Event structure

Why doesn't the event structure register local variable changes?

The main reason for not sending events for programmatic value changes is to avoid feedback.

A happens. In responding to A, you update B and C. If responding to B or C results in changing A, then you have feedback and your code will behave like a dog chasing its tail. Sometimes the feedback will die out because the value set will match what is already there, but often it will continue indefinitely in a new type of infinite loop.

One solution to this is what is called User Events. You define what data they carry and when they fire. Then you either have the value change and the set local both fire the user event, or you can combine the event diagram to handle both and just fire the event when writing to the locals. Today, you can accomplish this with the queued state machine using the state machine to do all the common work and just having the event structure pass things to the queue.

Locking The Panel

Given the synchronized mechanism of events, it is pretty easy to repost events to another queue or turn off locking and synchronization. If the event structure isn't synchronized, it would be impossible for the diagram to add it and become synchronized with the UI, so it is at least necessary for event diagrams to be able to lock the panel.

Should it be the default?

In our opinion, yes. When responding to an event, it is pretty common to enable/disable or show/hide some other part of the display. Until you finish doing this, it is wrong for LV to process user clicks on those controls, and LV doesn't know which controls you are changing until you are finished.

Additionally, it isn't the best idea, but what happens when the event handler does something expensive like write lots of stuff to a database inside the event structure? If the UI is locked, then the user's events don't do much, ideally the mouse is made to spin and this works the same as a C program. The user is waiting for the computer, and the UI more or less tells the user to be patient.

If the UI isn't locked, the user can change other things, but you can't execute another frame of your event structure until this one is finished. This is a node. It must finish and propagate data before it can run again, and the loop it is probably in can't go to the next iteration until it completes. You would have low level clicks being interpretted with the current state of the controls before the diagram has a chance to respond. This is sometimes the case, so it is possible to turn off event handling on the cases where you know that you may take awhile and you do not affect the state of the UI.

If you need to respond to the events in parallel, you can make a parallel loop, add an event handler for the other controls, and handle them there while this one churns away. That will work fine and it is IMO clear from the diagram what is synchronized and what is parallel. Taken to an extreme, each control has its own loop and this approach stinks, but it is a valid architecture. Note that for this to work well, you need to turn off the UI lock or have a node to release it.

Another way of doing expensive tasks is to have the Event Structure do the minimum amount necessary before unlocking -- treat them like interrupts. Have the event structure repost expensive operations to a parallel loop or fire up an asynchronous dynamic VI. Now your event structure is free to handle events, your UI is live, LV is still a nice parallel-friendly language, and your diagram just needs to keep track of what parallel tasks it has going on.

In the end, I'm not sure I can convince you, but if you continue to experiment with the different architectures that can be built using the Event Structure, I think you will come to agree that it normally doesn't matter whether it is locked or not. There are times where it is really nice that it is locked, and occasionally you may turn off locking so that the user can do additional UI things up to the point where synchronization is necessary again. For correctness, we decided that locking should be the default.

I'd suggest reading the article on devzone sooner or later. It is in the Developer Insights/LabVIEW Guru section, and it will help start you down the right path.