How to pass parameters between states of a QStateMachine

neterror

Hello,

I was wondering, what is the proper way of passing parameters between states in a QStateMachine. The transitions between states are triggered by signals, but the signal parameters are not passed to the other QState - only entered() is emitted without options to specify some input data.
One way I can think to solve the problem is to make QSignalTransition subclass, take the signal parameters in onTransition(QEvent* event) and set them to the targetState() using assignProperty() or some custom method of QState descendant.

Is there some default solution from QStateMachine framework which I am missing?

SGaist

Hi,

What kind of parameter are you thinking about ? On what kind of object would they work ?

neterror

QByteArray parameters. My task is to implement a network protocol working over TCP. I have a class SendMessage:public QState that wraps the state machine for sending a single message (transitions of the QTcpSocket for bytesWritten(), readyRead(), timeout()). I want to combine these states into higher level protocol state machine. When it leaves a state and enters another state it should pass what data it has received.

SGaist

Might be easier to have a central object that handles the network part and your state machine acts on it so you would be able to retrieve the information needed from that "central point"

neterror

Thanks for your feedback.
I was trying to think of a more general solution that I may easily specialise to the different network sessions that I have to implement. I think that the result from one state to be passed as input to the next state as part of the transition is a more natural flow of the state machine (without some global state maintenance). I can implement it with by specialising QSignalTransition, I was just curious if there wasn't something ready made.

frankiefrank

If anyone else worked on this aspect of state machines, would appreciate input - I'm wondering about the same issue.

neterror

For my problem, I did the following:
Created a subclass of QState, that implements the virtual method QState::onEntry(...) where it takes the parameter from the signal (QByteArray in my case) and keeps it in a local member:

@
class ParamState : public QState
{
public:
QByteArray data() {return mData;}
protected:
virtual void onEntry(QEvent* e);
QByteArray mData;
};

void ParamState::onEntry(QEvent* e)
{
if (e->type() == QEvent::StateMachineSignal) {
QStateMachine::SignalEvent* se = static_castQStateMachine::SignalEvent*(e);
if (se->arguments().size() > 0) {
QVariant arg = se->arguments().at(0);
if (arg.canConvert(QMetaType.QByteArray))
mData = arg.toByteArray();
}
}
}
@

Each state in my state machine is ParamState instance. In the slots for the QState::entered signal I use the following:
@
ParamState* state = (ParamState*)sender();
state->data() ... <--- the QByteArray that was passed to the state via the transition signal
@

frankiefrank

Thanks, this is a helpful example!