Simple state machine example in C

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Update  Again thanks for the examples  they have been very helpful and with the following  I don t mean to take anything away from them  Aren t the currently given examples  as far as I understand them  amp  state-machines  only half of what we usually understand by a state-machine  In the sense that the examples do change state but that s only represented by changing the value of a variable  and allowing different value- changes in different states   while usually  a state machine should also change its behavior  and behavior not  only  in the sense of allowing different value changes for a variable depending on the state  but in the sense of allowing different methods to be executed for different states  Or do I have a misconception of state machines and their common use  Best regards  Original question  I found this discussion about state machines  amp  iterator blocks in c  and tools to create state machines and whatnot for C   so I found a lot of abstract stuff but as a noob  all of this is a little confusing  So it would be great if someone could provide a C  source code-example that realizes a simple state machine with perhaps 3 4 states  just to get the gist of it

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Today i deep in State Design Pattern  I did and tested ThreadState  which equal    -  to Threading in C   as described in picture from Threading in C     You can easly add new states  configure moves from one state to other is very easy becouse it incapsulated in state implementation  Implementation and using at  Implements  NET ThreadState by State Design Pattern

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There are 2 popular state machine packages in NuGet   Appccelerate StateMachine  13 6K downloads   3 82K of legacy version  bbv Common StateMachine    StateMachineToolkit  1 56K downloads   The Appccelerate lib has good documentation  but it does not support  NET 4  so I chose StateMachineToolkit for my project

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In my opinion a state machine is not only meant for changing states but also  very important  for handling triggers events within a specific state  If you want to understand state machine design pattern better  a good description can be found within the book Head First Design Patterns  page 320     It is not only about the states within variables but also about handling triggers within the different states  Great chapter  and no  there is no fee for me in mentioning this  -  which contains just an easy to understand explanation

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What a bout StatePattern  Does that fit your needs   I think its context related  but its worth a shot for sure   http   en wikipedia org wiki State pattern  This let your states decide where to go and not the  object  class   Bruno

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Other alternative in this repo https   github com lingkodsoft StateBliss used fluent syntax  supports triggers       public class BasicTests                Fact          public void Tests                            Arrange             StateMachineManager Register new      typeof BasicTests  Assembly       Register at bootstrap of your application  i e  Startup             var currentState   AuthenticationState Unauthenticated              var nextState   AuthenticationState Authenticated              var data   new Dictionary lt string  object gt                     Act             var changeInfo   StateMachineManager Trigger currentState  nextState  data                   Assert             Assert True changeInfo StateChangedSucceeded               Assert Equal  ChangingHandler1   changeInfo Data  key1                 Assert Equal  ChangingHandler2   changeInfo Data  key2                          this class gets regitered automatically by calling StateMachineManager Register         public class AuthenticationStateDefinition   StateDefinition lt AuthenticationState gt                        public override void Define IStateFromBuilder lt AuthenticationState gt  builder                                builder From AuthenticationState Unauthenticated  To AuthenticationState Authenticated                       Changing this  a   gt  a ChangingHandler1                       Changed this  a   gt  a ChangedHandler1                    builder OnEntering AuthenticationState Authenticated  this  a   gt  a OnEnteringHandler1                   builder OnEntered AuthenticationState Authenticated  this  a   gt  a OnEnteredHandler1                    builder OnExiting AuthenticationState Unauthenticated  this  a   gt  a OnExitingHandler1                   builder OnExited AuthenticationState Authenticated  this  a   gt  a OnExitedHandler1                    builder OnEditing AuthenticationState Authenticated  this  a   gt  a OnEditingHandler1                   builder OnEdited AuthenticationState Authenticated  this  a   gt  a OnEditedHandler1                    builder ThrowExceptionWhenDiscontinued   true                             private void ChangingHandler1 StateChangeGuardInfo lt AuthenticationState gt  changeinfo                                var data   changeinfo DataAs lt Dictionary lt string  object gt  gt                     data  key1      ChangingHandler1                              private void OnEnteringHandler1 StateChangeGuardInfo lt AuthenticationState gt  changeinfo                                   changeinfo Continue   false    this will prevent changing the state                            private void OnEditedHandler1 StateChangeInfo lt AuthenticationState gt  changeinfo                                                           private void OnExitedHandler1 StateChangeInfo lt AuthenticationState gt  changeinfo                                                           private void OnEnteredHandler1 StateChangeInfo lt AuthenticationState gt  changeinfo                                                           private void OnEditingHandler1 StateChangeGuardInfo lt AuthenticationState gt  changeinfo                                           private void OnExitingHandler1 StateChangeGuardInfo lt AuthenticationState gt  changeinfo                                           private void ChangedHandler1 StateChangeInfo lt AuthenticationState gt  changeinfo                                                 public class AnotherAuthenticationStateDefinition   StateDefinition lt AuthenticationState gt                        public override void Define IStateFromBuilder lt AuthenticationState gt  builder                                builder From AuthenticationState Unauthenticated  To AuthenticationState Authenticated                       Changing this  a   gt  a ChangingHandler2                               private void ChangingHandler2 StateChangeGuardInfo lt AuthenticationState gt  changeinfo                                var data   changeinfo DataAs lt Dictionary lt string  object gt  gt                     data  key2      ChangingHandler2                                      public enum AuthenticationState               Unauthenticated          Authenticated

