[MacRuby-devel] Key Value Observing module

29 Nov 2010

      I hope people don't mind me posting this bit of code to the list, but I hope it will be of some use to others and would also be interested to hear if the technique has any problems which I've not foreseen.

Ruby has its own means of implementing the observer pattern, but as Key Value Observing (KVO) is so pervasive in cocoa, it would be good if it was easy to use. The module is an attempt at making KVO a bit more pleasant to use in macruby. 
The ideas have been adapted/stolen from here:

https://gist.github.com/153676

The following is an example of how to have a block of code executed asynchronously every time a property of an instance variable changes

self.add_observer_for_key_path('person.first_name', async:true) do |object,change|
    object.do_expensive_calculation #will not block main thread
end

A concern I do have is that the object which wants to receive KVO notifications holds an array of Observer objects. These Observer objects hold a reference to the object receiving the KVO notifications. Is this going to cause havoc with garbage collection?

module KVO

  class Observer
    require 'dispatch'

    attr_reader :observee, :path
    def initialize(object, path, async, block, opts=0)
      @observee = object
      @path = path
      @async = async
      @block = block		  
      @observee.addObserver(self, forKeyPath:path, options:opts, context:nil)
    end

    def observeValueForKeyPath(path, ofObject:object, change:change, context:context)
      if @async
        Dispatch::Job.new { @block.call(object, change) } 
      else
        @block.call(object, change) 
      end
    end

    def cancel_observation
      @observee.removeObserver(self, forKeyPath:@path)
    end
  end

  def add_observer_for_key_path(path, async:async, &block)
    (@_observers_array_ ||= []) << Observer.new(self, path, async, block)
  end

  def add_observer_for_key_path(path, async:async, options:opts, &block)
    (@_observers_array_ ||= []) << Observer.new(self, path, async, block, opts)
  end

  def remove_observer_for_key_path(path)
    observer = @_observers_array_.find {|o| o.path == path && o.observee == self} if @_observers_array_
    @_observers_array_.delete(observer) and observer.cancel_observation if observer
  end

end

[MacRuby-devel] Key Value Observing module

Alan Skipp