Semaphore abstract

abstract class Semaphore

A purely functional semaphore for controlling concurrent access to a shared resource.

A Semaphore maintains a count of available permits. Fibers can acquire permits (blocking if none are available) and release them when done. The permit method provides a Resource-based API that automatically releases the permit when the scope exits.

Supports both single-permit (acquire/release) and multi-permit (acquireN/releaseN) operations.

Constructors

Semaphore()

Semaphore()

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

acquire()

IO<Unit> acquire()

Acquires a single permit, blocking (semantically) until one is available.

Implementation

IO<Unit> acquire() => acquireN(1);

acquireN()

IO<Unit> acquireN(int n)

Acquires n permits, blocking (semantically) until they are available.

Implementation

IO<Unit> acquireN(int n);

available()

IO<int> available()

Returns the number of permits currently available.

Implementation

IO<int> available();

count()

IO<int> count()

Returns the number of permits currently available, or the negated number of permits being waited on if the semaphore is over-subscribed.

Implementation

IO<int> count();

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);

permit()

Resource<Unit> permit()

Returns a Resource that acquires a single permit on use and releases it on finalization.

Implementation

Resource<Unit> permit();

release()

IO<Unit> release()

Releases a single permit back to the semaphore.

Implementation

IO<Unit> release() => releaseN(1);

releaseN()

IO<Unit> releaseN(int n)

Releases n permits back to the semaphore, potentially unblocking waiting fibers.

Implementation

IO<Unit> releaseN(int n);

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();

tryAcquire()

IO<bool> tryAcquire()

Attempts to acquire a single permit without blocking.

Returns true if the permit was acquired, false otherwise.

Implementation

IO<bool> tryAcquire() => tryAcquireN(1);

tryAcquireN()

IO<bool> tryAcquireN(int n)

Attempts to acquire n permits without blocking.

Returns true if the permits were acquired, false otherwise.

Implementation

IO<bool> tryAcquireN(int n);

tryPermit()

Resource<bool> tryPermit()

Returns a Resource that attempts to acquire a single permit.

The resource value is true if the permit was acquired. The permit is released on finalization only if it was actually acquired.

Implementation

Resource<bool> tryPermit() =>
    Resource.make(tryAcquire(), (acquired) => release().whenA(acquired));

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

permits()

IO<Semaphore> permits(int n)

Creates a Semaphore with n available permits.

Throws an ArgumentError if n is negative.

Implementation

static IO<Semaphore> permits(int n) {
  if (n < 0) {
    throw ArgumentError('n must be nonnegative, was: $n');
  }

  return IO.ref(_State(n, IQueue.empty())).map(_SemaphoreImpl.new);
}