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I made this generic state machine out of Juliet s code  It s working awesome for me   These are the benefits    you can create new state machine in code with two enums TState and TCommand   added struct TransitionResult lt TState gt  to have more control over the output results of  Try GetNext   methods exposing nested class StateTransition only through AddTransition TState  TCommand  TState  making it easier to work with it   Code   public class StateMachine lt TState  TCommand gt      where TState   struct  IConvertible  IComparable     where TCommand   struct  IConvertible  IComparable       protected class StateTransition lt TS  TC gt          where TS   struct  IConvertible  IComparable         where TC   struct  IConvertible  IComparable               readonly TS CurrentState          readonly TC Command           public StateTransition TS currentState  TC command                        if   typeof TS  IsEnum     typeof TC  IsEnum                                throw new ArgumentException  TS TC must be an enumerated type                               CurrentState   currentState              Command   command                     public override int GetHashCode                         return 17   31   CurrentState GetHashCode     31   Command GetHashCode                       public override bool Equals object obj                        StateTransition lt TS  TC gt  other   obj as StateTransition lt TS  TC gt               return other    null                  amp  amp  this CurrentState CompareTo other CurrentState     0                  amp  amp  this Command CompareTo other Command     0                       private Dictionary lt StateTransition lt TState  TCommand gt   TState gt  transitions      public TState CurrentState   get  private set         protected StateMachine TState initialState                if   typeof TState  IsEnum     typeof TCommand  IsEnum                        throw new ArgumentException  TState TCommand must be an enumerated type                       CurrentState   initialState          transitions   new Dictionary lt StateTransition lt TState  TCommand gt   TState gt                     lt summary gt          Defines a new transition inside this state machine          lt  summary gt           lt param name  start  gt source state lt  param gt           lt param name  command  gt transition condition lt  param gt           lt param name  end  gt destination state lt  param gt      protected void AddTransition TState start  TCommand command  TState end                transitions Add new StateTransition lt TState  TCommand gt  start  command   end              public TransitionResult lt TState gt  TryGetNext TCommand command                StateTransition lt TState  TCommand gt  transition   new StateTransition lt TState  TCommand gt  CurrentState  command           TState nextState          if  transitions TryGetValue transition  out nextState               return new TransitionResult lt TState gt  nextState  true           else             return new TransitionResult lt TState gt  CurrentState  false              public TransitionResult lt TState gt  MoveNext TCommand command                var result   TryGetNext command           if result IsValid                          changes state             CurrentState   result NewState                    return result            This is the return type of TryGetNext method   public struct TransitionResult lt TState gt        public TransitionResult TState newState  bool isValid                NewState   newState          IsValid   isValid            public TState NewState      public bool IsValid      How to use   This is how you can create a OnlineDiscountStateMachine from the generic class   Define an enum OnlineDiscountState for its states and an enum OnlineDiscountCommand for its commands    Define a class OnlineDiscountStateMachine derived from the generic class using those two enums  Derive the constructor from base OnlineDiscountState InitialState  so that the initial state is set to OnlineDiscountState InitialState  Use AddTransition as many times as needed  public class OnlineDiscountStateMachine   StateMachine lt OnlineDiscountState  OnlineDiscountCommand gt        public OnlineDiscountStateMachine     base OnlineDiscountState Disconnected                AddTransition OnlineDiscountState Disconnected  OnlineDiscountCommand Connect  OnlineDiscountState Connected           AddTransition OnlineDiscountState Disconnected  OnlineDiscountCommand Connect  OnlineDiscountState Error AuthenticationError           AddTransition OnlineDiscountState Connected  OnlineDiscountCommand Submit  OnlineDiscountState WaitingForResponse           AddTransition OnlineDiscountState WaitingForResponse  OnlineDiscountCommand DataReceived  OnlineDiscountState Disconnected             use the derived state machine      odsm   new OnlineDiscountStateMachine        public void Connect                 var result   odsm TryGetNext OnlineDiscountCommand Connect              is result valid          if   result IsValid                if this happens you need to add transitions to the state machine               in this case result NewState is the same as before             Console WriteLine  cannot navigate from this state using OnlineDiscountCommand Connect               the transition was successfull           show messages for new states         else if result NewState    OnlineDiscountState Error AuthenticationError              Console WriteLine  invalid user pass            else if result NewState    OnlineDiscountState Connected              Console WriteLine  Connected            else             Console WriteLine  not implemented transition result for     result NewState

