNING = const ScheduleState._("RUNNING");

	/// The schedule has finished running all its tasks, either successfully or
	/// with an error.
	static const DONE = const ScheduleState._("DONE");

	/// The name of the state.
	final String name;

	const ScheduleState._(this.name);

	String toString() => name;
	}

	/// 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;

	/// The [SubstituteFuture] for the currently-running task in the queue, or
	/// null if no task is currently running.
	SubstituteFuture _taskFuture;

	TaskQueue._(this.name, this._schedule);

	/// Whether this queue is currently running.
	bool get isRunning => _schedule.state == ScheduleState.RUNNING &&
	_schedule.currentQueue == this;

	/// 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.
	///
	/// If this is called when this queue is currently running, it will run [fn]
	/// on the next event loop iteration rather than adding it to a queue--this is
	/// known as a "nested task". The current task will not complete until [fn]
	/// (and any [Future] it returns) has finished running. Any errors in [fn]
	/// will automatically be handled. Nested tasks run in parallel, unlike
	/// top-level tasks which run in sequence.
	Future schedule(fn(), [String description]) {
	if (isRunning) {
	var task = _schedule.currentTask;
	var wrappedFn = () => _schedule.wrapFuture(
	new Future.immediate(null).then((_) => fn()));
	if (task == null) return wrappedFn();
	return task.runChild(wrappedFn, description);
	}

	var task = new Task(fn, description, this);
	_contents.add(task);
	return task.result;
	}

	/// Runs all the tasks in this queue in order.
	Future _run() {
	_schedule._currentQueue = this;
	_schedule.heartbeat();
	return Future.forEach(_contents, (task) {
	_schedule._currentTask = task;
	if (_error != null) throw _error;

	_taskFuture = new SubstituteFuture(task.fn());
	return _taskFuture.whenComplete(() {
	_taskFuture = null;
	_schedule.heartbeat();
	}).catchError((e) {
	if (_error != null) _schedule._addError(_error);
	throw new ScheduleError.from(_schedule, e);
	});
	}).whenComplete(() {
	_schedule._currentTask = null;
	return _schedule._awaitNoPendingCallbacks();
	}).then((_) {
	_schedule.heartbeat();
	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) {
	// If multiple errors are detected while a task is running, make sure the
	// earlier ones are recorded in the schedule.
	if (_error != null) _schedule._addError(_error);
	_error = error;
	}

	/// Notifies the queue that it has timed out and it needs to terminate
	/// immediately with a timeout error.
	void _signalTimeout(ScheduleError error) {
	if (_taskFuture != null) {
	// Catch errors coming off the old task future, in case it completes after
	// timing out.
	_taskFuture.substitute(new Future.immediateError(error)).catchError((e) {
	_schedule._signalPostTimeoutError(e);
	});
	} else {
	// This branch probably won't be reached, but it's conceivable that the
	// event loop might get pumped when _taskFuture is null but we haven't yet
	// called _awaitNoPendingCallbacks.
	_signalError(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");
	}
	}