# class Channel(T)

- Channel(T)
- Reference
- Object

##
Overview

A `Channel` enables concurrent communication between fibers.

They allow communicating data between fibers without sharing memory and without having to worry about locks, semaphores or other special structures.

    channel = Channel(Int32).new

    spawn do
      channel.send(0)
      channel.send(1)
    end

    channel.receive # => 0
    channel.receive # => 1

NOTE Although a `Channel(Nil)` or any other nilable types like `Channel(Int32?)` are valid they are discouraged since from certain methods or constructs it receiving a `nil` as data will be indistinguishable from a closed channel.

##
Defined in:

channel.cr
channel/select.cr
channel/select/select_action.cr
channel/select/timeout_action.cr

##
Constructors

- .new(capacity : Int32 = 0)

##
Class Method Summary

- .receive_first(channels : Enumerable(Channel))
- .receive_first(*channels)
- .send_first(value, channels : Enumerable(Channel)) : Nil
- .send_first(value, *channels) : Nil

##
Instance Method Summary

- #close : Bool

  Closes the channel.

- #closed? : Bool
- #inspect(io : IO) : Nil

  Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

- #pretty_print(pp)
- #receive : T

  Receives a value from the channel.

- #receive? : T | Nil

  Receives a value from the channel.

- #send(value : T) : self

  Sends a value to the channel.

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, initialize initialize, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference) : Bool
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new, unsafe_construct(address : Pointer, *args, **opts) : self unsafe_construct

### Class methods inherited from class `Reference`

pre_initialize(address : Pointer) pre_initialize

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io, root : String)
from_json(string_or_io) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

def self.new(capacity : Int32 = 0) #

##
Class Method Detail

def self.receive_first(channels : Enumerable(Channel)) #
def self.receive_first(*channels) #
def self.send_first(value, channels : Enumerable(Channel)) : Nil #
def self.send_first(value, *channels) : Nil #

##
Instance Method Detail

def close : Bool #

Closes the channel. The method prevents any new value from being sent to / received from the channel. All fibers blocked in `#send` or `#receive` will be awakened with `Channel::ClosedError`

Both awaiting and subsequent calls to `#send` will consider the channel closed. All items successfully sent to the channel can be received, before `#receive` considers the channel closed. Calling `#close` on a closed channel does not have any effect.

It returns `true` when the channel was successfully closed, or `false` if it was already closed.

def closed? : Bool #
def inspect(io : IO) : Nil #
Description copied from class Reference

Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

    class Person
      def initialize(@name : String, @age : Int32)
      end
    end

    Person.new("John", 32).inspect # => #<Person:0x10fd31f20 @name="John", @age=32>

def pretty_print(pp) #
def receive : T #

Receives a value from the channel. If there is a value waiting, then it is returned immediately. Otherwise, this method blocks until a value is sent to the channel.

Raises `ClosedError` if the channel is closed or closes while waiting for receive.

    channel = Channel(Int32).new
    spawn do
      channel.send(1)
    end
    channel.receive # => 1

def receive? : T | Nil #

Receives a value from the channel. If there is a value waiting, it is returned immediately. Otherwise, this method blocks until a value is sent to the channel.

Returns `nil` if the channel is closed or closes while waiting for receive.

def send(value : T) : self #

Sends a value to the channel. If the channel has spare capacity, then the method returns immediately. Otherwise, this method blocks the calling fiber until another fiber calls `#receive` on the channel.

Raises `ClosedError` if the channel is closed or closes while waiting on a full channel.