[Interest] QStateMachine as C++ (or Python) backbone of QML app

[Elvis Stansvik]

2016-05-07 22:12 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
> 2016-05-07 21:39 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
>> Hi all,
>>
>> Back in 2010 there were a couple of interesting threads on this list
>> regarding the use of state machines:
>>
>>     http://comments.gmane.org/gmane.comp.lib.qt.general/29861
>>     http://comments.gmane.org/gmane.comp.lib.qt.general/36760
>>
>> In the second one, which was about using a C++ QStateMachine based
>> design for the backend logic, and making it available to QML, Sean
>> Harmer describes his approach in a very good post.
>
> To clarify, I'm talking about this answer:
>
>     http://permalink.gmane.org/gmane.comp.lib.qt.general/36869
>

Sorry for replying so much to myself here, but a question about the
approach you show in your trafficlight example:

In the example, you expose a QObject representing the traffic light,
with a single boolean property `powered` which, instead of being
represented by a boolean variable, is represented internally by the
state in which the internal state machine is in (the OnState or
OffState).

This seems very pure so to speak, but I was thinking, could this be
extended so that the QObject is given a general `state` QString
property (since I see you've given the states string names), so that
you could use property bindings like:

    color: trafficLight.state = "On" ? "green" : "red"

?

Is there something "bad" with an approach like that? I find myself
thinking that if I'd use an approach like this, there would be a lot
of boilerplate code to do all posting of events and emitting *Changed
signals on behalf of the QObject for all the states I want to model,
when I would rather be focusing on the state machine (it's logic and
actions) instead.

Hope I'm somewhat clear in what I mean.

Elvis

> Elvis
>
>>
>> Sean, you mentioned back in that thread that you would polish up what
>> you had written and put it up on the Nokia blog (or the wiki?). Do you
>> remember if you ever got around to this, and if so, if that material
>> was lost when Nokia sold Qt? (or if Sean is not here, does anyone else
>> know?).
>>
>> I'm very much interested in real-world examples/stories of using a
>> state machine-based C++ (or Python) backend for the logic, and QML for
>> the UI, especially embedded/hardware control use cases.
>>
>> Cheers,
>> Elvis