Why use property in a class?

Question

I was just wondering about why should I use property in a class instead of "normal" variables (class attributes?). What I mean is this:

TSampleClass = class
  public
    SomeInfo: integer;
end;

TPropertyClass = class
  private
    fSomeInfo: integer;
  public
    property SomeInfo: integer read fSomeInfo write fSomeInfo;
end;

What is the big difference? I know that I can define getter and setter methods for getting or saving the property respectively, but that is possible even without the variable being a "property".

I tried searching for why to use it, but nothing useful came up, so I'm asking here.

Thank you

Answer 1

This is just a very simple example of a specific case, but still, it is a very common case.

If you have a visual control, you might need to repaint the control when you change a variable/property. For instance, let's say your control has a BackgroundColor variable/property.

The simplest way of adding such a variable/property is to let it be a public variable:

TMyControl = class(TCustomControl)
public
  BackgroundColor: TColor;
...
end;

And in the TMyControl.Paint procedure, you paint the background using the value of the BackgroundColor. But this doesn't do it. Because if you change the BackgroundColor variable of an instance of the control, the control doesn't repaint itself. Instead, the new background colour will not be used until the next time the control redraws itself for some other reason.

So you have to do it like this:

TMyControl = class(TCustomControl)
private
  FBackgroundColor: TColor;
public
  function GetBackgroundColor: TColor;
  procedure SetBackgroundColor(NewColor: TColor);
...
end;

where

function TMyControl.GetBackgroundColor: TColor;
begin
  result := FBackgroundColor;
end;

procedure TMyControl.SetBackgroundColor(NewColor: TColor);
begin
  if FBackgroundColor <> NewColor then
  begin
    FBackgroundColor := NewColor;
    Invalidate;
  end;
end;

and then the programmer using the control has to use MyControl1.GetBackgroundColor to obtain the colour, and to use MyControl1.SetBackgroundColor to set it. That's awkward.

Using properties, you can have the best of both worlds. Indeed, if you do

TMyControl = class(TCustomControl)
private
  FBackgroundColor: TColor;
  procedure SetBackgroundColor(NewColor: TColor);
published
  property BackgroundColor: TColor read FBackgroundColor write SetBackgroundColor;
end;

...

procedure TMyControl.SetBackgroundColor(NewColor: TColor);
begin
  if FBackgroundColor <> NewColor then
  begin
    FBackgroundColor := NewColor;
    Invalidate;
  end;
end;

then

• from the programmer's point of view, he can both read and set the background colour using a single identifier, the MyControl1.BackgroundColor property, and
• the control is repainted when he sets it!