From: "Thomas B." <tpreal@...>
Date: 2008-09-21T03:14:44+09:00
Subject: Re: Question about Mixins and calling a method

Carlos Ortega wrote:
> 1)  Is it possible to invoke a method with a "<<" notation ?

It is not 'invoking a method with "<<" notation,' it is simply calling 
the method named observers without any arguments, and then calling << on 
the result of the method. This is equivalent to:
(@observer_list||=[])<<obj
or even:
(@observer_list||=[]).<<(obj)

Now: the method observers initialises the instance variable 
@observer_list. The operator ||= assigns the empty array to the variable 
if the variable is still nil at the moment of the call (so it will 
assign the empty array only on the first call), and then return the 
variable, no mater if the assignment was done or not. So the line 
observers<<obj does the following:
@observer_list=[] if not @observer_list
@observer_list<<obj

> On the other side while reviewing the "observers" method, I noticed that
> there is no parameter in its definition:
Parentheses can be skipped both in definition and in call of a methods, 
if there are no parameters.

> 2) Same situation happens in the "notify_observers" method, in it I can
> invoke "observers.each" although it is not defined anywhere. Why can I
> do that?

Again, this is a call to the method observers, and then a call to the 
method each of the result you got. The method observers never returns 
nil, because it either initialises or just returns the already 
initialised @observer_list.

The instance variable @observer_list itself is not defined anywhere 
though, because you don't have to declare instance variables in any way. 
Simply if you try to read a variable for the first time, it returns nil 
if the variable has not been assigned before.

You can read more about classes and their features here: 
http://al2o3-cr.blogspot.com/2008/08/class-class.html

TPR.
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