From: Enrique Comba Riepenhausen <ecomba@...>
Date: 2007-06-09T06:46:00+09:00
Subject: Re: Disadvantages of Dependency Inversion?


On 8 Jun 2007, at 21:05, Mark Volkmann wrote:

> On Jun 8, 2007, at 1:55 PM, sweetchuck74@gmail.com wrote:
>
>> Nowadays, DI (Dependency Inversion) is one of the widely used design
>> pattern.
>> It provides several benefits, such as (1) loose coupling between
>> component (2) effective and easy testing (component testing or unit
>> testing)
>>
>> What are the drawbacks or disadvantages of using DI?
>
> I'm not anti-DI, but I think one disadvantage is that it can be  
> hard to follow the code. You see a method in a class, you know  
> something is calling it to inject a value, but you're not sure  
> where it happens or what value is injected.

Actually Dependency Injection comes from less dynamic languages where  
you have the notion of interfaces, etc.

In Ruby though this does not exist at such a level (like Smalltalk an  
the like), but it is still advantageous to use in most cases.

Dependency Injection follows the Hollywood principle: "You won't call  
us, we will call you".

Let's suppose you have a method that calls a method from a command  
(it doesn't matter much what this command does; and sure, the example  
is pretty silly). You could create an instance of the particular  
command in the method like:

class CommandCaller

   def initialize
     @command = MySuperCommand.new
   end

   def call_command()
     @command.execute
   end
end

or, using a DI approach you would pass the fully initialized command  
itself to the method:

class CommandCaller

   def initialize(command)
     @command = command
   end

   def call_command(command)
     command.execute
   end
end

Obviously one of the strengths of DI (or IoC; Inversion of Control)  
is that you normally wire those objects dynamically on startup of  
your application...

I hope that helped a bit.

Cheers,


----
Enrique Comba Riepenhausen
ecomba@mac.com

I always thought Smalltalk would beat Java, I just didn't know it  
would be called 'Ruby' when it did.
-- Kent Beck