Inno Setup Checks for Update before installing

Question

I've fixed the error in your code, you had defined 2 InitializeWizard procedures

[Code]
procedure ExitProcess(uExitCode: UINT);
  external 'ExitProcess@kernel32.dll stdcall';

type
  TPositionStorage = array of Integer;

var
  NewInstallerPath: string;
  CompPageModified: Boolean;
  CompPagePositions: TPositionStorage;
  Progress: TOutputProgressWizardPage;

procedure SaveComponentsPage(out Storage: TPositionStorage);
begin
  SetArrayLength(Storage, 15);

  Storage[0] := WizardForm.Height;
  Storage[1] := WizardForm.NextButton.Top;
  Storage[2] := WizardForm.BackButton.Top;
  Storage[3] := WizardForm.CancelButton.Top;
  Storage[4] := WizardForm.ComponentsList.Height;
  Storage[5] := WizardForm.OuterNotebook.Height;
  Storage[6] := WizardForm.InnerNotebook.Height;
  Storage[7] := WizardForm.Bevel.Top;
  Storage[8] := WizardForm.BeveledLabel.Top;
  Storage[9] := WizardForm.ComponentsDiskSpaceLabel.Top;
end;

procedure LoadComponentsPage(const Storage: TPositionStorage;
  HeightOffset: Integer);
begin
  if GetArrayLength(Storage) <> 15 then
    RaiseException('Invalid storage array length.');

  WizardForm.Height := Storage[0] + HeightOffset;
  WizardForm.NextButton.Top := Storage[1] + HeightOffset;
  WizardForm.BackButton.Top := Storage[2] + HeightOffset;
  WizardForm.CancelButton.Top := Storage[3] + HeightOffset;
  WizardForm.ComponentsList.Height := Storage[4] + HeightOffset;
  WizardForm.OuterNotebook.Height := Storage[5] + HeightOffset;
  WizardForm.InnerNotebook.Height := Storage[6] + HeightOffset;
  WizardForm.Bevel.Top := Storage[7] + HeightOffset;
  WizardForm.BeveledLabel.Top := Storage[8] + HeightOffset;
  WizardForm.ComponentsDiskSpaceLabel.Top := Storage[9] + HeightOffset;
end;

procedure CurPageChanged(CurPageID: Integer);
begin
  if CurpageID = wpSelectComponents then
  begin
    SaveComponentsPage(CompPagePositions);
    LoadComponentsPage(CompPagePositions, 300);
    CompPageModified := True;
  end
  else
  if CompPageModified then
  begin
    LoadComponentsPage(CompPagePositions, 0);
    CompPageModified := False;
  end;
end;

procedure DownloadFinished(DownloadPage: TWizardPage);
var
  ErrorCode: Integer;
begin
  MsgBox(ITD_GetString(ITDS_Update_WillLaunch), mbInformation, MB_OK);

  if ShellExec('open', NewInstallerPath, '/updated',
    ExtractFilePath(NewInstallerPath), SW_SHOW, ewNoWait, ErrorCode)
  then
    ExitProcess(1);
end;

function CompareVersions(this, that:string):integer;
var thisField, thatField:integer;
begin
 while (length(this)>0) or (length(that)>0) do begin
   if (pos('.',this)>0) then begin
     //Read the first field from the string
     thisField:=StrToIntDef(Copy(this, 1, pos('.',this)-1),0);
     //Remove the first field from the string
     this:=Copy(this, pos('.',this)+1, length(this));
   end else begin
     thisField:=StrToIntDef(this, 0);
     this:='';
   end;

   if (pos('.',that)>0) then begin
     //Read the first field from the string
     thatField:=StrToIntDef(Copy(that, 1, pos('.',that)-1),0);
     //Remove the first field from the string
     that:=Copy(that, pos('.',that)+1, length(that));
   end else begin
     thatField:=StrToIntDef(that, 0);
     that:='';
   end;

   if thisField>thatField then begin
    result:=1;
    exit;
   end else if thisField<thatField then begin
    result:=-1;
    exit;
   end;
 end;

 result:=0;
end;

procedure InitializeWizard();
var
  downloadPage:TWizardpage;
begin
 itd_init;

