From: "Sean O'Dell" <sean@...[REMOVE_THE_SPAM]>
Date: 2003-09-04T04:03:27+09:00
Subject: Re: OOP flavors - was Re: Python vs. Ruby

Ged Byrne wrote:
> 

[...snip...]

> 
> Good OO involves separating all of these views so that
> it is possible for somebody using the objects to work
> consistantly within a single model of a real world
> object.  

[...snip...]

> When your dialog receives a repaint command, it says
> "Button, here is some canvas.  Draw yourself upon it."
>  [Button.paint(g)]  The button takes this  canvas and
> breaks it into 5 parts, passes each part to the
> necessary shape and tells them to draw themselves.
> 
> Finally the chain comes to a Rectangle or Rhombus
> object.  This shape object does not know that it is
> part of a button.  It only knows that it is a shape
> and how to draw itself upon a canvas.
> 
> Eventually a canvas will be given a command.  Whether
> it is Canvas.draw(Rectangle) or Canvas.plot(x,y)
> depends on the canvas object.  Can it just plot
> points, or does it know how to draw geometric shapes. 
> What you wouldn't expect the Canvas to know is how to
> draw a button or a dialog.
> 
> In this model you are dealing with a button as a group
> of geometric shapes.
> 
> When you use a dialog that has a button on it, you
> don't worry about any of this.  You say "Dialog, you
> have a button called Fred."  "Fred, when somebody
> clicks on you, do this."  
> 
> You are working in a logical model that views a button
> as something on dialog that causes an action when it
> is pushed.
> 
> So whether or not you say Window.draw(Square) or
> Square.draw(Window) depends upon the logical model you
> are dealing with.  OO design does not dictate one form
> over the other.  OO design dictates a systematic
> modeling of reality.

This is an excellent example!

The button itself is thought of as a solid object (physical) and knows 
how to do several things with itself; it knows how it is supposed to 
look, how to react to button clicks, etc.

It encapsulates this: a button.

Perfectly.

By contrast, a string does not encapsulate a solid object; it 
encapsulates information.  Therefore, a string is not expected to render 
itself on a canvas.

It all has to do what with you are modelling.  Are you modelling a 
solid-object-abstraction?  If so:

     button.draw(canvas)
     button.click()
     button.hide()

Or is button just a shape, and not modelled after a solid object?  If so:

     canvas.draw(button.polygon)

I really like your analogy.  I have written, myself, many custom UI 
controls and have come to understand the very class system you're 
describing.

	Sean O'Dell