HFSM2
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Quick Tutorial
1. Include the C/C++ headers
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#include <assert.h>
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2. Configure optional HFSM2 functionality using
#defines
(in this case we're using Plans to make transition cycle more straightforward):1
#define HFSM2_ENABLE_PLANS
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#include <hfsm2/machine.hpp>
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4. Define interface class between the state machine and its host
(also ok to use the host object itself):
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struct Context {
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bool powerOn;
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};
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5. (Optional) Define type config:
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using Config = hfsm2::Config::ContextT<Context>;
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6. (Optional, recommended) Define
hfsm2::Machine for convenience:1
using M = hfsm2::MachineT<Config>;
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7. Declare state machine structure.
States need to be forward declared, e.g. with a magic macro:
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#define S(s) struct s
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using FSM = M::PeerRoot<
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S(Off), // initial top-level state
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M::Composite<S(On), // sub-machine region with a head state (On) and and 3 sub-states
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S(Red), // initial sub-state of the region
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S(Yellow),
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S(Green)
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>,
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S(Done)
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>;
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#undef S
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8. (Optional) While HFSM2 transitions aren't event-based, events can be used to have FSM react to external stimuli:
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struct Event {};
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9. Define states and override required state methods:
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struct Off
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: FSM::State
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{
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void entryGuard(FullControl& control) { // called before state activation, use to re-route transitions
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if (control.context().powerOn) // access shared data
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control.changeTo<On>(); // initiate a transition into 'On' region
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}
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};
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struct On
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: FSM::State
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{
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void enter(PlanControl& control) { // called on state activation
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auto plan = control.plan(); // access the plan for the region
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plan.change<Red, Yellow>(); // sequence plan steps, executed when the previous state succeeds
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plan.change<Yellow, Green>();
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plan.change<Green, Yellow>();
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plan.change<Yellow, Red>();
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}
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void exit(PlanControl& /*control*/) {} // called on state deactivation
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void planSucceeded(FullControl& control) { // called on the successful completion of all plan steps
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control.changeTo<Done>();
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}
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void planFailed(FullControl& /*control*/) {} // called if any of the plan steps fails
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};
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struct Red
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: FSM::State
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{
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void update(FullControl& control) { // called on periodic state machine updates
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control.succeed(); // notify successful completion of the plan step
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} // plan will advance to the 'Yellow' state
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};
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struct Yellow
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: FSM::State
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{
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void update(FullControl& control) {
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control.succeed(); // plan will advance to the 'Green' state on the first entry
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// and 'Red' state on the second one
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}
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};
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struct Green
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: FSM::State
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{
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void react(const Event&, FullControl& control) { // called on external events
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control.succeed(); // advance to the next plan step
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}
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};
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struct Done
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: FSM::State
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{};
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10. Write the client code to use your new state machine:
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int main() {
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11. Create context and state machine instances:
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Context context;
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context.powerOn = true;
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FSM::Instance fsm{context};
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assert(fsm.isActive<On>()); // activated by Off::entryGuard()
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assert(fsm.isActive<Red>()); // On's initial sub-state
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12. Call
FSM::update() for the FSM to process transitions:1
fsm.update();
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assert(fsm.isActive<Yellow>()); // 1st setp of On's plan
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fsm.update();
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assert(fsm.isActive<Green>()); // 2nd setp of On's plan
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13. (Optional) Event reactions also cause transitions to be processed:
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fsm.react(Event{});
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assert(fsm.isActive<Yellow>()); // 3rd setp of On's plan
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14. Keep updating the FSM for as long as necessary:
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fsm.update();
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assert(fsm.isActive<Red>()); // 4th setp of On's plan
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fsm.update();
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assert(fsm.isActive<Done>()); // activated by On::planSucceeded()
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return 0;
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}
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