[SOLVED] How detect if i'm moving mouse to left, right, top or bottom inside TImage component on mousemove event?

How detect if i'm moving mouse to left, right, top or bottom inside TImage component on mousemove event?

I want know how detect to what side i'm moving mouse: to left, right, top, bottom inside TImage component on mousemove event?

Thank you.

Solution

Here's an example to be used in an FMX project. For a VCL project, you would use integer variables.

First, declare two variables Xold, Yold: single; for example in the private section of the form.

private
  Xold, Yold: Single;

Initialize these variables e.g. in the forms OnCreate() event. Using NaN requires System.Math in the uses clause.

procedure TForm5.FormCreate(Sender: TObject);
begin
  Xold := NaN;
  Yold := NaN;
end;

Then, in the OnMouseMove() event, calculate the movement horizontally and vertically, negative value indicate moving left or up, positive right or down.

procedure TForm5.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Single);
var
  horz, vert: Single;
begin
  if not IsNan(Xold) then horz := X - Xold else horz := 0;
  if not IsNan(Yold) then vert := Y - Yold else vert := 0;
  Xold := X; // save new values
  Yold := Y; //
  // use horz and vert as needed
  Label1.Text := Format('h: %f, v: %f',[horz, vert]);
end;

You may also want to reset the Xold and Yold variables to NaN when the mouse leaves the image.

procedure TForm5.Image1MouseLeave(Sender: TObject);
begin
  Xold := NaN;
  Yold := NaN;
end;

It was asked in comments, why initialize to NaN instead of just zero? Xold := 0; Yold := 0 is the top-left corner. If the mouse entry to the image happens at e.g. right side, the first move would be a jump from 0 to image width. Using NaN we can omit the first entry as a move and just store the entry point in Xold and Yold for use with next move.