sdk/lib/_internal/vm/lib/timer_impl.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.

 // part of "isolate_patch.dart";

 // Timer heap implemented as a array-based binary heap[0].
 // This allows for O(1) `first`, O(log(n)) `remove`/`removeFirst` and O(log(n))
 // `add`.
 //
 // To ensure the timers are ordered by insertion time, the _Timer class has a
 // `_id` field set when added to the heap.
 //
 // [0] http://en.wikipedia.org/wiki/Binary_heap
 class _TimerHeap {
   List<_Timer> _list;
   int _used = 0;

   _TimerHeap([int initSize = 7])
       : _list = List<_Timer>.filled(initSize, _Timer._sentinelTimer);

   bool get isEmpty => _used == 0;

   _Timer get first => _list[0];

   bool isFirst(_Timer timer) => timer._indexOrNext == 0;

   void add(_Timer timer) {
     if (_used == _list.length) {
       _resize();
     }
     var index = _used++;
     timer._indexOrNext = index;
     _list[index] = timer;
     _bubbleUp(timer);
   }

   _Timer removeFirst() {
     var f = first;
     remove(f);
     return f;
   }

   void remove(_Timer timer) {
     _used--;
     if (isEmpty) {
       _list[0] = _Timer._sentinelTimer;
       timer._indexOrNext = null;
       return;
     }
     var last = _list[_used];
     if (!identical(last, timer)) {
       var index = timer._indexOrNext as int;
       last._indexOrNext = index;
       _list[index] = last;
       if (last._compareTo(timer) < 0) {
         _bubbleUp(last);
       } else {
         _bubbleDown(last);
       }
     }
     _list[_used] = _Timer._sentinelTimer;
     timer._indexOrNext = null;
   }

   void _resize() {
     var newList =
         List<_Timer>.filled(_list.length * 2 + 1, _Timer._sentinelTimer);
     newList.setRange(0, _used, _list);
     _list = newList;
   }

   void _bubbleUp(_Timer timer) {
     while (!isFirst(timer)) {
       _Timer parent = _parent(timer);
       if (timer._compareTo(parent) < 0) {
         _swap(timer, parent);
       } else {
         break;
       }
     }
   }

   void _bubbleDown(_Timer timer) {
     while (true) {
       var leftIndex = _leftChildIndex(timer._indexOrNext as int);
       var rightIndex = _rightChildIndex(timer._indexOrNext as int);
       _Timer newest = timer;
       if (leftIndex < _used && _list[leftIndex]._compareTo(newest) < 0) {
         newest = _list[leftIndex];
       }
       if (rightIndex < _used && _list[rightIndex]._compareTo(newest) < 0) {
         newest = _list[rightIndex];
       }
       if (identical(newest, timer)) {
         // We are where we should be, break.
         break;
       }
       _swap(newest, timer);
     }
   }

   void _swap(_Timer first, _Timer second) {
     var newFirstIndex = second._indexOrNext as int;
     var newSecondIndex = first._indexOrNext as int;
     first._indexOrNext = newFirstIndex;
     second._indexOrNext = newSecondIndex;
     _list[newFirstIndex] = first;
     _list[newSecondIndex] = second;
   }

   _Timer _parent(_Timer timer) =>
       _list[_parentIndex(timer._indexOrNext as int)];

   static int _parentIndex(int index) => (index - 1) ~/ 2;
   static int _leftChildIndex(int index) => 2 * index + 1;
   static int _rightChildIndex(int index) => 2 * index + 2;
 }

 class _Timer implements Timer {
   // Cancels the timer in the event handler.
   static const _NO_TIMER = -1;

   // A generic null timer object that is used to populate unused slots
   // in TimerHeap.
   static final _sentinelTimer = _Timer._sentinel();

   // We distinguish what kind of message arrived based on the value being sent.
   static const _ZERO_EVENT = 1;
   static const _TIMEOUT_EVENT = null;

