Inno Setup: Download setup .bin slice files before installation starts

I am trying to create a distributable .exe file with Inno Setup tools and Inno Download Plugin. The resulting file is ~3GB in size, split in 6 parts (1 for the executable, 5 bins containing all the files).

Would it be possible to keep the 5 bins uploaded on some server and download them during installation with the remaining executable file?

My code is here :

procedure InitializeWizard();
var
  ResultCode: integer;
  TempAddress: String;
  FinalSavePath: String;
  UserName, UserCompany: String;
begin
  idpSetOption('DetailedMode',  '1');
  idpSetOption('AllowContinue', '1');

  idpSetLogin('aaa', 'aaa');

  idpAddFile('https://...', target_path);
  idpAddFile('https://...', target_path);
  idpAddFile('https://...', target_path);
  idpAddFile('https://...', target_path);
  idpDownloadAfter(wpWelcome);
end;

With idpDownloadAfter(wpWelcome) the installer starts downloading right after accepting to run the executable, if .bin files are already present. If not, the installer just keeps asking for the .bin to be present.

Inno Setup 6.1 has a built-in support for file downloads, which does not need any support files. So the solution below is obsolete now. See Inno Setup: Install file from Internet.

Inno Download Plugin uses idp.dll, which itself is stored in the mysetup-*.bin files. That's why you get prompted for the .bin files, even before anything starts. You need the idp.dll so that the download itself can start.

With some hacking you can have the idp.dll be stored in the [Code], hence directly in the mysetup.exe.
See Inno Setup: Reading a file from installer during uninstallation.

You will need to modify the idp.iss as follows:

• Remove the [Files] section with its reference to idp.dll.
• In all the external functions declarations:
  change @files:idp.dll cdecl to @{tmp}\idp.dll cdecl delayload.

To the front of your .iss script, copy the long code block from my answer to the previously mentioned question.

And now you can do:

procedure InitializeWizard();
begin
  SaveBinaryStringToFile(
    ExpandConstant('{tmp}\idp.dll'), {#FileToBinaryString("unicode\idp.dll")});

  idpAddFile(
    'https://www.example.com/mysetup-1.bin', ExpandConstant('{src}\mysetup-1.bin'));

  idpDownloadAfter( {whatever} );
end;

Make sure you update the path to the idp.dll to the correct location on your development machine in the call to FileToBinaryString.

The code is for Unicode version of Inno Setup (the only version as of Inno Setup 6).