How to see if two shapes overlap

Question

I'm trying to write a simple firemonkey test app.

I have a form, with a panel (align:= alClient).
On the form are 2 TCircle's. I have set TCircle.Dragmode:= dmAutomatic.

I would like to drag the circles around and have something happen when the circles overlap.
The question is: I don't see any method in TCircle called overlap, nor do I see an event called on overlap. I've tried all the xxxxDrag events, but that does not help me with the hittesting.

How can I see when a shape being dragged overlaps with another shape ?
I was expecting one of the DragOver, DragEnter events to detect this for me, but that does not seem to be the case.

Surely there must be some standard method for this in Firemonkey?

For now the pas file just looks like:

implementation

{$R *.fmx}

procedure TForm8.Circle1DragEnter(Sender: TObject; const Data: TDragObject;
  const Point: TPointF);
begin
  if Data.Source = Circle1 then Button1.Text:= 'DragEnter';

end;

procedure TForm8.Circle1DragOver(Sender: TObject; const Data: TDragObject;
  const Point: TPointF; var Accept: Boolean);
begin
  if (Data.Source = Circle2) then Button1.Text:= 'Circle2 drag';
end;

procedure TForm8.Circle2DragEnd(Sender: TObject);
begin
  Button1.Text:= 'DragEnd';
end;

procedure TForm8.Circle2DragEnter(Sender: TObject; const Data: TDragObject;
  const Point: TPointF);
begin
  Button1.Text:= 'DragEnter';
end;

procedure TForm8.Circle2DragLeave(Sender: TObject);
begin
  Button1.Text:= 'DragLeave';
end;

procedure TForm8.Circle2DragOver(Sender: TObject; const Data: TDragObject;
  const Point: TPointF; var Accept: Boolean);
begin
  if Data.Source = Circle2 then begin

    Button1.Text:= 'DragOver';
    Accept:= true;
  end;
end;

The dfm looks something like this:

object Form8: TForm8
  Left = 0
  Top = 0
  BiDiMode = bdLeftToRight
  Caption = 'Form8'
  ClientHeight = 603
  ClientWidth = 821
  Transparency = False
  Visible = False
  StyleLookup = 'backgroundstyle'
  object Panel1: TPanel
    Align = alClient
    Width = 821.000000000000000000
    Height = 603.000000000000000000
    TabOrder = 1
    object Button1: TButton
      Position.Point = '(16,16)'
      Width = 80.000000000000000000
      Height = 22.000000000000000000
      TabOrder = 1
      StaysPressed = False
      IsPressed = False
      Text = 'Button1'
    end
    object Circle1: TCircle
      DragMode = dmAutomatic
      Position.Point = '(248,120)'
      Width = 97.000000000000000000
      Height = 105.000000000000000000
      OnDragEnter = Circle1DragEnter
      OnDragOver = Circle1DragOver
    end
    object Circle2: TCircle
      DragMode = dmAutomatic
      Position.Point = '(168,280)'
      Width = 81.000000000000000000
      Height = 65.000000000000000000
      OnDragEnter = Circle2DragEnter
      OnDragLeave = Circle2DragLeave
      OnDragOver = Circle2DragOver
      OnDragEnd = Circle2DragEnd
    end
  end
end

Answer 1

The general problem is difficult and known as collision detection - you can google the term to find the related algorithms.

The particular case of circles collision detection is easy - just calculate a distance between the centers of the circles. If the distance obtained is less than the sum of the circle's radii, the circles overlap.