   // Timers are ordered by wakeup time. Timers with a timeout value of > 0 do
   // end up on the TimerHeap. Timers with a timeout of 0 are queued in a list.
   static final _heap = new _TimerHeap();
   static _Timer? _firstZeroTimer;
   static _Timer _lastZeroTimer = _sentinelTimer;

   // We use an id to be able to sort timers with the same expiration time.
   // ids are recycled after ID_MASK enqueues or when the timer queue is empty.
   static const _ID_MASK = 0x1fffffff;
   static int _idCount = 0;

   static _RawReceivePortImpl? _receivePort;
   static SendPort? _sendPort;
   static bool _receivePortActive = false;
   static int _scheduledWakeupTime = 0;

   static bool _handlingCallbacks = false;

   void Function(Timer)?
       _callback; // Closure to call when timer fires. null if canceled.
   int _wakeupTime; // Expiration time.
   final int _milliSeconds; // Duration specified at creation.
   final bool _repeating; // Indicates periodic timers.
   Object? _indexOrNext; // Index if part of the TimerHeap, link otherwise.
   int _id; // Incrementing id to enable sorting of timers with same expiry.

   int _tick = 0; // Backing for [tick],

   // Get the next available id. We accept collisions and reordering when the
   // _idCount overflows and the timers expire at the same millisecond.
   static int _nextId() {
     var result = _idCount;
     _idCount = (_idCount + 1) & _ID_MASK;
     return result;
   }

   _Timer._sentinel()
       : _callback = null,
         _wakeupTime = 0,
         _milliSeconds = 0,
         _repeating = false,
         _indexOrNext = null,
         _id = -1;

   _Timer._internal(
       this._callback, this._wakeupTime, this._milliSeconds, this._repeating)
       : _id = _nextId();

   static _Timer _createTimer(
       void callback(Timer timer), int milliSeconds, bool repeating) {
     // Negative timeouts are treated as if 0 timeout.
     if (milliSeconds < 0) {
       milliSeconds = 0;
     }
     // Add one because DateTime.now() is assumed to round down
     // to nearest millisecond, not up, so that time + duration is before
     // duration milliseconds from now. Using microsecond timers like
     // Stopwatch allows detecting that the timer fires early.
     int now = VMLibraryHooks.timerMillisecondClock();
     int wakeupTime = (milliSeconds == 0) ? now : (now + 1 + milliSeconds);

     _Timer timer =
         new _Timer._internal(callback, wakeupTime, milliSeconds, repeating);
     // Enqueue this newly created timer in the appropriate structure and
     // notify if necessary.
     timer._enqueue();
     return timer;
   }

   factory _Timer(int milliSeconds, void callback(Timer timer)) {
     return _createTimer(callback, milliSeconds, false);
   }

   factory _Timer.periodic(int milliSeconds, void callback(Timer timer)) {
     return _createTimer(callback, milliSeconds, true);
   }

   bool get _isInHeap => _indexOrNext is int;

   int _compareTo(_Timer other) {
     int c = _wakeupTime - other._wakeupTime;
     if (c != 0) return c;
     return _id - other._id;
   }

   bool get isActive => _callback != null;

   int get tick => _tick;

   // Cancels a set timer. The timer is removed from the timer heap if it is a
   // non-zero timer. Zero timers are kept in the list as they need to consume
   // the corresponding pending message.
   void cancel() {
     _callback = null;
     // Only heap timers are really removed. Zero timers need to consume their
     // corresponding wakeup message so they are left in the queue.
     if (!_isInHeap) return;
     bool update = _heap.isFirst(this);
     _heap.remove(this);
     if (update) {
       _notifyEventHandler();
     }
   }

   void _advanceWakeupTime() {
     // Recalculate the next wakeup time. For repeating timers with a 0 timeout
     // the next wakeup time is now.
     _id = _nextId();
     if (_milliSeconds > 0) {
       _wakeupTime += _milliSeconds;
     } else {
       _wakeupTime = VMLibraryHooks.timerMillisecondClock();
     }
   }

