Refactor to observer pattern

bin/trading

@@ -10,12 +10,20 @@ rescue LoadError
   require 'trading'
 end
 
+class LoggingObserver
+  def trade(obj)
+    puts obj
+  end
+
+  def order(obj)
+    puts obj
+  end
+end
+
 engine = Trading::Engine.new
+engine.add_observer(LoggingObserver.new)
 
 ARGF.each do |line|
   order = Trading::Order.parse(line.trim)
-  result = engine.submit_order(order)
-  if result
-    puts result
-  end
+  engine.submit_order(order)
 end

lib/trading/engine.rb

@@ -6,15 +6,30 @@ class Trading::Engine
 
   def initialize
     @order_book = {}
+    @observers = []
   end
 
   def submit_order(order)
+    notify_observers(:order, order)
+
     book = (order_book[order.commodity] ||= Trading::OrderBook.new)
     book.submit_order(order)
     if book.match?
-      return book.trade!
-    else
-      return false
+      trade = book.trade!
+      notify_observers(:trade, trade)
+    end
+  end
+
+  def add_observer(observer)
+    @observers << observer
+  end
+
+  private
+
+  def notify_observers(event, payload)
+    @observers.each do |observer|
+      observer.send(event, payload)
     end
   end
+
 end

lib/trading/order.rb

@@ -35,6 +35,10 @@ def <=>(other)
     return time_e
   end
 
+  def to_s
+    [time, order_type, price, commodity, quantity].join("\t")
+  end
+
   def split(amount)
     if amount >= quantity
       raise "Invalid split: desired amount #{amount} greater than available amount #{quantity}"

spec/engine_spec.rb

@@ -3,45 +3,58 @@
 describe 'Trading::Engine' do
   before do
     @engine = Trading::Engine.new
+    @observer = TestObserver.new
+    @engine.add_observer @observer
   end
 
   describe '#submit_order' do
     it "should return false for no match" do
       order = Trading::Order.parse("2013-12-01T16:19:00Z\tBUY\tBTC\t100.000\t100")
 
-      @engine.submit_order(order).should eq(false)
+      @engine.submit_order(order)
+      @observer.trades.length.should eq(0)
     end
 
     it "should return a pair of orders for a match" do
       buy = Trading::Order.parse("2013-12-01T16:19:00Z\tBUY\tBTC\t100.000\t100")
       sell = Trading::Order.parse("2013-12-01T16:20:00Z\tSELL\tBTC\t100.000\t100")
 
-      @engine.submit_order(buy).should eq(false)
-      @engine.submit_order(sell).should eq(Trading::Trade.new(buy, sell))
+      @engine.submit_order(buy)
+      @observer.trades.length.should eq(0)
+
+      @engine.submit_order(sell)
+      @observer.trades.length.should eq(1)
+      @observer.trades.first.should eq(Trading::Trade.new(buy, sell))
     end
 
     it "should not match differing commodities" do
       buy = Trading::Order.parse("2013-12-01T16:19:00Z\tBUY\tBTC\t100.000\t100")
       sell = Trading::Order.parse("2013-12-01T16:20:00Z\tSELL\tUSD\t100.000\t100")
 
-      @engine.submit_order(buy).should eq(false)
-      @engine.submit_order(sell).should eq(false)
+      @engine.submit_order(buy)
+      @observer.trades.length.should eq(0)
+      @engine.submit_order(sell)
+      @observer.trades.length.should eq(0)
     end
 
     it "should not match differing prices" do
       buy = Trading::Order.parse("2013-12-01T16:19:00Z\tBUY\tBTC\t100.000\t100")
       sell = Trading::Order.parse("2013-12-01T16:20:00Z\tSELL\tBTC\t100.001\t100")
 
-      @engine.submit_order(buy).should eq(false)
-      @engine.submit_order(sell).should eq(false)
+      @engine.submit_order(buy)
+      @observer.trades.length.should eq(0)
+      @engine.submit_order(sell)
+      @observer.trades.length.should eq(0)
     end
 
     it "should not match differing quantities" do
       buy = Trading::Order.parse("2013-12-01T16:19:00Z\tBUY\tBTC\t100.000\t100")
       sell = Trading::Order.parse("2013-12-01T16:20:00Z\tSELL\tBTC\t100.000\t200")
 
-      @engine.submit_order(buy).should eq(false)
-      @engine.submit_order(sell).should eq(false)
+      @engine.submit_order(buy)
+      @observer.trades.length.should eq(0)
+      @engine.submit_order(sell)
+      @observer.trades.length.should eq(0)
     end
   end
 end

spec/spec_helper.rb

@@ -1 +1,19 @@
 require_relative '../lib/trading'
+
+class TestObserver
+
+  attr_reader :orders, :trades
+
+  def initialize
+    @orders = []
+    @trades = []
+  end
+
+  def trade(obj)
+    @trades << obj
+  end
+
+  def order(obj)
+    @orders << obj
+  end
+end