Fix EventMachine::PeriodicTimer

EventMachine::PeriodicTimer doesn't give an accurate timing. For example, if I add a periodic timer with 1 second interval, in my Mac, it looks like this.

1. the test code.

 require 'eventmachine'  
 puts 'Start'  
 EM.run do  
   EM::add_periodic_timer 1.0 do   
     puts "time: #{Time.new.to_f.round(1)}"  
   end  
 end  

2. output in my MacBook Pro.

 time: 1298620726.9  
 time: 1298620728.0  
 time: 1298620729.1  
 time: 1298620730.1  
 time: 1298620731.2  
 time: 1298620732.3  
 time: 1298620733.4  
 time: 1298620734.5  
 time: 1298620735.5  
 time: 1298620736.6  
 time: 1298620737.7  
 time: 1298620738.8  
 time: 1298620739.9  
 time: 1298620741.0  
 time: 1298620742.0  
 time: 1298620743.1  
 time: 1298620744.2  
 time: 1298620745.3  
 time: 1298620746.4  
 time: 1298620747.4  
 time: 1298620748.5  
 time: 1298620749.6  

As you can see, it drifted 100ms every time. Which is not very good when you want to do some job every 1 second. You might omit some seconds. So, with using the same method that I used for the Periodic ruby service, I made a change to PeriodicTimer. It looks like this.

 # Override PeriodicTimer  
 module EventMachine  
   class PeriodicTimer  
     alias :old_initialize :initialize  
     def initialize interval, callback=nil, &block  
       # Added two additional instance variables to compensate difference.   
       @start = Time.now  
       @fixed_interval = interval  
       old_initialize interval, callback, &block  
     end  
     alias :old_schedule :schedule  
     def schedule  
       # print "Started at #{@start}..: "  
       compensation = (Time.now - @start) % @fixed_interval   
       @interval = @fixed_interval - compensation  
       # Schedule   
       old_schedule  
     end  
   end  
 end  

So, here is the result (this is not that accurate like the simple ruby service, but reasonably useable.

 time: 1298620683.4  
 time: 1298620684.4  
 time: 1298620685.4  
 time: 1298620686.4  
 time: 1298620687.4  
 time: 1298620688.4  
 time: 1298620689.4  
 time: 1298620690.4  
 time: 1298620691.3  
 time: 1298620692.3  
 time: 1298620693.4  
 time: 1298620694.4  
 time: 1298620695.4  
 time: 1298620696.4  
 time: 1298620697.4  
 time: 1298620698.4  
 time: 1298620699.4  

That's it. :-)