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Thread: Overriding Twice

  1. #1

    Overriding Twice

    System: Windows Vista
    Compiler/IDE: Lazarus .9.28.2, FPC 2.2.4
    Libraries/API: Andorra 2D (Not quite relevant, please try to ignore the superfluous code that deals with Andorra if you can.)

    Well, I'm trying to flesh out a very flexible system at the moment for game development with Andorra, but to do so, I need to override some key procedures that are already overridden...

    Here is the current class, taken from a tutorial and modified very slightly.
    [pascal]
    TSpr = class(TImageSpriteEx)
    protected
    procedure DoMove(timegap:double);override;
    procedure DoDraw;override;
    public
    procedure onDraw;virtual;

    col:boolean;
    xspeed,yspeed: integer;
    end;
    [/pascal]
    The thing is, I have other units that create instances of TSpr, but I'd like to override DoDraw again, for example.

    Instantiating a TSpr:
    [pascal]
    spr := TSpr.Create(engine);
    with spr do
    begin
    Image := gui.Items[1];
    X := 0;
    Y := 0;
    CollisionTester := ColTest;
    end;
    [/pascal]
    What I would like to do would be something along the lines of
    [pascal]
    spr := TSpr.Create(engine);
    elGUI.add(spr);
    with spr do
    begin
    Image := gui.Items[1];
    X := 0;
    Y := 0;
    CollisionTester := ColTest;
    DoDraw := SecondOverride;
    end;

    procedure SecondOverride;
    begin
    Color := rgb(200,200,200);
    end;
    [/pascal]
    The idea is that there would be one main "object" class, where I can specify which functions handle collisions, drawing, and so on, while also having that main object track some variables such as the current x and y speeds.

    With my hacked up idea above, a few things:
    1. Is that even possible?
    2. If that code was even going to work, wouldn't I have to make a class of TSpr, and change it to procedure TSpr.SecondOverride; ?
    3. PS: Color is a property of TImageSpriteEx, and thus of TSpr, so the thing I'd hope is that if I was able to override this function again, it would realize that Color was a property of TSpr which is a property of TImageSpriteEx, so I'd be able to throw that variable in there and have it affect the image as expected.

    The other idea that I had, that I've now thrown out would be this:
    (Forward declaration of DoDraw of TSpr, how it currently is)
    [pascal]
    procedure TSpr.DoDraw;
    begin
    if not col then
    Color := rgb(200,200,200);
    inherited;
    Color := rgb(255,255,255);
    end;
    [/pascal]
    I was thinking (and tried) doing this:
    [pascal]
    procedure TSpr.DoDraw;
    begin
    onDraw;
    inherited;
    end;

    procedure TSpr.onDraw;
    begin
    if not col then
    Color := rgb(200,200,200);
    Color := rgb(255,255,255);
    end;
    [/pascal]
    But this resulted in the TSpr not being visible at all.

    Still learning! Thanks for your patience.

  2. #2

    Re: Overriding Twice

    onDraw approach is propably what you are after. I'll try show you the details.

    [quote=dazappa ]
    [pascal]type
    TDrawProcedure = procedure();
    // add "of object" at end if you want to define procedure in a class
    // and you propably do want.. so onDraw knows the sprite's variables

    TSpr = class(TImageSpriteEx)
    protected
    procedure DoMove(timegap:double); override;
    procedure DoDraw; override;
    public
    onDraw: TDrawProcedure; // It is a pointer to procedure

    col:boolean;
    xspeed,yspeed: integer;
    end;
    [/pascal]

    [pascal]
    spr := TSpr.Create(engine);
    elGUI.add(spr);
    with spr do
    begin
    Image := gui.Items[1];
    X := 0;
    Y := 0;
    CollisionTester := ColTest;
    onDraw := SecondOverride;
    end;

    procedure SecondOverride;
    begin
    Color := rgb(200,200,200);
    end;
    [/pascal]

    [pascal]
    procedure TSpr.DoDraw;
    begin
    if assigned(onDraw) then onDraw;
    inherited;
    end;
    [/pascal]

  3. #3

    Re: Overriding Twice

    Hi, yes I would like SecondOverride to know of the sprite's variables, so I defined it as procedure of object. However, I'm fairly sure it needs tweaking to make it work with procedure of object, as just changing it to a procedure of object gives a compilation error (which I was not able to solve with trusty Google): "Main.pas(134,26) Error: Incompatible types: got "DecondOverride;" expected "<procedure variable type of procedure of object;Register>"" Where the line contains the onDraw:=SecondOverride;

    The code you did provide so far is compilable, however. It's just not useful if I have no access to the variables .

    - Thanks for the help so far.

  4. #4

    Re: Overriding Twice

    Object procedures are defined in the class like mentioned.

    so Not this:
    [pascal]procedure SecondOverride;
    begin
    Color := rgb(200,200,200);
    end;[/pascal]

    but this works:
    [pascal]type
    TMySprite = class(TSpr)
    public
    procedure SecondOverride; // It's defined within class, not in unit functions
    end;

    procedure TMySprite.SecondOverride;
    begin
    Color := rgb(200,200,200);
    end;
    ...
    spr := TMySprite.Create(engine);
    with spr do
    begin
    onDraw := SecondOverride;
    end;
    [/pascal]

  5. #5

    Re: Overriding Twice

    Hm, I was in the middle of doing that too. (I wasn't sure if you could have a class of a class, but wanted to try because it made sense for this)

    When I do assign SecondOverride (through "spr.onDraw := TMySprite.secondOverride;"), it seg faults on that line where it sets the color variable... to narrow it down, I just used another variable, "Alpha := 255;" to make sure the rgb function wasn't the cause, but it seems that any variable of the parent's parent (TImageSpriteEx) causes the crash. I believe I can set the parent's variables just fine (like xspeed)

    Is that typical for this type of situation? I believe I could devise some sort of dirty workaround, but just wanted to double check.

    I believe this one is my own fault, although I'm not sure exactly why setting the Alpha/Color variables in certain areas causes the seg fault, but doesn't in others (If I set Alpha in the actual DoDraw event, and not a function called from it, it works, as well as setting the Alpha during Andorra's drawing procedure)... but this is probably more on the side of an Andorra tweak now.

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