A pattern for state machine

State machine是很常見的應用/Pattern，這章節會根據下面的圖來實作State machine pattern

enum state {
 STATE_INITIAL = 0,
 STATE_1,
 STATE_2,
 STATE_3,
 STATE_4,
};

enum event {
 E1 = 1,
 E2,
 E3,
 E4,
};

先定義State Machine狀態與Event

struct instance_data {
 enum event evt;
 int data;
};

struct instance {
 enum state cur_state;
 struct instance_data data;
};

定義一個struct來儲存"現在狀態"，Event與data

typedef enum state state_func_t(struct instance_data *data);

state_func_t* const state_table[] = {
 [STATE_INITIAL] = in_init,
 [STATE_1] = in_state_1,
 [STATE_2] = in_state_2,
 [STATE_3] = in_state_3,
 [STATE_4] = in_state_4,
};

void run_state(struct instance *inst)
{
 inst->cur_state = state_table[inst->cur_state](&(inst->data));
};

建立一個Table，將每一個state對應的function填入，run_state()會根據目前的State處理該event/data，並回傳下一個狀態

enum state in_state_1(struct instance_data *data)
{
 printf("%s(#%d): got EVT:%d\n", __FUNCTION__, __LINE__, data->evt);
 switch (data->evt) {
 case E1:
  printf("change to S2\n");
  return do_s2();
 case E2:
  printf("change to S3\n");
  return do_s3();
 default:
  printf("keep the same STATE && do nothing\n");
  return STATE_1;
 }
}

enum state in_state_2(struct instance_data *data)
{
 printf("%s(#%d): got EVT:%d\n", __FUNCTION__, __LINE__, data->evt);
 switch (data->evt) {
 case E3:
  printf("change to S3\n");
  return do_s3();
 default:
  printf("keep the same STATE && do s2 again\n");
  return do_s2(); // do s2 again
 }
}

enum state in_state_3(struct instance_data *data)
{
 printf("%s(#%d): got EVT:%d\n", __FUNCTION__, __LINE__, data->evt);
 switch (data->evt) {
 case E2:
  printf("change to S4\n");
  return do_s4();
 default:
  printf("keep the same STATE && do nothing\n");
  return STATE_3;
 }
}

enum state in_state_4(struct instance_data *data)
{
 printf("%s(#%d): got EVT:%d\n", __FUNCTION__, __LINE__, data->evt);
 switch (data->evt) {
 case E1:
  printf("change to S2\n");
  return do_s2();
 case E3:
  printf("change to S1\n");
  return do_s1();
 default:
  printf("keep the same STATE && do again\n");
  return do_s4();
 }
}

定義每一個狀態中的行為

enum state do_s1(void)
{
 printf("%s(#%d)\n", __FUNCTION__, __LINE__);
 return STATE_1;
}

enum state do_s2(void)
{
 printf("%s(#%d)\n", __FUNCTION__, __LINE__);
 return STATE_2;
}

enum state do_s3(void)
{
 printf("%s(#%d)\n", __FUNCTION__, __LINE__);
 return STATE_3;
}

enum state do_s4(void)
{
 printf("%s(#%d)\n", __FUNCTION__, __LINE__);
 return STATE_4;
}

enum state in_init(struct instance_data *data)
{
 printf("%s(#%d): do some init. E:%d\n", __FUNCTION__, __LINE__, data->evt);
 printf("change to S1\n");
 return STATE_1;
}

定義進入每一個狀態要執行的動作

int main( void ) {
 int ch;
 struct instance inst = {STATE_INITIAL, 0};
 while ( 1 ) {
  run_state(&inst);
  // do other program logic, run other state machines, etc
  printf("MENU: E1/E2/E3/E4\n");
  while(((ch = getc(stdin)) == '\n') || (ch <= '0') || (ch > '4'));
  inst.data.evt = ch - '0';
 }
}

main function模擬State Machine收到不同event(1~4)


以下是收到event 1, 2, 3, 4的執行結果