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/// NOTE: This is a toy task interceptor, not one you'd use in a real app!
import 'dart:async';
import 'dart:collection';
/// A task scheduled with a timer.
class _TimerTask extends LinkedListEntry {
int scheduledAtTime;
Function task;
_TimerTask(this.scheduledAtTime, this.task);
/// Our implementation of the Timer interface.
class _Timer implements Timer {
bool _isActive = true;
bool get isActive => _isActive;
void cancel() {
_isActive = false;
/// The class that simulates Dart's event-loop and timer behavior.
class EventLoop {
/// Queue of micro tasks.
final _microtasks = new Queue();
/// Sorted linked list _TimerTasks.
final LinkedList<_TimerTask> _timerTasks = new LinkedList<_TimerTask>();
/// Simulate the current time in milliseconds.
int _currentTime = 0;
/// Boolean to make sure that this instance is only used once
/// for forking a new zone.
bool _forked = false;
/// Pops the next task to execute.
Function _popNextTask() {
// If there are still microtasks to do, execute them.
if (_microtasks.isNotEmpty) return _microtasks.removeFirst();
// Otherwise execute the first timer task (and update the
// current time).
if (_timerTasks.isEmpty) return null;
_TimerTask timerTask = _timerTasks.first;
_currentTime = timerTask.scheduledAtTime;
return timerTask.task;
/// The simulated scheduleMicrotask simply adds the task to the
/// end of the queue.
void _scheduleMicrotask(Zone self, ZoneDelegate parent, Zone zone,
Function microtask) {
// Note that the microtask is bound to the origin-zone.
_microtasks.add(zone.bindCallback(microtask, runGuarded: true));
/// Adds the given [task] at the correct position of the timer list.
// TODO: the performance could be improved by using a priority queue.
void _scheduleIn(Duration duration, Function task) {
int scheduledAtTime = _currentTime + duration.inMilliseconds;
var newTimerTask = new _TimerTask(scheduledAtTime, task);
// Increment the currentTime, so that scheduling multiple events
// in the same event will be scheduled in the right order.
if (_timerTasks.isEmpty) {
var timerTask = _timerTasks.first;
while (timerTask.scheduledAtTime < scheduledAtTime) {
timerTask =;
if (timerTask == null) {
/// Creates a new Timer for the simulated event loop.
Timer _createTimer(Zone self, ZoneDelegate parent, Zone zone,
Duration duration, Function callback) {
_Timer result = new _Timer();
void wrappedCallback() {
if (result.isActive) {
result._isActive = false;
// The callback is again executed in the origin zone.
_scheduleIn(duration, wrappedCallback);
Timer _createPeriodicTimer(Zone self, ZoneDelegate parent, Zone zone,
Duration duration, Function callback) {
throw new UnimplementedError('createPeriodicTimer');
void forkCurrentAndRun(void f()) {
if (_forked) throw new StateError('Can only fork once');
_forked = true;
// Creates a specification that overrides the microtask and
// timer operations.
var specification = new ZoneSpecification(
scheduleMicrotask: _scheduleMicrotask,
createTimer: _createTimer,
createPeriodicTimer: _createPeriodicTimer);
Zone zone = Zone.current.fork(specification: specification);
// Schedule the given function as if it was a microtask.
// Run the event loop to completion.
while (true) {
Function nextTask = _popNextTask();
if (nextTask == null) return;
// =========== Code to run and test the ZoneSpecification =========
void tests() { {
scheduleMicrotask(() {
new Timer(new Duration(milliseconds: 5), () {
print('t2 5ms');
new Timer(new Duration(milliseconds: 3), () {
print('t2 3ms');
throw 't2 3ms';
scheduleMicrotask(() {
scheduleMicrotask(() { print('1'); });
void main() {
runZoned(() {
var eventLoop = new EventLoop();
}, onError: print);