MultiThreading in 3D Studio Max using the BackgroundWorker class

Aug 29, 2008 by     10 Comments    Posted under: 3dsMax, DotNet, Maxscript

You will all be aware that when you perform any intensive calculation process within MaxScript that it pretty much ties up that session of max. The solution to this would be to run this process in a separate thread, much like 3DS Max does with all other operations. However, the system.threading class can be tricky to configure. Luckily the DotNet chaps have provided an alternative that does all the hard work for you – The BackgroundWorker.

You create an instance of this class in MXS via the following method –

MainThread = dotnetobject "System.ComponentModel.BackGroundWorker"

This is a small test that demonstrates use of this class. You will notice that when you run the example via MXS, the 3Dsmax UI will be completely tied up. Via the Dotnet method, you can still use the max interface and do other tasks within a single copy of max. It also supports cancellation so you can abort an intensive task should you need to.

The main thread is called via the DoWork() event – specifying this in MaxScript as a dotnethandler allows you to pass a function to operate as the work thread. So you can pretty much put what you like in here and it will run in a thread separate to the UI. This is obviously better suited to work that isn’t viewport related. It is perfect if you want to perform an intensive calculation without starting a separate instance of 3DS Max.

Updating the UI thread from the Worker Thread

Something to understand in VB/C# with this class is that like using other threading methods you have to use delegates if you want to update a UI that was started in a thread different to the one perform the work. If you attempt this in VB/C# it will throw an exception. The backgroundworker class has this delegate functionality built in, providing an easy way to pass progress (and other) information back to the UI thread. I haven’t implemented it here as it didn’t seem to be necessary for the script to work, although i have included it commented out in the code. In VB/C#, The backgroundworker class allows this to happen via the ProgressChanged handler. If you look in the script this can only report back if you set the object’s workerreportsprogress property to true., without this property set will also result in an exception.

Once started with the runworkerasync() function, The mainthread performs the work function. You choose when to update back to the UI thread via the reportprogress method. This takes two arguments, a percentage integer and a userstate object. So, you can put pretty much anything in this object – i have placed an array with various properties in so i can can the userstate[integer]method to retrieve these in the updatethread function. The ProgressChangedEventArgs therefore contains a progresspercentage property to update a progressbar or similar and the userstate property for anything else. For example I have used this to pass the e.progresspercentage argument to the painthandler of a control and use GDI+ to draw a custom progressbar over a bitmap that is loading. You don’t have to provide a userstate property, the eventarg is overloaded (meaning you can choose which arguments to supply) so that you can just return the progress percentage should you wish.

Insert Update Thread pun here…

Whilst the background worker seems to perform well in a VS environment, I had notived a couple of intermittent synchronisation errors when using a this class within 3dsMax. Following a discussion on CGTalk, the dotnet SDK mentions this issue. Here is what it says –

SynchronizingBackgroundWorker Class

Replaces the standard BackgroundWorker provided by the .NET Framework to fix what seems to be a bug causing the BackgroundWorker to fire events in the wrong thread.We have encountered cases where the BackgroundWorker seems to lose track of the main thread or just arbitrarily decide to fire ProgressChanged and RunWorkerCompleted in a new thread rather than in the main thread. This causes synchronization errors.

This replacement class allows the client to specify a ISynchronizeInvoke synchronizer object through which the events will be fired. All Controls implement ISynchronizeInvoke, so any Control should be adequate to act as a synchronizer to invoke the events in the main thread.

I have updated these classes to run with this component by adding a reference to CsharpUtilities in VS. In max, you can just reference it normally like so –

Worker = DotNetObject "CSharpUtilities.SynchronizingBackgroundWorker"

You don’t need to load the assembly as MaxDoes this automatically at startup.

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