I have implemented a small state machine in this way (in pseudo code):
class Input {}
class KeyInput inherits Input
{
public :
enum { Key_A, Key_B, ..., }
}
class GUIInput inherits Input
{
public :
enum { Button_A, Button_B, ..., }
}
enum Event { NewGame, Quit, OpenOptions, OpenMenu }
class BaseState
{
String name;
Event get_event (Input input);
void handle (Event e); //event handling function
}
class Menu inherits BaseState{...}
class InGame inherits BaseState{...}
class Options inherits BaseState{...}
class StateMachine
{
public :
BaseState get_current_state () { return current_state; }
void add_state (String name, BaseState state) { statesMap.insert(name, state);}
//raise an exception if state not found
BaseState get_state (String name) { return statesMap.find(name); }
//raise an exception if state or next_state not found
void add_transition (Event event, String state_name, String next_state_name)
{
BaseState state = get_state(state_name);
BaseState next_state = get_state(next_state_name);
transitionsMap.insert(pair<event, state>, next_state);
}
//raise exception if couple not found
BaseState get_next_state(Event event, BaseState state)
{
return transitionsMap.find(pair<event, state>);
}
void handle(Input input)
{
Event event = current_state.get_event(input)
current_state.handle(event);
current_state = get_next_state(event, current_state);
}
private :
BaseState current_state;
map<String, BaseState> statesMap; //map of all states in
the machine
//for each couple event/state, this map stores
the next state
map<pair<Event, BaseState>, BaseState> transitionsMap;
}
So, before getting
the transition, I need to convert
the key input or GUI input to
the proper event, given
the current state; thus
the same key 'W' can launch a new game in
the 'Menu' state or moving forward a character in
the 'InGame' state;
Then I get
the next state from
the transitionsMap and I update
the current state
Does this configuration seem valid to you ? Is it a good idea to separate states and transitions ?
And I have some kind of trouble to represent a 'null state' or a 'null event';
What initial value can I give to
the current state and which one should be returned by get_state if it fails ?
