How to validate input in a TEdit

Problem/Question/Abstract:

How to validate input in a TEdit

Answer:

Assuming you're using regular TEdit components, during OnExit, you will see irregular behavior from controls if you attempt to change focus at that time. The solution is to post a message to your form in the TEdit's OnExit event handler. This user-defined posted message will indicate that the coast is clear to begin validating input. Since posted messages are placed at the end of the message queue, this gives Windows the opportunity to complete the focus change before you attempt to change the focus back to another control:

unit Unit5;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, StdCtrls, Mask;

const
  {User-defined message}
  um_ValidateInput = wm_User + 100;

type
  TForm5 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    Edit4: TEdit;
    Button1: TButton;
    procedure EditExit(Sender: TObject);
    procedure EditEnter(Sender: TObject);
  private
    Refocusing: TObject;
    {User-defined message handler}
    procedure ValidateInput(var M: TMessage); message um_ValidateInput;
  end;

var
  Form5: TForm5;

implementation

{$R *.DFM}

procedure TForm5.ValidateInput(var M: TMessage);
var
  E: TEdit;
begin
  {The following line is my validation. I want to make sure the first character is a lower case
  alpha character. Note the typecast of lParam to a TEdit}
  E := TEdit(M.lParam);
  if not (E.Text[1] in ['a'..'z']) then
  begin
    Refocusing := E; {Avoid a loop}
    ShowMessage('Bad input'); {Yell at the user}
    TEdit(E).SetFocus; {Set focus back}
  end;
end;

procedure TForm5.EditExit(Sender: TObject);
begin
  {Post a message to myself which indicates it's time to validate the input. Pass the TEdit
  instance (Self) as the message lParam}
  if Refocusing = nil then
    PostMessage(Handle, um_ValidateInput, 0, longint(Sender));
end;

procedure TForm5.EditEnter(Sender: TObject);
begin
  if Refocusing = Sender then
    Refocusing := nil;
end;

end.