Question

I want to implement the GUI as a state machine. I think there are some benefits and some drawbacks of doing this, but this is not the topic of this questions.

After some reading about this I found several ways of modeling a state machine in C++ and I stuck on 2, but I don’t know what method may fit better for GUI modeling.

1. Represent the State Machine as a list of states with following methods:

   • OnEvent(...);
   • OnEnterState(...);
   • OnExitState(...);

   From StateMachine::OnEvent(...) I forward the event to CurrentState::OnEvent(...) and here the decision to make a transition or not is made. On transition I call CurrentState::OnExitState(...), NewState::OnEnterState() and CurrentState = NewState;

   With this approach the state will be tightly coupled with actions, but State might get complicated when from one state I can go to multiple states and I have to take different actions for different transitions.

2. Represent the state machine as list of transitions with following properties:

   • InitialState
   • FinalState
   • OnEvent(...)
   • DoTransition(...)

   From StateMachine::OnEvent(...) I forward the event to all transitions where InitialState has same value as CurrentState in the state machine. If the transition condition is met the loop is stopped, DoTransition method is called and CurrentState set to Transition::FinalState.

   With this approach Transition will be very simple, but the number of transition count might get very high. Also it will become harder to track what actions will be done when one state receives an event.

What approach do you think is better for GUI modeling. Do you know other representations that may be better for my problem?

Answer 1

Here is a third option:

• Represent the state machine as a transition matrix
  • Matrix column index represents a state
  • Matrix row index represents a symbol (see below)
  • Matrix cell represents the state machihe should transit to. This could be both new state or the same state
  • Every state has OnEvent method which returns a symbol

From StateMachine::OnEvent(...) events are forwarded to State::OnEvent which returns a symbol – a result of execution. StateMachine then based on current state and returned symbol decides whether

• Transition to different state must be made, or
• Current state is preserved
• Optionally, if transition is made, OnExitState and OnEnterState is called for a corresponsing states

Example matrix for 3 states and 3 symbols

0 1 2
1 2 0
2 0 1

In this example if if machine is in any od the states (0,1,2) and State::OnEvent returns symbol 0 (first row in the matrix) – it stays in the same state

Second row says, that if current state is 0 and returned symbol is 1 transition is made to state 1. For state 1 -> state 2 and for state 2 -> state 0.

Similary third row says that for symbol 2, state 0-> state 2, state 1 -> state 0, state 2 -> state 1

The point of this being:

1. Number of symbols will likely be much lower than that of states.
2. States are not aware of each other
3. All transition are controlled from one point, so the moment you want to handle symbol DB_ERROR differently to NETWORK_ERROR you just change the transition table and don’t touch states implementation.