我最喜欢的模式之一是状态设计模式。对相同的给定输入集做出不同的响应或行为。
将 switch/case 语句用于状态机的问题之一是，当您创建更多状态时，switch/case 变得更难/难以阅读/维护，促进无组织的意大利面条代码，并且越来越难以在不破坏某些内容的情况下进行更改。我发现使用设计模式可以帮助我更好地组织数据，这就是抽象的重点。与其围绕您来自的状态来设计您的状态代码，不如构建您的代码，以便在您进入新状态时记录状态。这样，您就可以有效地记录您之前的状态。我喜欢@JoshPetit 的回答，并且直接从 GoF 书中采取了他的解决方案：
stateCtxt.h：
#define STATE (void *)
typedef enum fsmSignal
{
eEnter =0,
eNormal,
eExit
}FsmSignalT;
typedef struct fsm
{
FsmSignalT signal;
// StateT is an enum that you can define any which way you want
StateT currentState;
}FsmT;
extern int STATECTXT_Init(void);
/* optionally allow client context to set the target state */
extern STATECTXT_Set(StateT stateID);
extern void STATECTXT_Handle(void *pvEvent);
stateCtxt.c:
#include "stateCtxt.h"
#include "statehandlers.h"
typedef STATE (*pfnStateT)(FsmSignalT signal, void *pvEvent);
static FsmT fsm;
static pfnStateT UsbState ;
int STATECTXT_Init(void)
{
UsbState = State1;
fsm.signal = eEnter;
// use an enum for better maintainability
fsm.currentState = '1';
(*UsbState)( &fsm, pvEvent);
return 0;
}
static void ChangeState( FsmT *pFsm, pfnStateT targetState )
{
// Check to see if the state has changed
if (targetState != NULL)
{
// Call current state's exit event
pFsm->signal = eExit;
STATE dummyState = (*UsbState)( pFsm, pvEvent);
// Update the State Machine structure
UsbState = targetState ;
// Call the new state's enter event
pFsm->signal = eEnter;
dummyState = (*UsbState)( pFsm, pvEvent);
}
}
void STATECTXT_Handle(void *pvEvent)
{
pfnStateT newState;
if (UsbState != NULL)
{
fsm.signal = eNormal;
newState = (*UsbState)( &fsm, pvEvent );
ChangeState( &fsm, newState );
}
}
void STATECTXT_Set(StateT stateID)
{
prevState = UsbState;
switch (stateID)
{
case '1':
ChangeState( State1 );
break;
case '2':
ChangeState( State2);
break;
case '3':
ChangeState( State3);
break;
}
}
statehandlers.h：
/* define state handlers */
extern STATE State1(void);
extern STATE State2(void);
extern STATE State3(void);
statehandlers.c：
#include "stateCtxt.h:"
/* Define behaviour to given set of inputs */
STATE State1(FsmT *fsm, void *pvEvent)
{
STATE nextState;
/* do some state specific behaviours
* here
*/
/* fsm->currentState currently contains the previous state
* just before it gets updated, so you can implement behaviours
* which depend on previous state here
*/
fsm->currentState = '1';
/* Now, specify the next state
* to transition to, or return null if you're still waiting for
* more stuff to process.
*/
switch (fsm->signal)
{
case eEnter:
nextState = State2;
break;
case eNormal:
nextState = null;
break;
case eExit:
nextState = State2;
break;
}
return nextState;
}
STATE State3(FsmT *fsm, void *pvEvent)
{
/* do some state specific behaviours
* here
*/
fsm->currentState = '2';
/* Now, specify the next state
* to transition to
*/
return State1;
}
STATE State2(FsmT *fsm, void *pvEvent)
{
/* do some state specific behaviours
* here
*/
fsm->currentState = '3';
/* Now, specify the next state
* to transition to
*/
return State3;
}
对于大多数状态机，尤其是。有限状态机，每个状态都会知道它的下一个状态应该是什么，以及转换到下一个状态的标准。对于松散状态设计，情况可能并非如此，因此可以选择公开 API 以转换状态。如果您需要更多抽象，可以将每个状态处理程序分离到自己的文件中，这相当于 GoF 书中的具体状态处理程序。如果您的设计很简单，只有几个状态，那么为了简单起见，stateCtxt.c 和 statehandlers.c 都可以合并到一个文件中。