   // Adds a timer to the heap or timer list. Timers with the same wakeup time
   // are enqueued in order and notified in FIFO order.
   void _enqueue() {
     if (_milliSeconds == 0) {
       if (_firstZeroTimer == null) {
         _lastZeroTimer = this;
         _firstZeroTimer = this;
       } else {
         _lastZeroTimer._indexOrNext = this;
         _lastZeroTimer = this;
       }
       // Every zero timer gets its own event.
       _notifyZeroHandler();
     } else {
       _heap.add(this);
       if (_heap.isFirst(this)) {
         _notifyEventHandler();
       }
     }
   }

   // Enqueue one message for each zero timer. To be able to distinguish from
   // EventHandler messages we send a _ZERO_EVENT instead of a _TIMEOUT_EVENT.
   static void _notifyZeroHandler() {
     if (!_receivePortActive) {
       _createTimerHandler();
     }
     _sendPort!.send(_ZERO_EVENT);
   }

   // Handle the notification of a zero timer. Make sure to also execute non-zero
   // timers with a lower expiration time.
   static List<_Timer> _queueFromZeroEvent() {
     var pendingTimers = <_Timer>[];
     final firstTimer = _firstZeroTimer;
     if (firstTimer != null) {
       // Collect pending timers from the timer heap that have an expiration prior
       // to the currently notified zero timer.
       _Timer timer;
       while (!_heap.isEmpty && (_heap.first._compareTo(firstTimer) < 0)) {
         timer = _heap.removeFirst();
         pendingTimers.add(timer);
       }
       // Append the first zero timer to the pending timers.
       timer = firstTimer;
       _firstZeroTimer = timer._indexOrNext as _Timer?;
       timer._indexOrNext = null;
       pendingTimers.add(timer);
     }
     return pendingTimers;
   }

   static void _notifyEventHandler() {
     if (_handlingCallbacks) {
       // While we are already handling callbacks we will not notify the event
       // handler. _handleTimeout will call _notifyEventHandler once all pending
       // timers are processed.
       return;
     }

     // If there are no pending timers. Close down the receive port.
     if ((_firstZeroTimer == null) && _heap.isEmpty) {
       // No pending timers: Close the receive port and let the event handler
       // know.
       if (_sendPort != null) {
         _cancelWakeup();
         _shutdownTimerHandler();
       }
       return;
     } else if (_heap.isEmpty) {
       // Only zero timers are left. Cancel any scheduled wakeups.
       _cancelWakeup();
       return;
     }
     // Only send a message if the requested wakeup time differs from the
     // already scheduled wakeup time.
     var wakeupTime = _heap.first._wakeupTime;
     if ((_scheduledWakeupTime == 0) || (wakeupTime != _scheduledWakeupTime)) {
       _scheduleWakeup(wakeupTime);
     }
   }

   static List<_Timer> _queueFromTimeoutEvent() {
     var pendingTimers = <_Timer>[];
     final firstTimer = _firstZeroTimer;
     if (firstTimer != null) {
       // Collect pending timers from the timer heap that have an expiration
       // prior to the next zero timer.
       // By definition the first zero timer has been scheduled before the
       // current time, meaning all timers which are "less than" the first zero
       // timer are expired. The first zero timer will be dispatched when its
       // corresponding message is delivered.
       while (!_heap.isEmpty && (_heap.first._compareTo(firstTimer) < 0)) {
         var timer = _heap.removeFirst();
         pendingTimers.add(timer);
       }
     } else {
       // Collect pending timers from the timer heap which have expired at this
       // time.
       var currentTime = VMLibraryHooks.timerMillisecondClock();
       while (!_heap.isEmpty && (_heap.first._wakeupTime <= currentTime)) {
         var timer = _heap.removeFirst();
         pendingTimers.add(timer);
       }
     }
     return pendingTimers;
   }

   static void _runTimers(List<_Timer> pendingTimers) {
     // If there are no pending timers currently reset the id space before we
     // have a chance to enqueue new timers.
     if (_heap.isEmpty && (_firstZeroTimer == null)) {
       _idCount = 0;
     }