 CompPageModified := False;

 //Where the new installer should be saved to, can be anywhere.
 NewInstallerPath:=ExpandConstant('{tmp}\addsmpi.exe');

 {Create our own progress page for the initial download of a small
  textfile from the server which says what the latest version is}
 progress:=CreateOutputProgressPage(ITD_GetString(ITDS_Update_Caption),
    ITD_GetString(ITDS_Update_Description));

 //Create the ITD GUI so that we have it if we decide to download a new intaller version
 downloadPage:=itd_downloadafter(wpWelcome);

 {If the download succeeds, we will need to launch the new installer. The
 callback is called if the download is successful.}
 itd_afterSuccess:=@downloadfinished;

 {If the download of the new installer fails, we still want to give the
  user the option of continuing with the original installation}
 itd_setoption('UI_AllowContinue','1');
end;

function NextButtonClick(curPageID:integer):boolean;
var
 list, line:TStringList;
 newavail:boolean;
 i:integer;
 ourVersion:string;
 checkedSuccessfully:boolean;
 text:string;
begin
 result:=true;
 if curPageID=wpWelcome then begin

   //Are we being called by an updating setup? If so, don't ask to check for updates again!
   for i:=1 to ParamCount do begin
    if uppercase(ParamStr(i))='/UPDATED' then begin
     exit;
    end;
   end;

   //Offer to check for a new version for the user..
   if MsgBox(ITD_GetString(ITDS_Update_WantToCheck), mbConfirmation, MB_YESNO) = IDYES then
    begin
      wizardform.show;
      progress.Show;
      progress.SetText(ITD_GetString(ITDS_Update_Checking),'');
      progress.SetProgress(2,10);
      try
        newavail:=false;

        checkedSuccessfully:=false;
        GetVersionNumbersString(expandconstant('{srcexe}'), ourVersion);

        if itd_downloadfile('http://www.sherlocksoftware.org/innotools/latestver.txt',expandconstant('{tmp}\latestver.txt'))=ITDERR_SUCCESS then begin
          { Now read the version from that file and see if it is newer.
            The file has a really simple format:

            2.0,"http://www.sherlocksoftware.org/innotools/example3%202.0.exe"

            The installer version, a comma, and the URL where the new version can be downloaded.
          }
          list:=TStringList.create;
          try
            list.loadfromfile(expandconstant('{tmp}\latestver.txt'));

            if list.count>0 then begin
              line:=TStringList.create;
              try
                line.commatext:=list[0]; //Break down the line into its components

                if line.count>=2 then begin
                checkedSuccessfully:=true;
                if CompareVersions(trim(line[0]), trim(ourVersion))>0 then begin
                  //Version is newer
                    text:=ITD_GetString(ITDS_Update_NewAvailable);

                    StringChangeEx(text, '%1', ourVersion, true); //"Current version" part of the string
                    StringChangeEx(text, '%2', line[0], true); //"New version" part of the string

                    if MsgBox(text, mbConfirmation, MB_YESNO)=IDYES then begin
                      itd_addFile(trim(line[1]), NewInstallerPath);
                    end;
                end else begin
                  MsgBox(ITD_GetString(ITDS_Update_NoNewAvailable), mbInformation, MB_OK);
                end;
                end;
              finally
                line.free;
              end;
            end;
          finally
            list.free;
          end;
        end;

        if not checkedSuccessfully then begin
          text:=ITD_GetString(ITDS_Update_Failed);
                  StringChangeEx(text, '%1', ourVersion, true);
          MsgBox(text, mbInformation, MB_OK);
        end;
      finally
        progress.Hide;
      end;
    end;
  end;
 end;