State machine crashing

[sulemankm]

Hi.

I'm trying to send some data to my QP application using a control on QView and QSPY. Based on the received data I call the isIn() function of the QHsm to check if the it is in a particular state, before performing an action on the input data. However, the state-machine is crashing on me. When I remove the call to the isIn() function, it no longer crashes.

I came across the following note in the online QP documentation regarding the isIn() function:

Attention
This function must be called only on a state machine that is in the "stable state configuration". Among others, this means that the state machine cannot call it in the middle of its own transition.

My question is, how can we ensure that we don't call the isIn() function during a transition?

[quantum-leaps]

As you already noticed in the documentation, it makes no sense for a state machine to ask itself "Am I in state XYZ?" (using the isIn() query). If any action depends on the state, it should be placed in that state. This is the whole purpose of a state machine... It's like you were asking yourself "Am I in the awake state or the sleeping state?". If you're asking such a question, then probably you're awake! I hope you get the point.

Frankly, something like isIn() should not exist at all. But there is one compelling use case, and that is the "Orthogonal Component" state pattern. In that pattern, the isIn() function can be legitimately used to query the state of the other state machines (the Components).

My question is, how can we ensure that we don't call the isIn() function during a transition?

It is difficult to forbid applications from doing something that makes no sense. For example, how can you ensure that we can't divide by zero, dereference a NULL pointer, or index an array out of bounds? Well, you can't. The best you can do is to assert inside the framework when it detects something illegal. And that's precisely what QP Framework did in this case. You call it "crashing", but this was an assertion, which is very different from a "nebulous crash". An assertion shows you immediately what's wrong so that you can correct it. A "nebulous crash" leaves you completely in the dark.

I hope that these comments make sense to you.

--MMS

P.S.
Please consider posting future questions like that to the QP/QM Free Support Forum, where the discussion can benefit the whole QP/QM community.

[sulemankm]

OK, thanks.