     // Fast exit if no pending timers.
     if (pendingTimers.length == 0) {
       return;
     }

     // Trigger all of the pending timers. New timers added as part of the
     // callbacks will be enqueued now and notified in the next spin at the
     // earliest.
     _handlingCallbacks = true;
     var i = 0;
     try {
       for (; i < pendingTimers.length; i++) {
         // Next pending timer.
         var timer = pendingTimers[i];
         timer._indexOrNext = null;

         // One of the timers in the pending_timers list can cancel
         // one of the later timers which will set the callback to
         // null. Or the pending zero timer has been canceled earlier.
         var callback = timer._callback;
         if (callback != null) {
           if (!timer._repeating) {
             // Mark timer as inactive.
             timer._callback = null;
           } else if (timer._milliSeconds > 0) {
             var ms = timer._milliSeconds;
             int overdue =
                 VMLibraryHooks.timerMillisecondClock() - timer._wakeupTime;
             if (overdue > ms) {
               int missedTicks = overdue ~/ ms;
               timer._wakeupTime += missedTicks * ms;
               timer._tick += missedTicks;
             }
           }
           timer._tick += 1;

           callback(timer);
           // Re-insert repeating timer if not canceled.
           if (timer._repeating && (timer._callback != null)) {
             timer._advanceWakeupTime();
             timer._enqueue();
           }
           // Execute pending micro tasks.
           _runPendingImmediateCallback();
         }
       }
     } finally {
       _handlingCallbacks = false;
       // Re-queue timers we didn't get to.
       for (i++; i < pendingTimers.length; i++) {
         var timer = pendingTimers[i];
         timer._enqueue();
       }
       _notifyEventHandler();
     }
   }

   static void _handleMessage(msg) {
     List<_Timer> pendingTimers;
     if (msg == _ZERO_EVENT) {
       pendingTimers = _queueFromZeroEvent();
       assert(pendingTimers.length > 0);
     } else {
       assert(msg == _TIMEOUT_EVENT);
       _scheduledWakeupTime = 0; // Consumed the last scheduled wakeup now.
       pendingTimers = _queueFromTimeoutEvent();
     }
     _runTimers(pendingTimers);
     // Notify the event handler or shutdown the port if no more pending
     // timers are present.
     _notifyEventHandler();
   }

   // Tell the event handler to wake this isolate at a specific time.
   static void _scheduleWakeup(int wakeupTime) {
     if (!_receivePortActive) {
       _createTimerHandler();
     }
     VMLibraryHooks.eventHandlerSendData(null, _sendPort!, wakeupTime);
     _scheduledWakeupTime = wakeupTime;
   }

   // Cancel pending wakeups in the event handler.
   static void _cancelWakeup() {
     if (_sendPort != null) {
       VMLibraryHooks.eventHandlerSendData(null, _sendPort!, _NO_TIMER);
       _scheduledWakeupTime = 0;
     }
   }

   // Create a receive port and register a message handler for the timer
   // events.
   static void _createTimerHandler() {
     var receivePort = _receivePort;
     if (receivePort == null) {
       assert(_sendPort == null);
       final port = _RawReceivePortImpl('Timer');
       port.handler = _handleMessage;
       _sendPort = port.sendPort;
       _receivePort = port;
       _scheduledWakeupTime = 0;
     } else {
       receivePort._setActive(true);
     }
     _receivePortActive = true;
   }

   static void _shutdownTimerHandler() {
     _scheduledWakeupTime = 0;
     _receivePort!._setActive(false);
     _receivePortActive = false;
   }

   // The Timer factory registered with the dart:async library by the embedder.
   static Timer _factory(
       int milliSeconds, void callback(Timer timer), bool repeating) {
     if (repeating) {
       return new _Timer.periodic(milliSeconds, callback);
     }
     return new _Timer(milliSeconds, callback);
   }
 }

 @pragma("vm:entry-point", "call")
 _setupHooks() {
   VMLibraryHooks.timerFactory = _Timer._factory;
 }
	// 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.

