Future is inspired by Clojure's future function. A future represents a promise to complete an action at some time in the future. The action is atomic and permanent. The idea behind a future is to send an operation for asynchronous completion, do other stuff, then return and retrieve the result of the async operation at a later time. Futures run on the global thread pool.

Feature:
  As a highly responsive Ruby application
  I want long-running tasks on a separate thread
  So I can perform other tasks without waiting

Futures have several possible states: :unscheduled, :pending, :processing, :rejected, or :fulfilled. These are also aggregated as #incomplete? and #complete?. When a Future is created it is set to :unscheduled. Once the #execute method is called the state becomes :pending. Once a job is pulled from the thread pool's queue and is given to a thread for processing (often immediately upon #post) the state becomes :processing. The future will remain in this state until processing is complete. A future that is in the :unscheduled, :pending, or :processing is considered #incomplete?. A #complete? Future is either :rejected, indicating that an exception was thrown during processing, or :fulfilled, indicating success. If a Future is :fulfilled its #value will be updated to reflect the result of the operation. If :rejected the reason will be updated with a reference to the thrown exception. The predicate methods #unscheduled?, #pending?, #rejected?, and #fulfilled? can be called at any time to obtain the state of the Future, as can the #state method, which returns a symbol.

Retrieving the value of a Future is done through the #value (alias: #deref) method. Obtaining the value of a Future is a potentially blocking operation. When a Future is :rejected a call to #value will return nil immediately. When a Future is :fulfilled a call to #value will immediately return the current value. When a Future is :pending a call to #value will block until the Future is either :rejected or :fulfilled. A timeout value can be passed to #value to limit how long the call will block. If nil the call will block indefinitely. If 0 the call will not block. Any other integer or float value will indicate the maximum number of seconds to block.

The constructor can also be given zero or more processing options. Currently the only supported options are those recognized by the Dereferenceable module.

The Future class also includes the behavior of the Ruby standard library Observable module, but does so in a thread-safe way. On fulfillment or rejection all observers will be notified according to the normal Observable behavior. The observer callback function will be called with three parameters: the Time of fulfillment/rejection, the final value, and the final reason. Observers added after fulfillment/rejection will still be notified as normal. The notification will occur on the same thread that processed the job.

Examples

A fulfilled example:

require 'concurrent'
require 'csv'
require 'open-uri'

class Ticker
  def get_year_end_closing(symbol, year, api_key)
    uri = "https://www.alphavantage.co/query?function=TIME_SERIES_MONTHLY&symbol=#{symbol}&apikey=#{api_key}&datatype=csv"
    data = []
    csv = URI.parse(uri).read
    if csv.include?('call frequency')
      return :rate_limit_exceeded
    end
    CSV.parse(csv, headers: true) do |row|
      data << row['close'].to_f if row['timestamp'].include?(year.to_s)
    end
    year_end = data.first
    year_end
  rescue => e
    p e
  end
end

api_key = ENV['ALPHAVANTAGE_KEY']
abort(error_message) unless api_key

# Future
price = Concurrent::Future.execute{ Ticker.new.get_year_end_closing('TWTR', 2013, api_key) }
p price.state #=> :pending
p price.pending? #=> true
p price.value(0) #=> nil (does not block)

sleep(1)    # do other stuff

p price.value #=> 63.65 (after blocking if necessary)
p price.state #=> :fulfilled
p price.fulfilled? #=> true
p price.value #=> 63.65

A rejected example:

count = Concurrent::Future.execute{ sleep(10); raise StandardError.new("Boom!") }
count.state #=> :pending
count.pending? #=> true

count.value #=> nil (after blocking)
count.rejected? #=> true
count.reason #=> #<StandardError: Boom!>

An example with observation:

class Ticker
  Stock = Struct.new(:symbol, :name, :exchange)

  def update(time, value, reason)
    ticker = value.collect do |symbol|
      Stock.new(symbol['symbol'], symbol['name'], symbol['exch'])
    end

    output = ticker.join("\n")
    print "#{output}\n"
  end
end

yahoo = Ticker.new('YAHOO')
future = Concurrent::Future.new { yahoo.update.suggested_symbols }
future.add_observer(Ticker.new)
future.execute

# do important stuff...

#>> #<struct Ticker::Stock symbol="YHOO", name="Yahoo! Inc.", exchange="NMS">
#>> #<struct Ticker::Stock symbol="YHO.DE", name="Yahoo! Inc.", exchange="GER">
#>> #<struct Ticker::Stock symbol="YAHOY", name="Yahoo Japan Corporation", exchange="PNK">
#>> #<struct Ticker::Stock symbol="YAHOF", name="YAHOO JAPAN CORP", exchange="PNK">
#>> #<struct Ticker::Stock symbol="YOJ.SG", name="YAHOO JAPAN", exchange="STU">
#>> #<struct Ticker::Stock symbol="YHO.SG", name="YAHOO", exchange="STU">
#>> #<struct Ticker::Stock symbol="YHOO.BA", name="Yahoo! Inc.", exchange="BUE">
#>> #<struct Ticker::Stock symbol="YHO.DU", name="YAHOO", exchange="DUS">
#>> #<struct Ticker::Stock symbol="YHO.HM", name="YAHOO", exchange="HAM">
#>> #<struct Ticker::Stock symbol="YHO.BE", name="YAHOO", exchange="BER">