A simple Stopwatch interface to measure elapsed time.

class Stopwatch {
 // The _start and _stop fields capture the time when [start] and [stop]
 // are called respectively.
 // If _start is null, then the [Stopwatch] has not been started yet.
 // If _stop is null, then the [Stopwatch] has not been stopped yet,
 // or is running.
 int _start;
 int _stop;

 /**
  * Creates a [Stopwatch] in stopped state with a zero elapsed count.
  *
  * The following example shows how to start a [Stopwatch]
  * immediately after allocation.
  *
  *     Stopwatch stopwatch = new Stopwatch()..start();
  */
 Stopwatch() : _start = null, _stop = null {}

 /**
  * Starts the [Stopwatch].
  *
  * The [elapsed] count is increasing monotonically. If the [Stopwatch] has
  * been stopped, then calling start again restarts it without resetting the
  * [elapsed] count.
  *
  * If the [Stopwatch] is currently running, then calling start does nothing.
  */
 void start() {
   if (isRunning) return;
   if (_start == null) {
     // This stopwatch has never been started.
     _start = _now();
   } else {
     // Restart this stopwatch. Prepend the elapsed time to the current
     // start time.
     _start = _now() - (_stop - _start);
     _stop = null;
   }
 }

 /**
  * Stops the [Stopwatch].
  *
  * The [elapsedTicks] count stops increasing after this call. If the
  * [Stopwatch] is currently not running, then calling this method has no
  * effect.
  */
 void stop() {
   if (!isRunning) return;
   _stop = _now();
 }

 /**
  * Resets the [elapsed] count to zero.
  *
  * This method does not stop or start the [Stopwatch].
  */
 void reset() {
   if (_start == null) return;
   // If [_start] is not null, then the stopwatch had already been started. It
   // may running right now.
   _start = _now();
   if (_stop != null) {
     // The watch is not running. So simply set the [_stop] to [_start] thus
     // having an elapsed time of 0.
     _stop = _start;
   }
 }

 /**
  * Returns the elapsed number of clock ticks since calling [start] while the
  * [Stopwatch] is running.
  *
  * Returns the elapsed number of clock ticks between calling [start] and
  * calling [stop].
  *
  * Returns 0 if the [Stopwatch] has never been started.
  *
  * The elapsed number of clock ticks increases by [frequency] every second.
  */
 int get elapsedTicks {
   if (_start == null) {
     return 0;
   }
   return (_stop == null) ? (_now() - _start) : (_stop - _start);
 }

 /**
  * Returns the [elapsedTicks] counter converted to a [Duration].
  */
 Duration get elapsed {
   return new Duration(microseconds: elapsedMicroseconds);
 }

 /**
  * Returns the [elapsedTicks] counter converted to microseconds.
  */
 int get elapsedMicroseconds {
   return (elapsedTicks * 1000000) ~/ frequency;
 }

 /**
  * Returns the [elapsedTicks] counter converted to milliseconds.
  */
 int get elapsedMilliseconds {
   return (elapsedTicks * 1000) ~/ frequency;
 }

 /**
  * Returns the frequency of the elapsed counter in Hz.
  */
 int get frequency => _frequency();

 /**
  * Returns wether the [StopWatch] is currently running.
  */
 bool get isRunning => _start != null && _stop == null;

 external static int _frequency();
 external static int _now();
}