	// part of "isolate_patch.dart";

	// Timer heap implemented as a array-based binary heap[0].
	// This allows for O(1) `first`, O(log(n)) `remove`/`removeFirst` and O(log(n))
	// `add`.
	//
	// To ensure the timers are ordered by insertion time, the _Timer class has a
	// `_id` field set when added to the heap.
	//
	// [0] http://en.wikipedia.org/wiki/Binary_heap
	class _TimerHeap {
	List<_Timer> _list;
	int _used = 0;

	_TimerHeap([int initSize = 7])
	: _list = List<_Timer>.filled(initSize, _Timer._sentinelTimer);

	bool get isEmpty => _used == 0;

	_Timer get first => _list[0];

	bool isFirst(_Timer timer) => timer._indexOrNext == 0;

	void add(_Timer timer) {
	if (_used == _list.length) {
	_resize();
	}
	var index = _used++;
	timer._indexOrNext = index;
	_list[index] = timer;
	_bubbleUp(timer);
	}

	_Timer removeFirst() {
	var f = first;
	remove(f);
	return f;
	}

	void remove(_Timer timer) {
	_used--;
	if (isEmpty) {
	_list[0] = _Timer._sentinelTimer;
	timer._indexOrNext = null;
	return;
	}
	var last = _list[_used];
	if (!identical(last, timer)) {
	var index = timer._indexOrNext as int;
	last._indexOrNext = index;
	_list[index] = last;
	if (last._compareTo(timer) < 0) {
	_bubbleUp(last);
	} else {
	_bubbleDown(last);
	}
	}
	_list[_used] = _Timer._sentinelTimer;
	timer._indexOrNext = null;
	}

	void _resize() {
	var newList =
	List<_Timer>.filled(_list.length * 2 + 1, _Timer._sentinelTimer);
	newList.setRange(0, _used, _list);
	_list = newList;
	}

	void _bubbleUp(_Timer timer) {
	while (!isFirst(timer)) {
	_Timer parent = _parent(timer);
	if (timer._compareTo(parent) < 0) {
	_swap(timer, parent);
	} else {
	break;
	}
	}
	}

	void _bubbleDown(_Timer timer) {
	while (true) {
	var leftIndex = _leftChildIndex(timer._indexOrNext as int);
	var rightIndex = _rightChildIndex(timer._indexOrNext as int);
	_Timer newest = timer;
	if (leftIndex < _used && _list[leftIndex]._compareTo(newest) < 0) {
	newest = _list[leftIndex];
	}
	if (rightIndex < _used && _list[rightIndex]._compareTo(newest) < 0) {
	newest = _list[rightIndex];
	}
	if (identical(newest, timer)) {
	// We are where we should be, break.
	break;
	}
	_swap(newest, timer);
	}
	}

	void _swap(_Timer first, _Timer second) {
	var newFirstIndex = second._indexOrNext as int;
	var newSecondIndex = first._indexOrNext as int;
	first._indexOrNext = newFirstIndex;
	second._indexOrNext = newSecondIndex;
	_list[newFirstIndex] = first;
	_list[newSecondIndex] = second;
	}

	_Timer _parent(_Timer timer) =>
	_list[_parentIndex(timer._indexOrNext as int)];

	static int _parentIndex(int index) => (index - 1) ~/ 2;
	static int _leftChildIndex(int index) => 2 * index + 1;
	static int _rightChildIndex(int index) => 2 * index + 2;
	}

	class _Timer implements Timer {
	// Cancels the timer in the event handler.
	static const _NO_TIMER = -1;

	// A generic null timer object that is used to populate unused slots
	// in TimerHeap.
	static final _sentinelTimer = _Timer._sentinel();

	// We distinguish what kind of message arrived based on the value being sent.
	static const _ZERO_EVENT = 1;
	static const _TIMEOUT_EVENT = null;