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You can code an iterator block that lets you execute a code block in an orchestrated fashion  How the code block is broken up really doesn t have to correspond to anything  it s just how you want to code it  For example   IEnumerable lt int gt  CountToTen         System Console WriteLine  1        yield return 0      System Console WriteLine  2        System Console WriteLine  3        System Console WriteLine  4        yield return 0      System Console WriteLine  5        System Console WriteLine  6        System Console WriteLine  7        yield return 0      System Console WriteLine  8        yield return 0      System Console WriteLine  9        System Console WriteLine  10        In this case  when you call CountToTen  nothing actually executes  yet  What you get is effectively a state machine generator  for which you can create a new instance of the state machine  You do this by calling GetEnumerator    The resulting IEnumerator is effectively a state machine that you can drive by calling MoveNext         Thus  in this example  the first time you call MoveNext      you will see  1  written to the console  and the next time you call MoveNext      you will see 2  3  4  and then 5  6  7 and then 8  and then 9  10  As you can see  it s a useful mechanism for orchestrating how things should occur

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Some shameless self-promo here  but a while ago I created a library called YieldMachine which allows a limited-complexity state machine to be described in a very clean and simple way  For example  consider a lamp     Notice that this state machine has 2 triggers and 3 states  In YieldMachine code  we write a single method for all state-related behavior  in which we commit the horrible atrocity of using goto for each state  A trigger becomes a property or field of type Action  decorated with an attribute called Trigger  I ve commented the code of the first state and its transitions below  the next states follow the same pattern   public class Lamp   StateMachine          Triggers  or events  or actions  whatever  that our        state machine understands       Trigger      public readonly Action PressSwitch        Trigger      public readonly Action GotError          Actual state machine logic     protected override IEnumerable WalkStates             off                                                 Console WriteLine  off             yield return null           if  Trigger    PressSwitch  goto on          InvalidTrigger         on          Console WriteLine   shiiine              yield return null           if  Trigger    GotError  goto error          if  Trigger    PressSwitch  goto off          InvalidTrigger         error          Console WriteLine  -err-            yield return null           if  Trigger    PressSwitch  goto off          InvalidTrigger              Short and nice  eh    This state machine is controlled simply by sending triggers to it   var sm   new Lamp    sm PressSwitch      go on sm PressSwitch      go off  sm PressSwitch      go on sm GotError         get error sm PressSwitch      go off   Just to clarify  I ve added some comments to the first state to help you understand how to use this       protected override IEnumerable WalkStates             off                                           Each goto label is a state          Console WriteLine  off                    State entry actions          yield return null                         This means  Wait until a                                                    trigger is called                                                     Ah  we got triggered                                                       perform state exit actions                                                       none  in this case           if  Trigger    PressSwitch  goto on       Transitions go here                                                     depending on the trigger                                                    that was called  go to                                                   the right state          InvalidTrigger                            Throw exception on                                                    invalid trigger                This works because the C  compiler actually created a state machine internally for each method that uses yield return  This construct is usually used to lazily create sequences of data  but in this case we re not actually interested in the returned sequence  which is all nulls anyway   but in the state behaviour that gets created under the hood    The StateMachine base class does some reflection on construction to assign code to each  Trigger  action  which sets the Trigger member and moves the state machine forward   But you don t really need to understand the internals to be able to use it

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I m posting another answer here as this is state machines from a different perspective  very visual   My original answer is classic imperative code   I think its quite visual as code goes because of the array which makes visualizing the state machine simple   The downside is you have to write all this   Remos s answer alleviates the effort of writing the boiler-plate code but is far less visual   There is the third alternative  really drawing the state machine   If you are using  NET and can target version 4 of the run time then you have the option of using workflow s state machine activities   These in essence let you draw the state machine  much as in Juliet s diagram  and have the WF run-time execute it for you   See the MSDN article Building State Machines with Windows Workflow Foundation for more details  and this CodePlex site for the latest version   That s the option I would always prefer when targeting  NET because its easy to see  change and explain to non programmers  pictures are worth a thousand words as they say

