Dispatcher abstract

abstract class Dispatcher

Provides an unsafe API for running IO effects from outside the IO world, bridging purely functional code to callback-based or Future-based interfaces.

A Dispatcher is created as a Resource that scopes the lifecycle of any effects submitted to it. On finalization, active effects are either awaited or canceled depending on waitForAll.

Two execution modes are available:

• parallel: each submitted effect runs as its own fiber concurrently.
• sequential: submitted effects are queued and run one at a time in FIFO order by a single worker fiber.

Constructors

Dispatcher()

Dispatcher()

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;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

Methods

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();

unsafeRunAndForget()

void unsafeRunAndForget<A>(IO<A> fa)

Submits fa for execution, discarding the result. Fire-and-forget.

Implementation

void unsafeRunAndForget<A>(IO<A> fa) {
  unsafeToFutureCancelable(fa);
}

unsafeRunCancelable()

Future<Unit> Function() unsafeRunCancelable<A>(IO<A> fa)

Submits fa for execution and returns a function to cancel it.

Implementation

Function0<Future<Unit>> unsafeRunCancelable<A>(IO<A> fa) => unsafeToFutureCancelable(fa).$2;

unsafeToFuture()

Future<A> unsafeToFuture<A>(IO<A> fa)

Submits fa for execution and returns a Future that resolves to the result.

Implementation

Future<A> unsafeToFuture<A>(IO<A> fa) => unsafeToFutureCancelable(fa).$1;

unsafeToFutureCancelable()

Record unsafeToFutureCancelable<A>(IO<A> fa)

Submits fa for execution and returns a Future that resolves to the result along with a cancel function.

Implementation

(Future<A>, Function0<Future<Unit>>) unsafeToFutureCancelable<A>(IO<A> fa);

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

parallel()

Resource<Dispatcher> parallel({bool waitForAll = false})

Creates a parallel Dispatcher where each submitted effect runs as its own fiber concurrently.

If waitForAll is true, finalization waits for all active effects to complete. If false (the default), active effects are canceled.

Implementation

static Resource<Dispatcher> parallel({bool waitForAll = false}) {
  return Resource.make(
    IO.delay(() => _ParallelDispatcher()),
    (dispatcher) {
      final impl = dispatcher as _ParallelDispatcher;

      &#47;&#47; Defer all mutations into IO so they only run at finalization time,
      &#47;&#47; not when Resource.make calls release(d) eagerly to build the IO.
      return IO
          .delay(() {
            final active = impl._active ?? {};
            impl._active = null; &#47;&#47; mark as finalized

            return active;
          })
          .flatMap((Map<int, (Future<dynamic>, Function0<Future<Unit>>)> active) {
            if (active.isEmpty) {
              return IO.unit;
            } else if (waitForAll) {
              &#47;&#47; Wait for all active futures, ignoring errors (canceled = error).
              final silenced =
                  active.values.map((e) => e.$1.then((_) => null, onError: (_) => null)).toList();

              return IO.fromFutureF(() => Future.wait(silenced).then((_) => Unit()));
            } else {
              &#47;&#47; Cancel all fibers concurrently.
              return active.values.fold<IO<Unit>>(
                IO.unit,
                (IO<Unit> acc, (Future<dynamic>, Function0<Future<Unit>>) e) =>
                    IO.both(acc, IO.fromFutureF(e.$2)).voided(),
              );
            }
          });
    },
  );
}

sequential()

Resource<Dispatcher> sequential({bool waitForAll = false})

Creates a sequential Dispatcher where submitted effects are run in FIFO order by a single worker fiber.

If waitForAll is true, finalization drains the queue and waits for all pending effects to complete before stopping. If false (the default), the worker is stopped immediately.

Note: individual task cancellation sets a skip flag. If the task has already started running, it continues but its result is discarded.

Implementation

static Resource<Dispatcher> sequential({bool waitForAll = false}) {
  return Resource.eval(Queue.unbounded<IO<Unit>>()).flatMap(
    (queue) {
      final worker =
          queue.take().flatMap((task) => task.handleErrorWith((_) => IO.unit)).foreverM();

      return Resource.make(
        worker.start(),
        (workerFiber) {
          if (!waitForAll) {
            return workerFiber.cancel();
          } else {
            &#47;&#47; Yield first so any pending unsafeRunFuture offer calls can
            &#47;&#47; complete and their tasks are in the queue before the sentinel.
            return IO.cede.productR(
              Deferred.of<Unit>().flatMap((sentinel) {
                return queue
                    .offer(sentinel.complete(Unit()).voided())
                    .productR(sentinel.value())
                    .productR(workerFiber.cancel());
              }),
            );
          }
        },
      ).map((_) => _SequentialDispatcher(queue));
    },
  );
}