How to create Multi Producer-Single Consumer concurrency

[adnan338]

I have a list of files to download from the internet. Each of 
those downloads are a time consuming task.

What I want to do is download them concurrently and when each of 
those downloads finish, I want to activate something in my main 
thread, which will update a progressbar in the GUI thread.

So there are multiple "download finished" message producers, and 
one consumer of those messages. Furthermore, that producer has a 
callback that triggers an UI object.

For example,

auto list = [...]; // list of URLs
auto downloadableItems = list.length;
__gshared int downloaded = 0;
auto progressBar = // a gtk progressbar;

How should I proceed? I can do a parallel foreach and call 
download() on each of the URL and update `downloaded`, but I 
don't know how to listen to `downloaded` when it gets updated so 
I can make changes in `progressBar`

Not that my UI library is not thread safe and I cannot access UI 
object from different threads.