	// Timers are ordered by wakeup time. Timers with a timeout value of > 0 do
	// end up on the TimerHeap. Timers with a timeout of 0 are queued in a list.
	static final _heap = new _TimerHeap();
	static _Timer? _firstZeroTimer;
	static _Timer _lastZeroTimer = _sentinelTimer;

	// We use an id to be able to sort timers with the same expiration time.
	// ids are recycled after ID_MASK enqueues or when the timer queue is empty.
	static const _ID_MASK = 0x1fffffff;
	static int _idCount = 0;

	static _RawReceivePortImpl? _receivePort;
	static SendPort? _sendPort;
	static bool _receivePortActive = false;
	static int _scheduledWakeupTime = 0;

	static bool _handlingCallbacks = false;

	void Function(Timer)?
	_callback; // Closure to call when timer fires. null if canceled.
	int _wakeupTime; // Expiration time.
	final int _milliSeconds; // Duration specified at creation.
	final bool _repeating; // Indicates periodic timers.
	Object? _indexOrNext; // Index if part of the TimerHeap, link otherwise.
	int _id; // Incrementing id to enable sorting of timers with same expiry.

	int _tick = 0; // Backing for [tick],

	// Get the next available id. We accept collisions and reordering when the
	// _idCount overflows and the timers expire at the same millisecond.
	static int _nextId() {
	var result = _idCount;
	_idCount = (_idCount + 1) & _ID_MASK;
	return result;
	}

	_Timer._sentinel()
	: _callback = null,
	_wakeupTime = 0,
	_milliSeconds = 0,
	_repeating = false,
	_indexOrNext = null,
	_id = -1;

	_Timer._internal(
	this._callback, this._wakeupTime, this._milliSeconds, this._repeating)
	: _id = _nextId();

	static _Timer _createTimer(
	void callback(Timer timer), int milliSeconds, bool repeating) {
	// Negative timeouts are treated as if 0 timeout.
	if (milliSeconds < 0) {
	milliSeconds = 0;
	}
	// Add one because DateTime.now() is assumed to round down
	// to nearest millisecond, not up, so that time + duration is before
	// duration milliseconds from now. Using microsecond timers like
	// Stopwatch allows detecting that the timer fires early.
	int now = VMLibraryHooks.timerMillisecondClock();
	int wakeupTime = (milliSeconds == 0) ? now : (now + 1 + milliSeconds);

	_Timer timer =
	new _Timer._internal(callback, wakeupTime, milliSeconds, repeating);
	// Enqueue this newly created timer in the appropriate structure and
	// notify if necessary.
	timer._enqueue();
	return timer;
	}

	factory _Timer(int milliSeconds, void callback(Timer timer)) {
	return _createTimer(callback, milliSeconds, false);
	}

	factory _Timer.periodic(int milliSeconds, void callback(Timer timer)) {
	return _createTimer(callback, milliSeconds, true);
	}

	bool get _isInHeap => _indexOrNext is int;

	int _compareTo(_Timer other) {
	int c = _wakeupTime - other._wakeupTime;
	if (c != 0) return c;
	return _id - other._id;
	}

	bool get isActive => _callback != null;

	int get tick => _tick;

	// Cancels a set timer. The timer is removed from the timer heap if it is a
	// non-zero timer. Zero timers are kept in the list as they need to consume
	// the corresponding pending message.
	void cancel() {
	_callback = null;
	// Only heap timers are really removed. Zero timers need to consume their
	// corresponding wakeup message so they are left in the queue.
	if (!_isInHeap) return;
	bool update = _heap.isFirst(this);
	_heap.remove(this);
	if (update) {
	_notifyEventHandler();
	}
	}

	void _advanceWakeupTime() {
	// Recalculate the next wakeup time. For repeating timers with a 0 timeout
	// the next wakeup time is now.
	_id = _nextId();
	if (_milliSeconds > 0) {
	_wakeupTime += _milliSeconds;
	} else {
	_wakeupTime = VMLibraryHooks.timerMillisecondClock();
	}
	}

