Another object question

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bstripp

(Posted 2005) [#1]

Ok I am trying to wrap my head around objects...

I have a base type that can display an object and handle input. When it is created, it goes into a master list for display items.

Now I extend that type do make the varrious display objects do different functions. One type of display object displays a player, another shows stats, etc. All of the functionality is handled here as to what happens when the player interacts with the object.

When I create I assume that I call the extended type to create both.

Now when I loop through the master display list, do I get the extended type as well as the base display type? Or do I need to have some kind of index to tie them together?

Example:

type displayitem
  method showitem()
    stuff...
  end method

  method new()
    displist.addlast self
  end method
end type

type playerselection extends basetype
  method handleinput()
    stuff
  end method

  method create()
    return new playerselection
  end method
end type

local p:playerselection
local a

for a = 1 to 1000
  p:playerselection = p.create()
next

Now when I loop through my display list do I do my eachin on playerselection objects, or displayobjects? If I do the loop on displayobjects (since that is what is added to the list) do I get the associated playerselection object as well?

I love this object stuff, but this is my first go with it. Neat and really functional!

tonyg

(Posted 2005) [#2]

Your code is a bit confusing.
If you do your eachin against playerselection it will show all playerselection objects. If you do it against displayitem it will do all playerselection AND any other objects on displist which has been extended from displayitem.
I was hoping the colours would show this better but this might help...

SetGraphicsDriver GLMax2DDriver()
Type displayitem
  Global displist:TList 
  Field x
  Field y
  Method showitem()
    DrawRect x,y,10,10
  End Method

  Method New()
    displist.addlast Self
  End Method
End Type

Type player1 Extends displayitem

  Function create()
     If displist=Null displist=CreateList()
     p1:player1 = New player1
     p1.x = Rand(1,200)
     p1.y = Rand(1,200)  
     displist.addlast p1  
  End Function
End Type
Type player2 Extends displayitem

  Function create()
     If displist=Null displist=CreateList()
     p2:player2= New player2
     p2.x = Rand(1,200)
     p2.y = Rand(1,200) 
     displist.addlast p2   
  End Function
End Type


Graphics 640,480
SeedRnd MilliSecs()
Local p1:player1
Local p2:player2
Local a

For a = 1 To 10
   player1.create()
Next
For a = 1 To 10
   player2.create()
Next
  Cls
  SetColor 255,0,0
  DrawText "Player 1 types",0,0
  For my1:player1 = EachIn player1.displist
    my1.showitem()
  Next
  Flip
  WaitChar
  SetColor 0,0,255
  DrawText "and Player 2 types",0,10
  DrawText "Hit any key",0,30
  For my2:player2 = EachIn player2.displist
    my2.showitem()
  Next
  Flip
  WaitChar
  Cls
  SetColor 0,255,0
  DrawText "and displayitem",0,0
  For my3:displayitem = EachIn displayitem.displist
    my3.showitem()
  Next
  Flip
  WaitChar
End

<edit> that's better. Good ole GLdriver

FlameDuck

(Posted 2005) [#3]

[quote]If I do the loop on displayobjects (since that is what is added to the list) do I get the associated playerselection object as well?[quote]Ys and no. Here's how polymorphism works:

If you itterate through your basetype (in your example), you do get the extended types, however you can only directly manipulate objects through methods defined in the basetypes interface.

So in your case you would not be able to handleinput, without a specific downcast, however if the playerselection objects had a different implementation of showitem() than the basetype, then the playerselection version would be the one invoked.

bstripp

(Posted 2005) [#4]

Ah I think I understand.

If I wanted to call the extended type version of a method, I need to have one in the base method.

Then when I loop through the base menthod, I get the associated extended method and can use the code for handling input (from my example).

Thanks again, this is a really odd way of thinking :)