VCL下可以在OnKeyPress事件下这样写

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if not (key in ['0'..'9','.',#8]) then
    key:=#0;
  if (key='.') and (Pos('.',TEdit(Sender).Text)>0)   then  //TEdit(Sender)写法，这段代码可以重用，不必在每个Edit下都写一段代码，其他Edit控件只要引用这个Edit的事件即可
    key:=#0;
end;

XE的FMX里没有OnKeyPress事件，所以在OnKeyDown下写，而Key参数是Word类型，所以要用到KeyChar

procedure TForm1.Edit1KeyDown(Sender: TObject; var Key: Word; var KeyChar: Char; Shift: TShiftState);
var
  i: Boolean;
begin
  i := (KeyChar <= #8) or (KeyChar <= #8) and (KeyChar < #48) or (KeyChar > #57);
  if i then
    KeyChar := #0;

  if not (KeyChar in ['0'..'9', '.', #13]) then
    KeyChar := #0;

  if (KeyChar = '.') and (Pos('.', TEdit(Sender).Text) > 0) then
    KeyChar := #0;
end;