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You might want to use one of the existing open source Finite State Machines  E g  bbv Common StateMachine found at http   code google com p bbvcommon wiki StateMachine  It has a very intuitive fluent syntax and a lot of features such as  enter exit actions  transition actions  guards  hierarchical  passive implementation  executed on the thread of the caller  and active implementation  own thread on which the fsm runs  events are added to a queue    Taking Juliets example the definition for the state machine gets very easy   var fsm   new PassiveStateMachine lt ProcessState  Command gt     fsm In ProcessState Inactive      On Command Exit  Goto ProcessState Terminated  Execute SomeTransitionAction      On Command Begin  Goto ProcessState Active   fsm In ProcessState Active      ExecuteOnEntry SomeEntryAction      ExecuteOnExit SomeExitAction      On Command End  Goto ProcessState Inactive      On Command Pause  Goto ProcessState Paused   fsm In ProcessState Paused      On Command End  Goto ProcessState Inactive  OnlyIf SomeGuard      On Command Resume  Goto ProcessState Active   fsm Initialize ProcessState Inactive   fsm Start     fsm Fire Command Begin     Update  The project location has moved to  https   github com appccelerate statemachine

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Not sure whether I miss the point  but I think none of the answers here are  quot simple quot  state machines  What i usually call a simple state machine is using a loop with a switch inside  That is what we used in PLC   microchip programming or in C C   programming at the university  advantages   easy to write  no special objects and stuff required  you dont even need object orientation for it  when it is small  it is easy to understand   disadvantages   can become quite big and hard to read  when there are many states   It looked like that  public enum State       First      Second      Third     static void Main string   args        var state   State First         x and i are just examples for stuff that you could change inside the state and use for state transitions     var x       0       var i       0          does not have to be a while loop  you could loop over the characters of a string too     while  true                  switch  state                        case State First                     Do sth here                 if  x    2                      state   State Second                           you may or may not add a break  right after setting the next state                    or do sth here                 if  i    3                      state   State Third                     or here                 break              case State Second                     Do sth here                 if  x    10                      state   State First                     or do sth here                 break              case State Third                     Do sth here                 if  x    10                      state   State First                     or do sth here                 break              default                     you may wanna throw an exception here                  break                      if it should be really a state machine on which you call methods which react depending on which state you are in differently  state design pattern is the better approach

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Here s an example of a very classic finite state machine  modelling a very simplified electronic device  like a TV   using System  using System Collections Generic  using System Linq  using System Text   namespace fsm   class Program       static void Main string   args                var fsm   new FiniteStateMachine            Console WriteLine fsm State           fsm ProcessEvent FiniteStateMachine Events PlugIn           Console WriteLine fsm State           fsm ProcessEvent FiniteStateMachine Events TurnOn           Console WriteLine fsm State           fsm ProcessEvent FiniteStateMachine Events TurnOff           Console WriteLine fsm State           fsm ProcessEvent FiniteStateMachine Events TurnOn           Console WriteLine fsm State           fsm ProcessEvent FiniteStateMachine Events RemovePower           Console WriteLine fsm State           Console ReadKey               class FiniteStateMachine               public enum States   Start  Standby  On            public States State   get  set             public enum Events   PlugIn  TurnOn  TurnOff  RemovePower             private Action    fsm           public FiniteStateMachine                         this fsm   new Action 3  4                       PlugIn        TurnOn                  TurnOff             RemovePower                  this PowerOn   null                    null                null                  start                  null           this StandbyWhenOff     null                this PowerOff         standby                  null           null                    this StandbyWhenOn  this PowerOff         on                   public void ProcessEvent Events theEvent                        this fsm  int this State   int theEvent  Invoke                       private void PowerOn     this State   States Standby            private void PowerOff     this State   States Start            private void StandbyWhenOn     this State   States Standby            private void StandbyWhenOff     this State   States On

