# class WaitGroup

- WaitGroup
- Reference
- Object

##
Overview

Suspend execution until a collection of fibers are finished.

The wait group is a declarative counter of how many concurrent fibers have been started. Each such fiber is expected to call `#done` to report that they are finished doing their work. Whenever the counter reaches zero the waiters will be resumed.

This is a simpler and more efficient alternative to using a `Channel(Nil)` then looping a number of times until we received N messages to resume execution.

Basic example:

    require "wait_group"
    wg = WaitGroup.new(5)

    5.times do
      spawn do
        do_something
      ensure
        wg.done # the fiber has finished
      end
    end

    # suspend the current fiber until the 5 fibers are done
    wg.wait

##
Defined in:

wait_group.cr

##
Constructors

- .new(n : Int32 = 0)

##
Class Method Summary

- .wait(&) : Nil

  Yields a `WaitGroup` instance and waits at the end of the block for all of the work enqueued inside it to complete.

##
Instance Method Summary

- #add(n : Int32 = 1) : Nil

  Increments the counter by how many fibers we want to wait for.

- #done : Nil

  Decrements the counter by one.

- #spawn(*, name : String | Nil = nil, &block) : Fiber

  Increment the counter by 1, perform the work inside the block in a separate fiber, decrementing the counter after it completes or raises.

- #wait : Nil

  Suspends the current fiber until the counter reaches zero, at which point the fiber will be resumed.

### 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(n : Int32 = 0) #

##
Class Method Detail

def self.wait(&) : Nil #

Yields a `WaitGroup` instance and waits at the end of the block for all of the work enqueued inside it to complete.

    WaitGroup.wait do |wg|
      items.each do |item|
        wg.spawn { process item }
      end
    end

##
Instance Method Detail

def add(n : Int32 = 1) : Nil #

Increments the counter by how many fibers we want to wait for.

A negative value decrements the counter. When the counter reaches zero, all waiting fibers will be resumed. Raises `RuntimeError` if the counter reaches a negative value.

Can be called at any time, allowing concurrent fibers to add more fibers to wait for, but they must always do so before calling `#done` that would decrement the counter, to make sure that the counter may never inadvertently reach zero before all fibers are done.

def done : Nil #

Decrements the counter by one. Must be called by concurrent fibers once they have finished processing. When the counter reaches zero, all waiting fibers will be resumed.

def spawn(*, name : String | Nil = nil, &block) : Fiber #

Increment the counter by 1, perform the work inside the block in a separate fiber, decrementing the counter after it completes or raises. Returns the `Fiber` that was spawned.

    wg = WaitGroup.new
    wg.spawn { do_something }
    wg.wait

def wait : Nil #

Suspends the current fiber until the counter reaches zero, at which point the fiber will be resumed.

Can be called from different fibers.