	// Adds a timer to the heap or timer list. Timers with the same wakeup time
	// are enqueued in order and notified in FIFO order.
	void _enqueue() {
	if (_milliSeconds == 0) {
	if (_firstZeroTimer == null) {
	_lastZeroTimer = this;
	_firstZeroTimer = this;
	} else {
	_lastZeroTimer._indexOrNext = this;
	_lastZeroTimer = this;
	}
	// Every zero timer gets its own event.
	_notifyZeroHandler();
	} else {
	_heap.add(this);
	if (_heap.isFirst(this)) {
	_notifyEventHandler();
	}
	}
	}

	// Enqueue one message for each zero timer. To be able to distinguish from
	// EventHandler messages we send a _ZERO_EVENT instead of a _TIMEOUT_EVENT.
	static void _notifyZeroHandler() {
	if (!_receivePortActive) {
	_createTimerHandler();
	}
	_sendPort!.send(_ZERO_EVENT);
	}

	// Handle the notification of a zero timer. Make sure to also execute non-zero
	// timers with a lower expiration time.
	static List<_Timer> _queueFromZeroEvent() {
	var pendingTimers = <_Timer>[];
	final firstTimer = _firstZeroTimer;
	if (firstTimer != null) {
	// Collect pending timers from the timer heap that have an expiration prior
	// to the currently notified zero timer.
	_Timer timer;
	while (!_heap.isEmpty && (_heap.first._compareTo(firstTimer) < 0)) {
	timer = _heap.removeFirst();
	pendingTimers.add(timer);
	}
	// Append the first zero timer to the pending timers.
	timer = firstTimer;
	_firstZeroTimer = timer._indexOrNext as _Timer?;
	timer._indexOrNext = null;
	pendingTimers.add(timer);
	}
	return pendingTimers;
	}

	static void _notifyEventHandler() {
	if (_handlingCallbacks) {
	// While we are already handling callbacks we will not notify the event
	// handler. _handleTimeout will call _notifyEventHandler once all pending
	// timers are processed.
	return;
	}

	// If there are no pending timers. Close down the receive port.
	if ((_firstZeroTimer == null) && _heap.isEmpty) {
	// No pending timers: Close the receive port and let the event handler
	// know.
	if (_sendPort != null) {
	_cancelWakeup();
	_shutdownTimerHandler();
	}
	return;
	} else if (_heap.isEmpty) {
	// Only zero timers are left. Cancel any scheduled wakeups.
	_cancelWakeup();
	return;
	}
	// Only send a message if the requested wakeup time differs from the
	// already scheduled wakeup time.
	var wakeupTime = _heap.first._wakeupTime;
	if ((_scheduledWakeupTime == 0) \|\| (wakeupTime != _scheduledWakeupTime)) {
	_scheduleWakeup(wakeupTime);
	}
	}

	static List<_Timer> _queueFromTimeoutEvent() {
	var pendingTimers = <_Timer>[];
	final firstTimer = _firstZeroTimer;
	if (firstTimer != null) {
	// Collect pending timers from the timer heap that have an expiration
	// prior to the next zero timer.
	// By definition the first zero timer has been scheduled before the
	// current time, meaning all timers which are "less than" the first zero
	// timer are expired. The first zero timer will be dispatched when its
	// corresponding message is delivered.
	while (!_heap.isEmpty && (_heap.first._compareTo(firstTimer) < 0)) {
	var timer = _heap.removeFirst();
	pendingTimers.add(timer);
	}
	} else {
	// Collect pending timers from the timer heap which have expired at this
	// time.
	var currentTime = VMLibraryHooks.timerMillisecondClock();
	while (!_heap.isEmpty && (_heap.first._wakeupTime <= currentTime)) {
	var timer = _heap.removeFirst();
	pendingTimers.add(timer);
	}
	}
	return pendingTimers;
	}

	static void _runTimers(List<_Timer> pendingTimers) {
	// If there are no pending timers currently reset the id space before we
	// have a chance to enqueue new timers.
	if (_heap.isEmpty && (_firstZeroTimer == null)) {
	_idCount = 0;
	}