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Found this great tutorial online and it helped me wrap my head around finite state machines   http   gamedevelopment tutsplus com tutorials finite-state-machines-theory-and-implementation--gamedev-11867  The tutorial is language agnostic  so it can easily be adapted to your C  needs    Also  the example used  an ant looking for food  is easy to understand    From the tutorial     public class FSM       private var activeState  Function     points to the currently active state function      public function FSM                public function setState state  Function   void           activeState   state             public function update    void           if  activeState    null                activeState                        public class Ant       public var position    Vector3D      public var velocity    Vector3D      public var brain       FSM       public function Ant posX  Number  posY  Number            position      new Vector3D posX  posY           velocity      new Vector3D  -1  -1           brain         new FSM                Tell the brain to start looking for the leaf          brain setState findLeaf                        The  findLeaf  state        It makes the ant move towards the leaf             public function findLeaf    void              Move the ant towards the leaf          velocity   new Vector3D Game instance leaf x - position x  Game instance leaf y - position y            if  distance Game instance leaf  this   lt   10                   The ant is extremelly close to the leaf  it s time                to go home              brain setState goHome                      if  distance Game mouse  this   lt   MOUSE THREAT RADIUS                   Mouse cursor is threatening us  Let s run away                 It will make the brain start calling runAway   from                now on              brain setState runAway                                  The  goHome  state        It makes the ant move towards its home             public function goHome    void              Move the ant towards home         velocity   new Vector3D Game instance home x - position x  Game instance home y - position y            if  distance Game instance home  this   lt   10                   The ant is home  let s find the leaf again              brain setState findLeaf                                  The  runAway  state        It makes the ant run away from the mouse cursor             public function runAway    void              Move the ant away from the mouse cursor         velocity   new Vector3D position x - Game mouse x  position y - Game mouse y               Is the mouse cursor still close          if  distance Game mouse  this   gt  MOUSE THREAT RADIUS                   No  the mouse cursor has gone away  Let s go back looking for the leaf              brain setState findLeaf                        public function update   void              Update the FSM controlling the  brain   It will invoke the currently            active state function  findLeaf    goHome   or runAway            brain update                Apply the velocity vector to the position  making the ant move          moveBasedOnVelocity

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FiniteStateMachine is a Simple State Machine  written in C  Link  Advantages tu use my library FiniteStateMachine    Define a  context  class to present a single interface to the outside world  Define a State abstract base class  Represent the different  states  of the state machine as derived classes of the State base class  Define state-specific behavior in the appropriate State derived classes  Maintain a pointer to the current  state  in the  context  class  To change the state of the state machine  change the current  state  pointer    Download DLL Download  Example on LINQPad   void Main                 var machine   new SFM Machine new StatePaused                 var output   machine Command  Input Start   Command Start               Console WriteLine Command Start ToString      - gt   State      machine Current               Console WriteLine output                output   machine Command  Input Pause   Command Pause               Console WriteLine Command Pause ToString      - gt   State      machine Current               Console WriteLine output               Console WriteLine  -------------------------------------------------          public enum Command               Start          Pause             public class StateActive   SFM State                public override void Handle SFM IContext context                           Gestione parametri             var input    String context Input              context Output   input                 Gestione Navigazione             if   Command context Command    Command Pause  context Next   new StatePaused                if   Command context Command    Command Start  context Next   this                     public class StatePaused   SFM State         public override void Handle SFM IContext context                      Gestione parametri          var input    String context Input           context Output   input              Gestione Navigazione          if   Command context Command    Command Start  context Next   new  StateActive             if   Command context Command    Command Pause  context Next   this

