Why is the output of Process List getting jumbled?

(Since several folks have asked about essentially the same problem for Process List and Build List, I’m posting this question+answer as a reference. If you’re having trouble with Process List and Build List, make sure you understand “Tip: How to use the Process List node” in https://community.vuo.org/t/-/5088 and this question+answer.)

This composition with a Process List works as expected. It shows a row of rainbow-colored spheres.


But when I add mouse interaction, something’s not right. When I move the mouse, some of the spheres (esp. on the ends of the row) flicker with the wrong color. It looks like the list output by the Process List node’s Processed List port is out of order or has repeated items.


ProcessList-nomouse.vuo (3.43 KB)

ProcessList-bad.vuo (4.29 KB)

The Process List node’s Processed Item input port needs to receive exactly as many events as the Process Item port fired. Too few, and the Processed List port won’t fire. Too many, and the Processed List port will fire a list containing unintended items.

In the composition above, the problem is too many events hitting the Processed Item input port. The extra events are coming from the Receive Mouse Moves node.

To fix the problem, insert a Hold Value node, as in the composition below. The Hold Value node’s New Value port blocks events (it has a wall), so the events from Receive Mouse Moves no longer go through Make HSL Color and Make Sphere to hit the Process List node’s Processed Item port. The Hold Value node’s refresh port allows events through, so the event from the Process List node’s Process Item port can grab the value stored in Hold Value and pass it along to Make HSL Color. (If you’re not familiar with the terms or concepts in this paragraph, see the Vuo Manual under the Vuo Editor’s Help menu.)

In general, there are several ways to troubleshoot problems with Process List/Build List and events. One is just to look carefully at your composition and see if there are any extra cables going into the Process List/Build List node’s feedback loop; if so, do they all have Hold Value or other event-blocking nodes? Another strategy is to temporarily disconnect all cables into the Process List/Build List node’s Fire port, turn on the Show Events button in the toolbar, and run the composition; do any nodes in the feedback loop light up?

ProcessList-good.vuo (4.58 KB)

@jstrecker - Thanks for this!

I had posted this bug report regarding this very issue. Of course, after reading this explanation I was able to go back to my composition and add a few Hold Value nodes to make the issue go away.

Since my bug report no longer appears to report a bug, I’ll go to it and add a comment there that includes a fixed example composition and a link back here. Hopefully that will increase the number of people who come across this helpful Q&A.



Yep, this stuff has been confusing, thanks so much for this.

I thought I had figured out this issue related to my ToriRings composition, but my solution ended up slightly different for the subcomposition version pictured here (Hold nodes in different place), and seems to work fine.

To clarify this concept, I was hoping that the event blocking parentheses between Process Item and the Hold nodes could be my guide for where to insert Hold nodes when I feed things into the feedback loop. Should I also make sure to insert a Hold after the Process Item in this pic? (All hidden cables here are published ports, not shown because it’s messy to look at).

Screen Shot 2016-02-21 at 7.21.37 PM.png

@jersmi, your composition looks OK to me. Looks like all of the extra events potentially coming into the Make Torus node from published input ports are successfully blocked at the Hold Value nodes.