How to make the main (calling) thread wait for a child thread to complete execution?

Question

Using: Delphi 10 Seattle, Win32 VCL forms application

I'm developing an updater application that checks for updates to one or more installed software applications, and when updates are found will download the updates in sequence. After each update is downloaded, it will install the update before proceeding to download the next update. The downloading bit is implemented as a thread class (descendant of TThread) and its constructor is as follows:

constructor TWebFileDownloaderThread.Create(CreateSuspended: Boolean; const AWebFileURL, ALocalFilePath: String;
  ACallBackProc: TProgressCallback; AProxySetting: TProxySetting);
begin
  inherited Create(CreateSuspended);

  FWorkResult := False;

  FWebFileURL := AWebFileURL;
  FProxySetting := AProxySetting;
  FLocalFilePath := ALocalFilePath;

  FUpdateCallbackProc := ACallBackProc;
end;

The main thread creates and starts the downloader thread as follows:

procedure TfmMain.DownloadUpdateFromWeb(const AInstallerFileURL: String);
var
  internet_file_download_thread: TWebFileDownloaderThread;
begin
  internet_file_download_thread := TWebFileDownloaderThread.Create(True, AInstallerFileURL, FUpdateDownloadDir,
    UpdateProgressCallback, FProxySetting);

  internet_file_download_thread.OnTerminate := WebFileDownloaderThread_TerminatedMethod;
  internet_file_download_thread.FreeOnTerminate := True;
  internet_file_download_thread.Start;
end;

My specific question is: How to make the main (calling) UI thread wait until a downloader thread completes, before creating a new downloader thread to start the next download?

I believe that there is some form of queuing required, but not sure how to implement it. Your tips and advice are much appreciated.

You should try as hard as you can to avoid this design. The completion itself should trigger the next job without a thread having to wait for the completion to trigger the next job. — David Schwartz
@DavidSchwartz shouldn't threads be as independent from the 'outer world' as possible? Triggering another download thread seems to be without the responsibility of a download thread to me — mjn
@mjn That's completely the wrong way to think about it. Threads are just the vehicles that run the code. It's code that has responsibility, not threads. Whatever code does the downloading should then go on to do what needs to be done after the downloading without needing a thread to sit around waiting. It makes no sense to create a thread to do work and then wait for that thread to finish -- that extra thread serves no purpose. — David Schwartz
@DavidSchwartz imho this is a task where a 'queue of threads' is a solution, and the main thread is controlling how many threads are executed in parallel (could be depending on CPU load or core count). Mixing the 'global' queue management and 'worker' code (in the thread execute method) then would be bad style imho. — mjn
I am at no time creating all the threads in advance, so no threads remain idle. Read my question. When the download/install thread completes, then I need to create a new thread, and so on, until all the installers from all URLs have been downloaded/installed. — Steve F

Ondrej Kelle Ondrej Kelle · Accepted Answer · 2016-01-07T09:17:36

If I understood it correctly, you have a list of URLs to download and install (obtained by previously performing a check for updates). You want to download updates from these URLs and install them one by one: download update 1, install update 1, download update 2, install update 2, etc.

Here's a possible design:

In your WebFileDownloaderThread_TerminatedMethod, start installing the just downloaded update (to keep the main thread responsive, do this in a separate thread).

In the OnTerminate handler of the installer thread, remove the just completed URL (or mark it as processed) and start downloading the next one, by calling DownloadUpdateFromWeb again, unless the list is already empty (or contains no more unprocessed items).

(BTW, the method DownloadUpdateFromWeb would better be named something like BeginDownloadUpdateFromWeb, to indicate its asynchronous nature.)

How to make the main (calling) thread wait for a child thread to complete execution?

2 Answers