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I ve just contributed this   https   code google com p ysharp source browse  svn 2Ftrunk 2FStateMachinesPoC  Here s one of the examples demoing direct and indirect sending of commands  with states as IObserver of signal   thus responders to a signal source  IObservable of signal    using System  using System Collections Generic  using System Linq  using System Text   namespace Test       using Machines       public static class WatchingTvSampleAdvanced                  Enum type for the transition triggers  instead of System String            public enum TvOperation   Plug  SwitchOn  SwitchOff  Unplug  Dispose               The state machine class type is also used as the type for its possible states constants           public class Television   NamedState lt Television  TvOperation  DateTime gt                           Declare all the possible states constants               public static readonly Television Unplugged   new Television   Unplugged TV                 public static readonly Television Off   new Television   TV Off                 public static readonly Television On   new Television   TV On                 public static readonly Television Disposed   new Television   Disposed TV                     For convenience  enter the default start state when the parameterless constructor executes               public Television     this Television Unplugged                      To create a state machine instance  with a given start state               private Television Television value    this null  value                      To create a possible state constant               private Television string moniker    this moniker  null                   private Television string moniker  Television value                                if  moniker    null                                           Build the state graph programmatically                         instead of declaratively via custom attributes                        Handler lt Television  TvOperation  DateTime gt  stateChangeHandler   StateChange                      Build                                               new                                                         new   From   Television Unplugged  When   TvOperation Plug  Goto   Television Off  With   stateChangeHandler                                new   From   Television Unplugged  When   TvOperation Dispose  Goto   Television Disposed  With   stateChangeHandler                                new   From   Television Off  When   TvOperation SwitchOn  Goto   Television On  With   stateChangeHandler                                new   From   Television Off  When   TvOperation Unplug  Goto   Television Unplugged  With   stateChangeHandler                                new   From   Television Off  When   TvOperation Dispose  Goto   Television Disposed  With   stateChangeHandler                                new   From   Television On  When   TvOperation SwitchOff  Goto   Television Off  With   stateChangeHandler                                new   From   Television On  When   TvOperation Unplug  Goto   Television Unplugged  With   stateChangeHandler                                new   From   Television On  When   TvOperation Dispose  Goto   Television Disposed  With   stateChangeHandler                                                      false                                                          else                        Name the state constant                       Moniker   moniker                  Start value    this                                 Because the states  value domain is a reference type  disallow the null value for any start state value                protected override void OnStart Television value                                if  value    null                      throw new ArgumentNullException  value    cannot be null                                  When reaching a final state  unsubscribe from all the signal source s   if any               protected override void OnComplete bool stateComplete                                   Holds during all transitions into a final state                     i e   stateComplete implies IsFinal                    System Diagnostics Debug Assert  stateComplete    IsFinal                    if  stateComplete                      UnsubscribeFromAll                                  Executed before and after every state transition               private void StateChange IState lt Television gt  state  ExecutionStep step  Television value  TvOperation info  DateTime args                                   Holds during all possible transitions defined in the state graph                     i e    step equals ExecutionStep LeaveState  implies  not state IsFinal                   System Diagnostics Debug Assert  step    ExecutionStep LeaveState      state IsFinal                       Holds in instance  i e   non-static  transition handlers like this one                   System Diagnostics Debug Assert this    state                    switch  step                                        case ExecutionStep LeaveState                          var timeStamp     args    default DateTime     String Format   t t    0     args    String Empty                           Console WriteLine                                value  is the state value that we are transitioning TO                           Console WriteLine   tLeave   t 0  --  1  - gt   2  3    this  info  value  timeStamp                           break                      case ExecutionStep EnterState                              value  is the state value that we have transitioned FROM                           Console WriteLine   tEnter   t 0  --  1  - gt   2    value  info  this                           break                      default                          break                                               public override string ToString     return  IsConstant   Moniker   Value ToString                          public static void Run                         Console Clear                    Create a signal source instance  here  a k a   remote control   that implements                IObservable lt TvOperation gt  and IObservable lt KeyValuePair lt TvOperation  DateTime gt  gt                var remote   new SignalSource lt TvOperation  DateTime gt                     Create a television state machine instance  automatically set in a default start state                  and make it subscribe to a compatible signal source  such as the remote control  precisely               var tv   new Television   Using remote               bool done                  Always holds  assuming the call to Using      didn t throw an exception  in case of subscription failure                System Diagnostics Debug Assert tv    null   There s a bug somewhere  this message should never be displayed                     As commonly done  we can trigger a transition directly on the state machine               tv MoveNext TvOperation Plug  DateTime Now                   Alternatively  we can also trigger transitions by emitting from the signal source   remote control                that the state machine subscribed to   is an observer of               remote Emit TvOperation SwitchOn  DateTime Now               remote Emit TvOperation SwitchOff               remote Emit TvOperation SwitchOn               remote Emit TvOperation SwitchOff  DateTime Now                done                                         tv                          MoveNext TvOperation Unplug                           MoveNext TvOperation Dispose     MoveNext      returns null iff tv IsFinal    true                        null                                 remote Emit TvOperation Unplug      Ignored by the state machine thanks to the OnComplete      override above              Console WriteLine                Console WriteLine  Is the TV s state   0   a final state   1    tv Value  done                Console WriteLine                Console WriteLine  Press any key                   Console ReadKey                        Note   this example is rather artificial and mostly meant to demo a number of orthogonal features  There should seldomly be a real need to implement the state value domain itself by a full blown class  using the CRTP   see   http   en wikipedia org wiki Curiously recurring template pattern   like this   Here s for a certainly simpler and likely much more common implementation use case  using a simple enum type as the states value domain   for the same state machine  and with the same test case    https   code google com p ysharp source browse trunk StateMachinesPoC WatchingTVSample cs  using System  using System Collections Generic  using System Linq  using System Text   namespace Test       using Machines       public static class WatchingTvSample               public enum Status   Unplugged  Off  On  Disposed            public class DeviceTransitionAttribute   TransitionAttribute                       public Status From   get  set                public string When   get  set                public Status Goto   get  set                public object With   get  set                          State lt Status gt  is a shortcut for   derived from State lt Status  string gt              which in turn is a shortcut for   derived from State lt Status  string  object gt            public class Device   State lt Status gt                           Executed before and after every state transition               protected override void OnChange ExecutionStep step  Status value  string info  object args                                if  step    ExecutionStep EnterState                                            value  is the state value that we have transitioned FROM                       Console WriteLine   t 0  --  1  - gt   2    value  info  this                                                public override string ToString     return Value ToString                            Since  Device  has no state graph of its own  define one for derived  Television             DeviceTransition From   Status Unplugged  When    Plug   Goto   Status Off            DeviceTransition From   Status Unplugged  When    Dispose   Goto   Status Disposed            DeviceTransition From   Status Off  When    Switch On   Goto   Status On            DeviceTransition From   Status Off  When    Unplug   Goto   Status Unplugged            DeviceTransition From   Status Off  When    Dispose   Goto   Status Disposed            DeviceTransition From   Status On  When    Switch Off   Goto   Status Off            DeviceTransition From   Status On  When    Unplug   Goto   Status Unplugged            DeviceTransition From   Status On  When    Dispose   Goto   Status Disposed           public class Television   Device              public static void Run                         Console Clear                    Create a television state machine instance  and return it  set in some start state               var tv   new Television   Start Status Unplugged               bool done                  Holds iff the chosen start state isn t a final state               System Diagnostics Debug Assert tv    null   The chosen start state is a final state                     Trigger some state transitions with no arguments                  args  is ignored by this state machine s OnChange       anyway                done                                         tv                          MoveNext  Plug                            MoveNext  Switch On                            MoveNext  Switch Off                            MoveNext  Switch On                            MoveNext  Switch Off                            MoveNext  Unplug                            MoveNext  Dispose      MoveNext      returns null iff tv IsFinal    true                        null                                 Console WriteLine                Console WriteLine  Is the TV s state   0   a final state   1    tv Value  done                Console WriteLine                Console WriteLine  Press any key                   Console ReadKey                         HTH

