# Secret Agent Man

Agents are inspired by [Clojure's](http://clojure.org/) [agent](http://clojure.org/agents) keyword.
An agent is a single atomic value that represents an identity. The current value
of the agent can be requested at any time (`deref`). Each agent has a work queue and operates on
the global thread pool (see below). Consumers can `post` code blocks to the
agent. The code block (function) will receive the current value of the agent as its sole
parameter. The return value of the block will become the new value of the agent. Agents support
two error handling modes: fail and continue. A good example of an agent is a shared incrementing
counter, such as the score in a video game.

An agent must be initialize with an initial value. This value is always accessible via the `value`
(or `deref`) methods. Code blocks sent to the agent will be processed in the order received. As
each block is processed the current value is updated with the result from the block. This update
is an atomic operation so a `deref` will never block and will always return the current value.

When an agent is created it may be given an optional `validate` block and zero or more `rescue`
blocks. When a new value is calculated the value will be checked against the validator, if present.
If the validator returns `true` the new value will be accepted. If it returns `false` it will be
rejected. If a block raises an exception during execution the list of `rescue` blocks will be
seacrhed in order until one matching the current exception is found. That `rescue` block will
then be called an passed the exception object. If no matching `rescue` block is found, or none
were configured, then the exception will be suppressed.

Agents also implement Ruby's [Observable](http://ruby-doc.org/stdlib-1.9.3/libdoc/observer/rdoc/Observable.html).
Code that observes an agent will receive a callback with the new value any time the value
is changed.

## Examples

A simple example:

```ruby
require 'concurrent'

score = Concurrent::Agent.new(10)
score.value #=> 10

score << proc{|current| current + 100 }
sleep(0.1)
score.value #=> 110

score << proc{|current| current * 2 }
sleep(0.1)
deref score #=> 220

score << proc{|current| current - 50 }
sleep(0.1)
score.value #=> 170
```

With validation and error handling:

```ruby
score = agent(0).validate{|value| value <= 1024 }.
          rescue(NoMethodError){|ex| puts "Bam!" }.
          rescue(ArgumentError){|ex| puts "Pow!" }.
          rescue{|ex| puts "Boom!" }
score.value #=> 0

score << proc{|current| current + 2048 }
sleep(0.1)
score.value #=> 0

score << proc{|current| raise ArgumentError }
sleep(0.1)
#=> puts "Pow!"
score.value #=> 0

score << proc{|current| current + 100 }
sleep(0.1)
score.value #=> 100
```

With observation:

```ruby
bingo = Class.new{
  def update(time, score)
    puts "Bingo! [score: #{score}, time: #{time}]" if score >= 100
  end
}.new

score = agent(0)
score.add_observer(bingo)

score << proc{|current| sleep(0.1); current += 30 }
score << proc{|current| sleep(0.1); current += 30 }
score << proc{|current| sleep(0.1); current += 30 }
score << proc{|current| sleep(0.1); current += 30 }

sleep(1)
#=> Bingo! [score: 120, time: 2013-07-22 21:26:08 -0400]
```

## Copyright

*Concurrent Ruby* is Copyright &copy; 2013 [Jerry D'Antonio](https://twitter.com/jerrydantonio).
It is free software and may be redistributed under the terms specified in the LICENSE file.

## License

Released under the MIT license.

http://www.opensource.org/licenses/mit-license.php  

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> furnished to do so, subject to the following conditions:  
> 
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