复杂的状态转换：最佳实践

Question

我从事嵌入式工作，并且我有一些管理硬件的软件模块。这个模块有状态，状态转换很复杂：根据事件，模块可以从状态A 到状态B 或者可能到C。但是，当它退出某个状态时，它应该对硬件执行一些操作，以使其也保持正确的状态。

对于相当简单的模块，我只有几个这样的函数：

enum state_e {
    MY_STATE__A,
    MY_STATE__B,
};

static enum state_e _cur_state;

void state_on_off(enum state_e state, bool on)
{
    switch (state){
        case MY_STATE__A:
            if (on){
                //-- entering the state A
                prepare_hardware_1(for_state_a);
                prepare_hardware_2(for_state_a);
            } else {
                //-- exiting the state A
                finalize_hardware_2(for_state_a);
                finalize_hardware_1(for_state_a);
            }
            break;
        case MY_STATE__B:
            if (on){
                //-- entering the state B
                prepare_hardware_1(for_state_b);
                prepare_hardware_2(for_state_b);
            } else {
                //-- exiting the state B
                finalize_hardware_2(for_state_b);
                finalize_hardware_1(for_state_b);
            }
            break;
    }
}

void state_set(enum state_e new_state)
{
    state_on_off(_cur_state, false);
    _cur_state = new_state;
    state_on_off(_cur_state, true);
}

显然，我们需要在 _state_on_off() 函数中为所有状态保留所有必要的操作，当我们需要移动到另一个状态时，我们只需调用 _state_set(new_state) 并且状态转换会顺利进行，与方向无关：所有需要执行操作。

但它只适用于简单的情况。如果我们在状态MY_STATE__B 和MY_STATE__C 之间有一些共同点，那么当状态从MY_STATE__B 更改为MY_STATE__C 并返回时，我们应该只执行缩短的析构/构造？但是当我们进入其他状态（例如，MY_STATE__A）时，我们应该执行完全销毁。

想到的是子状态。所以我们有一个状态MY_STATE__BC，还有像MY_BC_SUBSTATE__B和MY_BC_SUBSTATE__C这样的子状态；当然我们也有它自己的函数，比如_state_bc_on_off()。即使这已经是一种痛苦，但想象一下更复杂的事情：它变得可怕。

那么，这样的最佳做法是什么？

Answer 1

更通用的状态机有

• 原语 -- 对特定硬件执行特定操作的子例程
• 序列 -- 以特定顺序调用的一个或多个原语
• 转换 -- 以特定顺序执行的一个或多个序列

转换被编码在一个结构数组中。序列由switch语句选择，每个序列调用一个或多个原语。

#define stA    0x00000001  // bit mask for state A
#define stB    0x00000002  // bit mask for state B
#define stC    0x00000004  // bit mask for state C
#define stAny  0xffffffff  // matches any state

enum { seqXtoY, seqError, seqEnterA, seqExitA, seqEnterB, seqExitB, seqEnableC, seqDisableC, seqEnd };

typedef struct
{
    int oldState;     // bit mask that represents one or more states that we're transitioning from
    int newState;     // bit mask that represents one or more states that we're transitioning to
    int seqList[10];  // an array of sequences that need to be executed
}
stTransition;

static stTransition transition[] =
{
    // transitions from state A to B or C
    { stA, stB, { seqExitA, seqXtoY, seqEnterB, seqEnd } },
    { stA, stC, { seqExitA, seqXtoY, seqEnableC, seqEnterB, seqEnd } },

    // transitions from state B to A or C
    { stB, stA, { seqExitB, seqXtoY, seqEnterA, seqEnd } },
    { stB, stC, { seqXtoY, seqEnableC, seqEnd } },

    // transitions from states C to A or B
    { stC, stA, { seqDisableC, seqExitB, seqXtoY, seqEnterA, seqEnd } },
    { stC, stB, { seqDisableC, seqXtoY, seqEnd } },

    // any other transition (should never get here)
    { stAny, stAny, { seqError, seqEnd } }
};

static int currentState = stA;

void executeSequence( int sequence )
{
    switch ( sequence )
    {
        case seqEnterA:
            prepare_hardware_1(for_state_a);
            prepare_hardware_2(for_state_a);
            break;

        case seqExitA:
            finalize_hardware_2(for_state_a);
            finalize_hardware_1(for_state_a);
            break;

        case seqEnterB:
            prepare_hardware_1(for_state_b);
            prepare_hardware_2(for_state_b);
            break;

        case seqExitB:
            finalize_hardware_2(for_state_b);
            finalize_hardware_1(for_state_b);
            break;

        case seqEnableC:
            enable_hardware_3();
            break;

        case seqDisableC:
            disable_hardware_3();
            break;
    }
}

void executeTransition( int newState )
{
    if ( newState == currentState )
        return;

    // search the transition table to find the entry that matches the old and new state
    stTransition *tptr;
    for ( tptr = transition; tptr->seqList[0] != seqError; tptr++ )
        if ( (tptr->oldState & currentState) && (tptr->newState & newState) )
            break;

    // execute the sequence list
    int *seqptr;
    for ( seqptr = tptr->seqList; *seqptr != seqEnd; seqptr++ )
    {
        if ( *seqptr == seqXtoY )
            currentState = newState;
        else if ( *seqptr == seqError )
            printf( "The state table is missing the transition from %d to %d\n", currentState, newState );
        else
            executeSequence( *seqptr );
    }

    // if the seqList doesn't have an explicit update, then we update at the end
    currentState = newState;
}