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Let s start with this simple state diagram     We have    4 states  Inactive  Active  Paused  and Exited  5 types of state transitions  Begin Command  End Command  Pause Command  Resume Command  Exit Command     You can convert this to C  in a handful of ways  such as performing a switch statement on the current state and command  or looking up transitions in a transition table  For this simple state machine  I prefer a transition table  which is very easy to represent using a Dictionary   using System  using System Collections Generic   namespace Juliet       public enum ProcessState               Inactive          Active          Paused          Terminated            public enum Command               Begin          End          Pause          Resume          Exit            public class Process               class StateTransition                       readonly ProcessState CurrentState              readonly Command Command               public StateTransition ProcessState currentState  Command command                                CurrentState   currentState                  Command   command                             public override int GetHashCode                                 return 17   31   CurrentState GetHashCode     31   Command GetHashCode                               public override bool Equals object obj                                StateTransition other   obj as StateTransition                  return other    null  amp  amp  this CurrentState    other CurrentState  amp  amp  this Command    other Command                                   Dictionary lt StateTransition  ProcessState gt  transitions          public ProcessState CurrentState   get  private set             public Process                         CurrentState   ProcessState Inactive              transitions   new Dictionary lt StateTransition  ProcessState gt                                  new StateTransition ProcessState Inactive  Command Exit   ProcessState Terminated                      new StateTransition ProcessState Inactive  Command Begin   ProcessState Active                      new StateTransition ProcessState Active  Command End   ProcessState Inactive                      new StateTransition ProcessState Active  Command Pause   ProcessState Paused                      new StateTransition ProcessState Paused  Command End   ProcessState Inactive                      new StateTransition ProcessState Paused  Command Resume   ProcessState Active                                     public ProcessState GetNext Command command                        StateTransition transition   new StateTransition CurrentState  command               ProcessState nextState              if   transitions TryGetValue transition  out nextState                   throw new Exception  Invalid transition      CurrentState     - gt      command               return nextState                     public ProcessState MoveNext Command command                        CurrentState   GetNext command               return CurrentState                        public class Program               static void Main string   args                        Process p   new Process                Console WriteLine  Current State       p CurrentState               Console WriteLine  Command Begin  Current State       p MoveNext Command Begin                Console WriteLine  Command Pause  Current State       p MoveNext Command Pause                Console WriteLine  Command End  Current State       p MoveNext Command End                Console WriteLine  Command Exit  Current State       p MoveNext Command Exit                Console ReadLine                        As a matter of personal preference  I like to design my state machines with a GetNext function to return the next state deterministically  and a MoveNext function to mutate the state machine

