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// Copyright (c) 2011, 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 dart.core;
/// A simple stopwatch interface to measure elapsed time.
class Stopwatch {
/// Cached frequency of the system in Hz (ticks per second).
/// Value must be returned by [_initTicker], which is called only once.
static final int _frequency = _initTicker();
// The _start and _stop fields capture the time when [start] and [stop]
// are called respectively.
// If _stop is null, the stopwatch is running.
int _start = 0;
int? _stop = 0;
/// Creates a [Stopwatch] in stopped state with a zero elapsed count.
/// The following example shows how to start a [Stopwatch]
/// immediately after allocation.
/// ```dart
/// var stopwatch = Stopwatch()..start();
/// ```
Stopwatch() {
_frequency; // Ensures initialization before using any method.
/// Frequency of the elapsed counter in Hz.
int get frequency => _frequency;
/// 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() {
int? stop = _stop;
if (stop != null) {
// (Re)start this stopwatch.
// Don't count the time while the stopwatch has been stopped.
_start += _now() - stop;
_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() {
_stop ??= _now();
/// Resets the [elapsed] count to zero.
/// This method does not stop or start the [Stopwatch].
void reset() {
_start = _stop ?? _now();
/// The elapsed number of clock ticks since calling [start] while the
/// [Stopwatch] is running.
/// This is the elapsed number of clock ticks between calling [start] and
/// calling [stop].
/// Is 0 if the [Stopwatch] has never been started.
/// The elapsed number of clock ticks increases by [frequency] every second.
int get elapsedTicks {
return (_stop ?? _now()) - _start;
/// The [elapsedTicks] counter converted to a [Duration].
Duration get elapsed {
return Duration(microseconds: elapsedMicroseconds);
/// The [elapsedTicks] counter converted to microseconds.
external int get elapsedMicroseconds;
/// The [elapsedTicks] counter converted to milliseconds.
external int get elapsedMilliseconds;
/// Whether the [Stopwatch] is currently running.
bool get isRunning => _stop == null;
/// Initializes the time-measuring system. *Must* return the [_frequency]
/// variable. May do other necessary initialization.
external static int _initTicker();
external static int _now();