Timer abstract interface

abstract interface class Timer

Annotations: @vmIsolateUnsendable

A countdown timer that can be configured to fire once or repeatedly.

The timer counts down from the specified duration to 0. When the timer reaches 0, the timer invokes the specified callback function. Use a periodic timer to repeatedly count down the same interval.

A negative duration is treated the same as Duration.zero. If the duration is statically known to be 0, consider using run.

void main() {
  Timer(const Duration(seconds: 5), handleTimeout);
}

void handleTimeout() {  // callback function
  // Do some work.
}

Note: If Dart code using Timer is compiled to JavaScript, the finest granularity available in the browser is 4 milliseconds.

See also:

• Stopwatch for measuring elapsed time.

Constructors

Timer() factory

factory Timer(Duration duration, void Function() callback)

Creates a new timer.

The callback function is invoked after the given duration.

Example:

final timer =
    Timer(const Duration(seconds: 5), () => print('Timer finished'));
// Outputs after 5 seconds: "Timer finished".

Implementation

factory Timer(Duration duration, void Function() callback) {
  if (Zone.current == Zone.root) {
    // No need to bind the callback. We know that the root's timer will
    // be invoked in the root zone.
    return Zone.current.createTimer(duration, callback);
  }
  return Zone.current.createTimer(
    duration,
    Zone.current.bindCallbackGuarded(callback),
  );
}

Timer.periodic() factory

factory Timer.periodic(Duration duration, void Function(Timer timer) callback)

Creates a new repeating timer.

The callback is invoked repeatedly with duration intervals until canceled with the cancel function.

The exact timing depends on the underlying timer implementation. No more than n callbacks will be made in duration * n time, but the time between two consecutive callbacks can be shorter and longer than duration.

In particular, an implementation may schedule the next callback, e.g., a duration after either when the previous callback ended, when the previous callback started, or when the previous callback was scheduled for - even if the actual callback was delayed.

A negative duration is treated the same as Duration.zero.

Example:

var counter = 3;
Timer.periodic(const Duration(seconds: 2), (timer) {
  print(timer.tick);
  counter--;
  if (counter == 0) {
    print('Cancel timer');
    timer.cancel();
  }
});
// Outputs:
// 1
// 2
// 3
// "Cancel timer"

Implementation

factory Timer.periodic(Duration duration, void callback(Timer timer)) {
  if (Zone.current == Zone.root) {
    // No need to bind the callback. We know that the root's timer will
    // be invoked in the root zone.
    return Zone.current.createPeriodicTimer(duration, callback);
  }
  var boundCallback = Zone.current.bindUnaryCallbackGuarded<Timer>(callback);
  return Zone.current.createPeriodicTimer(duration, boundCallback);
}

Properties

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

isActive no setter

bool get isActive

Returns whether the timer is still active.

A non-periodic timer is active if the callback has not been executed, and the timer has not been canceled.

A periodic timer is active if it has not been canceled.

Implementation

bool get isActive;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

tick no setter

int get tick

The number of durations preceding the most recent timer event.

The value starts at zero and is incremented each time a timer event occurs, so each callback will see a larger value than the previous one.

If a periodic timer with a non-zero duration is delayed too much, so more than one tick should have happened, all but the last tick in the past are considered "missed", and no callback is invoked for them. The tick count reflects the number of durations that have passed and not the number of callback invocations that have happened.

Example:

final stopwatch = Stopwatch()..start();
Timer.periodic(const Duration(seconds: 1), (timer) {
  print(timer.tick);
  if (timer.tick == 1) {
    while (stopwatch.elapsedMilliseconds < 4500) {
      // Run uninterrupted for another 3.5 seconds!
      // The latest due tick after that is the 4-second tick.
    }
  } else {
    timer.cancel();
  }
});
// Outputs:
// 1
// 4

Implementation

int get tick;

Methods

cancel()

void cancel()

Cancels the timer.

Once a Timer has been canceled, the callback function will not be called by the timer. Calling cancel more than once on a Timer is allowed, and will have no further effect.

Example:

final timer =
    Timer(const Duration(seconds: 5), () => print('Timer finished'));
// Cancel timer, callback never called.
timer.cancel();

Implementation

void cancel();

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toString() inherited

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

Implementation

external String toString();

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);

Static Methods

run()

void run(void Function() callback)

Runs the given callback asynchronously as soon as possible.

This function is equivalent to new Timer(Duration.zero, callback).

Example:

Timer.run(() => print('timer run'));

Implementation

static void run(void Function() callback) {
  Timer(Duration.zero, callback);
}