	// Fast exit if no pending timers.
	if (pendingTimers.length == 0) {
	return;
	}

	// Trigger all of the pending timers. New timers added as part of the
	// callbacks will be enqueued now and notified in the next spin at the
	// earliest.
	_handlingCallbacks = true;
	var i = 0;
	try {
	for (; i < pendingTimers.length; i++) {
	// Next pending timer.
	var timer = pendingTimers[i];
	timer._indexOrNext = null;

	// One of the timers in the pending_timers list can cancel
	// one of the later timers which will set the callback to
	// null. Or the pending zero timer has been canceled earlier.
	var callback = timer._callback;
	if (callback != null) {
	if (!timer._repeating) {
	// Mark timer as inactive.
	timer._callback = null;
	} else if (timer._milliSeconds > 0) {
	var ms = timer._milliSeconds;
	int overdue =
	VMLibraryHooks.timerMillisecondClock() - timer._wakeupTime;
	if (overdue > ms) {
	int missedTicks = overdue ~/ ms;
	timer._wakeupTime += missedTicks * ms;
	timer._tick += missedTicks;
	}
	}
	timer._tick += 1;

	callback(timer);
	// Re-insert repeating timer if not canceled.
	if (timer._repeating && (timer._callback != null)) {
	timer._advanceWakeupTime();
	timer._enqueue();
	}
	// Execute pending micro tasks.
	_runPendingImmediateCallback();
	}
	}
	} finally {
	_handlingCallbacks = false;
	// Re-queue timers we didn't get to.
	for (i++; i < pendingTimers.length; i++) {
	var timer = pendingTimers[i];
	timer._enqueue();
	}
	_notifyEventHandler();
	}
	}

	static void _handleMessage(msg) {
	List<_Timer> pendingTimers;
	if (msg == _ZERO_EVENT) {
	pendingTimers = _queueFromZeroEvent();
	assert(pendingTimers.length > 0);
	} else {
	assert(msg == _TIMEOUT_EVENT);
	_scheduledWakeupTime = 0; // Consumed the last scheduled wakeup now.
	pendingTimers = _queueFromTimeoutEvent();
	}
	_runTimers(pendingTimers);
	// Notify the event handler or shutdown the port if no more pending
	// timers are present.
	_notifyEventHandler();
	}

	// Tell the event handler to wake this isolate at a specific time.
	static void _scheduleWakeup(int wakeupTime) {
	if (!_receivePortActive) {
	_createTimerHandler();
	}
	VMLibraryHooks.eventHandlerSendData(null, _sendPort!, wakeupTime);
	_scheduledWakeupTime = wakeupTime;
	}

	// Cancel pending wakeups in the event handler.
	static void _cancelWakeup() {
	if (_sendPort != null) {
	VMLibraryHooks.eventHandlerSendData(null, _sendPort!, _NO_TIMER);
	_scheduledWakeupTime = 0;
	}
	}

	// Create a receive port and register a message handler for the timer
	// events.
	static void _createTimerHandler() {
	var receivePort = _receivePort;
	if (receivePort == null) {
	assert(_sendPort == null);
	final port = _RawReceivePortImpl('Timer');
	port.handler = _handleMessage;
	_sendPort = port.sendPort;
	_receivePort = port;
	_scheduledWakeupTime = 0;
	} else {
	receivePort._setActive(true);
	}
	_receivePortActive = true;
	}

	static void _shutdownTimerHandler() {
	_scheduledWakeupTime = 0;
	_receivePort!._setActive(false);
	_receivePortActive = false;
	}

	// The Timer factory registered with the dart:async library by the embedder.
	static Timer _factory(
	int milliSeconds, void callback(Timer timer), bool repeating) {
	if (repeating) {
	return new _Timer.periodic(milliSeconds, callback);
	}
	return new _Timer(milliSeconds, callback);
	}
	}

	@pragma("vm:entry-point", "call")
	_setupHooks() {
	VMLibraryHooks.timerFactory = _Timer._factory;
	}