How can a parallel region change state of the other region in UML state machine diagram
I have a state machine diagram for an application that has a UI and a thread that runs parallel. I model this using a state with 2 parallel regions. Now I want when that thread gives some message, for example an error, that the state of the UI changes, for example to an error screen state.
I was wondering what the correct method to do this is, as I was not able to find any example of this situation online.
The options that I thought of were creating a transition from every state to this error state when the error state is called, which in a big diagram would mean a lot of transitions. Or by just creating a transition from the thread to the error state (As shown in the image), which could possible be unclear.
Example of second option
You don't need a transition from every state. Just use a transition from the State State to the Error window state. It will then fire no matter what states are currently active.
Such a transition will also leave the Some process state. Since you didn't define initial states in the orthogonal regions, the right region of the state machine will then be in an undefined state. Some people allow this to happen, but for me it is not good practice. So, I suggest to add initial states to both regions. Then it is clear, which sub state will become active, when the State state is entered for any reason. The fork would also not be necessary then.
How you have done it in your example would also be Ok. However, since the Some process state is left by this transition, this region is now in an undefined state. Again, the solution to this is to have an initial state here. I would only use such a transition between regions, when it contains more than one state and the On error event shall not trigger a transition in all of the states.
If the Some process state is only there to allow for concurrent execution, there is an easier way to achieve this: The State state can have a do behavior, that is running while the sub states are active. Orthogonal regions should be used wisely, since they add a lot complexity.