If you need to execute some long running task on the background in Unity, you may find this tool I just wrote, useful. This is BackgroundWorker for Unity 3D

BackGround Worker for Unity3D

You will only need the class BackgroundWorker

It worth noting that a lot of seemingly background tasks can be implemented with Unity Coroutines. For example using WWW service to send or receive data from a remote server. Or any case where we just wait for a response from a remote computer.

However because coroutines run on the main thread if you really have a long running, not just waiting for response but computing intensely something, you will need to run it on a background thread. Here is how you use this tool:

if (_backgroundWorker != null) _backgroundWorker.CancelAsync();
_backgroundWorker = new BackgroundWorker();

_backgroundWorker.DoWork += (o, a) =>
{
    // executed on background thread
    // do slow running computationaly intense work, check periodically 
    // if (a.IsCanceled) return;
    
    // assign result as 
    // a.Result = a.Argument+"!";
};
_backgroundWorker.RunWorkerCompleted += (o, a) =>
{
    // executed on main thread
    // you can use a.Result
};

_backgroundWorker.RunWorkerAsync(argumetToPassOrNull);

And then on each frame execute:

if (_backgroundWorker != null) _backgroundWorker.Update();

The tool has been tested on Windows, Mac, iPhone, and Android.

This is a sample usage of the BackgroundWorker class.