[Setup]
AppName=My Program
AppVerName=My Program v.1.2
DefaultDirName={pf}\My Program
LicenseFile=compiler:License.txt

[Files]
Source: compiler:InnoCallback.dll; Flags: dontcopy

[Code]
type
    TProc = procedure(HandleW, msg, idEvent, TimeSys: LongWord);
const
    EM_GETFIRSTVISIBLELINE = $CE; TestLine = 5;
var
    TimerID: LongWord;

function WrapTimerProc(callback:TProc; paramcount:integer):longword; external 'wrapcallback@files:innocallback.dll stdcall';
function SetTimer(hWnd: LongWord; nIDEvent, uElapse: LongWord; lpTimerFunc: LongWord): LongWord; external 'SetTimer@user32.dll stdcall';
function KillTimer(hWnd: LongWord; nIDEvent: LongWord): LongWord; external 'KillTimer@user32.dll stdcall';

procedure OnTimer(HandleW, msg, idEvent, TimeSys: LongWord);
var
    CurrLine: integer;
begin
    CurrLine:= SendMessage(WizardForm.LicenseMemo.Handle, EM_GETFIRSTVISIBLELINE, 0, 0);
    If CurrLine >= TestLine then
      begin
        WizardForm.LicenseAcceptedRadio.Enabled:= True;
        KillTimer(0, TimerID);
      end;
    //WizardForm.Caption:= IntToStr(CurrLine);
end;

procedure InitializeWizard();
begin
    WizardForm.LicenseAcceptedRadio.Enabled:= False;
end;

procedure CurPageChanged(CurPageID: Integer);
var
    pfunc: LongWord;
begin
    if CurPageID = wpLicense then
      begin
        pfunc:= WrapTimerProc(@OnTimer, 4);
        TimerID:= SetTimer(0, 0, 500, pfunc);
      end else
        KillTimer(0, TimerID);
end;

procedure DeinitializeSetup();
begin
    KillTimer(0, TimerID);
end;