From: Joel VanderWerf <vjoel@...>
Date: 2006-10-21T06:57:17+09:00
Subject: Re: [ANN] Dependency 1.0.0

Tim Fletcher wrote:
> A Smalltalk-inspired dependency mechanism for Ruby. Like this:
> 
>     class StockWatcher
>       include Dependency::Methods
>       def initialize(stock)
>         stock.express_interest_in :price, :for => self, :send_back =>
> :price_changed
>       end
>       def price_changed(old_price, stock)
>         # gets called whenever the stock object changes it's price
>       end
>     end
> 
> Completely unrelated to Rails' dependency code - similar to observers
> though.
> 
> http://rubyforge.org/projects/dependency
> 
> gem install dependency
> 

You can do this with http://raa.ruby-lang.org/project/observable:

require 'observable'

include Observable
include Observable::Match

class Stock
   extend Observable

   observable :price

   def initialize price
     @price = price
   end

   def sell
     puts "selling at #{price}"
   end
end

class StockWatcher
   GREATER_THAN = proc do |price|
     MatchProc.new { |test_price| test_price > price }
   end

   def initialize(stock)
     stock.when_price CHANGES do |price, old_price|
       puts "price = #{price}  (was #{old_price.inspect})"
     end

     stock.when_price GREATER_THAN[20] do
       stock.sell
     end
   end
end

acme = Stock.new(10)
watcher = StockWatcher.new(acme)

10.step(30, 5) do |acme.price|
   puts "---"
end

__END__

Output:

price = 10  (was nil)
---
price = 15  (was 10)
---
price = 20  (was 15)
---
selling at 25
price = 25  (was 20)
---
selling at 30
price = 30  (was 25)
---


-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407