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I would recommend state cs  I personally used state js  the JavaScript version  and am very happy with it  That C  version works in a similar way    You instantiate states              create the state machine         var player   new StateMachine lt State gt    player                 create some states         var initial   player CreatePseudoState   initial   PseudoStateKind Initial            var operational   player CreateCompositeState   operational                   You instantiate some transitions           var t0   player CreateTransition  initial  operational            player CreateTransition  history  stopped            player CreateTransition lt String gt   stopped  running    state  command     gt  command Equals   play               player CreateTransition lt String gt   active  stopped    state  command     gt  command Equals   stop         You define actions on states and transitions       t0 Effect    DisengageHead      t0 Effect    StopMotor    And that s  pretty much  it  Look at the website for more information

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I haven t tried implementing a FSM in C  yet  but these all sound  or look  very complicated to the way I handled FSM s in the past in low-level languages like C or ASM     I believe the method I ve always known is called something like an  Iterative Loop    In it  you essentially have a  while  loop that periodically exits based on events  interrupts   then returns to the main loop again    Within the interrupt handlers  you would pass a CurrentState and return a NextState  which then overwrites the CurrentState variable in the main loop   You do this ad infinitum until the program closes  or the microcontroller resets     What I m seeing other answers all look very complicated compared with how a FSM is  in my mind  intended to be implemented  its beauty lies in its simplicity and FSM can be very complicated with many  many states and transitions  but they allow complicated process to be easily broken down and digested    I realize my response shouldn t include another question  but I am forced to ask  why do these other proposed solutions appear to be so complicated  They seem to be akin to hitting a small nail with a giant sledge hammer

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I think the state machine proposed by Juliet has a mistake  the method GetHashCode can return the same hash code for two different transitions  for example      State   Active  1    Command   Pause  2     HashCode   17   31   62     110      State   Paused  2    Command   End  1     HashCode   17   62   31     110   To avoid this error  the method should be like this   public override int GetHashCode                    return 17   23   CurrentState GetHashCode     31   Command GetHashCode           Alex

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You can use my solution  this is the most convenient way  It   s also free  Create state machine in three steps   1  Create scheme in  node editor and load it in your project using library StateMachine stateMachine   new StateMachine  quot scheme xml quot     2  Describe your app logic on events  stateMachine GetState  quot State1 quot   OnExit Action1   stateMachine GetState  quot State2 quot   OnEntry Action2   stateMachine GetTransition  quot Transition1 quot   OnInvoke Action3   stateMachine OnChangeState Action4    3  Run the state machine stateMachine Start     Links  Node editor  https   github com SimpleStateMachine SimpleStateMachineNodeEditor Library  https   github com SimpleStateMachine SimpleStateMachineLibrary

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It s useful to remember that state machines are an abstraction  and you don t need particular tools to create one  however tools can be useful   You can for example realise a state machine with functions   void Hunt IList lt Gull gt  gulls        if  gulls Empty           return       var target   gulls First        TargetAcquired target  gulls      void TargetAcquired Gull target  IList lt Gull gt  gulls        var balloon   new WaterBalloon weightKg  20        this Cannon Fire balloon        if  balloon Hit               TargetHit target  gulls             else        TargetMissed target  gulls      void TargetHit Gull target  IList lt Gull gt  gulls        Console WriteLine  Suck on it  0     target Name       Hunt gulls      void TargetMissed Gull target  IList lt Gull gt  gulls        Console WriteLine  I ll get ya         TargetAcquired target  gulls       This machine would hunt for gulls and try to hit them with water balloons  If it misses it will try firing one until it hits  could do with some realistic expectations      otherwise it will gloat in the console  It continues to hunt until it s out of gulls to harass   Each function corresponds to each state  the start and end  or accept  states are not shown  There are probably more states in there than modelled by the functions though  For example after firing the balloon the machine is really in another state than it was before it  but I decided this distinction was impractical to make   A common way is to use classes to represent states  and then connect them in different ways

[c#] Simple state machine example in C#?

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