提升状态图：Return 到上一个状态

Question

我有一个状态机，如果我进入一个特定的状态，有时我需要定期转换到另一个状态，而其他时候我需要 return 到之前的状态。

例如，有状态 A B C，假设转换 S 将状态 A 移动到 C，然后从 B 移动到 C。我需要转换 T 将 C 移动到 A，当 S 发生在状态 A 中时，C 移动到 B。在状态 B.

在下面的代码中，转换 S 发生在状态 B 中，因此我希望转换 T 到 return 到状态 B（而目前，它是 return 到状态 A）。

#include <boost/mpl/list.hpp>

#include <boost/statechart/state_machine.hpp>
#include <boost/statechart/simple_state.hpp>
#include <boost/statechart/event.hpp>
#include <boost/statechart/transition.hpp>

// states
struct A;
struct B;
struct C;
struct D;

// events
struct S : boost::statechart::event<S> {};
struct T : boost::statechart::event<T> {};
struct U : boost::statechart::event<U> {};

// fsm
struct FSM : boost::statechart::state_machine<FSM, B> {};

// fully defined states/transitions
struct A : boost::statechart::simple_state<A, FSM> {
    typedef boost::statechart::transition<S, C> reactions;

    A() { std::cout << "entered A" << std::endl; }
};

struct B : boost::statechart::simple_state<B, FSM> {
    typedef boost::statechart::transition<S, C> reactions;

    B() { std::cout << "entered B" << std::endl; }
};

struct C : boost::statechart::simple_state<C, FSM> {
    typedef boost::mpl::list<
                boost::statechart::transition<T, A>,
                boost::statechart::transition<T, B>,
                boost::statechart::transition<U, D> > reactions;

    C() { std::cout << "entered C" << std::endl; }
};

struct D : boost::statechart::simple_state<D, FSM> {
    D() { std::cout << "entered D" << std::endl; } 
};

int main() {
    FSM fsm;

    fsm.initiate();

    fsm.process_event(S());
    fsm.process_event(T());
    fsm.process_event(S());
    fsm.process_event(U());

    return 0;
}

以上代码returns:

entered B
entered C
entered A
entered C
entered D

我想改为查看：

entered B
entered C
entered B
entered C
entered D

有没有什么干净的方法可以使用 Boost::Statechart 来做到这一点？

Answer 1

我找到了一种~好~的方法，方法是为状态创建一个枚举映射，将先前的状态存储在最外层的上下文（顶级 fsm）中，然后对 T 事件使用自定义反应:

#include <boost/mpl/list.hpp>

#include <boost/statechart/state_machine.hpp>
#include <boost/statechart/simple_state.hpp>
#include <boost/statechart/event.hpp>
#include <boost/statechart/transition.hpp>
#include <boost/statechart/custom_reaction.hpp>

// states
struct A;
struct B;
struct C;
struct D;

// state enum mapping
enum class state_mapping {
    A = 0,
    B,
    C,
    D
};

// events
struct S : boost::statechart::event<S> {};
struct T : boost::statechart::event<T> {};
struct U : boost::statechart::event<U> {};

// fsm
struct FSM : boost::statechart::state_machine<FSM, B> {
    state_mapping previous_state = state_mapping::B;
};

// fully defined states/transitions
struct A : boost::statechart::simple_state<A, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    A() { std::cout << "entered A" << std::endl; }
    virtual ~A() { outermost_context().previous_state = state_mapping::A; }
};

struct B : boost::statechart::simple_state<B, FSM> {
    typedef boost::statechart::transition<S, C> reactions;
    
    B() { std::cout << "entered B" << std::endl; }
    virtual ~B() { outermost_context().previous_state = state_mapping::B; }
};

struct C : boost::statechart::simple_state<C, FSM> {
    typedef boost::mpl::list<
                boost::statechart::custom_reaction<T>,
                boost::statechart::transition<U, D> > reactions;
                
    C() { std::cout << "entered C" << std::endl; }
    
    boost::statechart::result react(const T&) {
        
        switch(outermost_context().previous_state) {
            
        case state_mapping::A:
            return transit<A>();
        case state_mapping::B:
            return  transit<B>();
        default:
            return discard_event();
            
        }
        
    }
};

struct D : boost::statechart::simple_state<D, FSM> {
    D() { std::cout << "entered D" << std::endl; }
};

int main() {
    FSM fsm;
    
    fsm.initiate();
    
    fsm.process_event(S());
    fsm.process_event(T());
    fsm.process_event(S());
    fsm.process_event(U());

    return 0;
}

提升状态图：Return 到上一个状态

Boost Statechart: Return to Previous State

c++

boost

boost-statechart