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If you are designing a new Delphi control and you need to catch or limit changes to Left, Top, Width, or Height, there is one simple way to do it. The key to catching changes to Left, Top, Width, Height, and even BoundsRect in Delphi is the SetBounds() method (found in TControl and all descendants). SetBounds() is a virtual function used to set the position and size of a control all in one easy step. However, what they do not tell you in the documentation is that TControl.SetBounds() is called by TControl's SetLeft(), SetTop(), SetWidth(), and SetHeight() property access methods every time a value is assigned to the Left, Top, Width, Height, and BoundsRect properties.

Thus, to catch changes to these properties, simply override the SetBounds() method, do whatever you need with the new values, and then pass them on to the inherited SetBounds() method.

In the following example, I have a control that automatically updates its X and Y custom properties to refer to the center of the control whenever Left, Top, Width, Height or BoundsRect are changed. Conversely, Left and Top will be updated whenever X and Y are changed:

    type
      TMyControl = class(TControl)
      private
        FX, FY: integer;
        {property access methods}
        procedure SetX(value: integer);
        procedure SetY(value: integer);
        { ... }
      public
        procedure SetBounds(aLeft, aTop, aWidth, aHeight: integer); override;
        { ... }
        property X: integer read FX write SetX;
        property Y: integer read FY write SetY;
      end;
    { ... }

    procedure TMyControl.SetX(value: integer);
    begin
      if FX <> value then
        SetBounds(value - Width div 2, Top, Width, Height);
    end;

    procedure TMyControl.SetY(value: integer);
    begin
      if FY <> value then
        SetBounds(Left, value - Height div 2, Width, Height);
    end;

    procedure TMyControl.SetBounds(aLeft, aTop, aWidth, aHeight: integer);
    begin
      {Go ahead and let SetBounds() do its thing...}
      inherited SetBounds(aLeft, aTop, aWidth, aHeight);
      {Now adjust FX and FY according to our new bounds.}
      FX := Width div 2;
      FY := Height div 2;
    end;

Also not mentioned in the documentation is the fact that the FLeft, FTop, FWidth, and FHeight private fields that TControl uses to keep track of its bounding rectangle are not updated in the corresponding SetLeft(), SetTop(), etc. property access methods. These variables, in fact, do not get updated anywhere except in TControl's SetBounds() method (as with FX and FY in the above example).

So, to limit changes to the bounds of your control, you can override SetBounds() to check and modify any of the properties before passing the values on to the inherited SetBounds() method.

In the following example, I have a control that limits its width and height to no more than 100 pixels:

    type
      TMyControl = class(TControl)
        { ... }
      public
        procedure SetBounds(aLeft, aTop, aWidth, aHeight: integer); override;
        { ... }
      end;
    { ... }

    procedure TMyControl.SetBounds(aLeft, aTop, aWidth, aHeight: integer);
    begin
      if aWidth > 100 then
        aWidth := 100;
      if aHeight > 100 then
        aHeight := 100;
      inherited SetBounds(aLeft, aTop, aWidth, aHeight);
    end;