Refactoring to the State Design Pattern

Refactoring to the
State Design Pattern
Jim Roepcke <jr@uvic.ca>
CSC 578D Fall 2009

Motivation

• Improve the design of the GameStats
application
• Make it easier to understand at a glance
• Make it easier to make improvements

Backup Backup
Replica Replica

Primary
Replica

Backup Backup
Replica Replica

Primary-Backup
• Bonjour can guarantee no more than one
Primary replica can publish its service
• Multiple Backup replicas communicate with
the Primary to keep all replicas synchronized
• Writes are made to the Primary and
propagated back to the Backup replicas
• If the Primary fails, a Backup can take over.
The remaining backups sync to the new
Primary

failed to
initial become
primary

start primary failed to start

trying to trying to
become connect to
primary primary

backup did start
primary did start backup failed to start

failed to
primary connect to backup
primary

stop stop

stopping stopping
error
primary backup

primary did stop backup did stop

stopped

Context State

Request() Handle()

state->Handle()
ConcreteStateA ConcreteStateB

Handle() Handle()

State
Context
TransitionTo(Context c, State s)
Enter(Context c)
SetState(State s)
Leave(Context c)
Foo()
Bar()
Foo(StateContext c)
Baz()
Bar(StateContext c)
....
Baz(StateContext c)
...

state->Foo()
BaseState
this->Leave(c)
TransitionTo(Context c, State s) c->SetState(s)
Enter(Context c) s->Enter(c)
Leave(Context c)

Foo(StateContext c)
Bar(StateContext c)
Baz(StateContext c)
...

ConcreteStateA ConcreteStateB
TransitionTo
Enter(Context c) Foo(StateContext c)
(c, ConcreteStateA)
Baz(StateContext c) Bar(StateContext c)

primary

stop

stopping
error
primary

primary did stop

stopped

Original Code
- (void) stop
{
! if (self.state == GSGameControllerStatePrimary) {
! ! self.state = GSGameControllerStateStopping;
! ! [self uninstallServerTargets];
! ! [_server stop];
! ! self.state = GSGameControllerStateStopped;
! } else if (self.state == GSGameControllerStateBackup) {
! ! self.state = GSGameControllerStateStopping;
! ! [_memberManager stopMonitoring:_primaryService];
! ! [_clientToPrimary stop];
! } else if (self.state != GSGameControllerStateStopped) {
! ! self.state = GSGameControllerStateError;
! }
}

First Refactoring
- (oneway void) stop
{
self.state = GSGameControllerStateStopping;
}

The Devil (is in the details)

- (void) setState: (GSGameControllerState)newState
{
! GSGameControllerState oldState = _state;
! _state = newState;
! if ( _state == GSGameControllerStateTryingToFindPrimary) {
! ! // [self findPrimary];
! ! self.state = GSGameControllerStateTryingToBecomePrimary; // TODO: remove the findprimary state
! } else if (_state == GSGameControllerStateTryingToBecomePrimary) {
! ! [self startPrimaryServer];
! } else if (_state == GSGameControllerStateFailedToBecomePrimary) {
! ! [self tearDownPrimary];
! ! // FIXME: this could be an endless loop of failing to become primary, put in a limit or something
! ! self.state = GSGameControllerStateTryingToBecomePrimary;
! } else if (_state == GSGameControllerStateTryingToConnectToPrimary) {
! ! [self connectToPrimary];
! } else if (_state == GSGameControllerStateFailedToConnectToPrimary) {
! ! [self tearDownBackup];
! ! self.state = GSGameControllerStateTryingToBecomePrimary;
! } else if (_state == GSGameControllerStatePrimary) {
! ! [self installServerTargets];
! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidBecomePrimary:)];
! } else if (_state == GSGameControllerStateBackup) {
! ! [self tellPrimaryWhoIAm];
! ! [self monitorPrimary];
! ! [self synchronizeWithPrimaryFromVersion:_game.version];
! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidBecomeBackup:)];
! } else if (_state == GSGameControllerStateStopping) {
! ! if (oldState == GSGameControllerStatePrimary) {
! ! ! self.state = GSGameControllerStateStoppingPrimary;
! ! } else if (oldState == GSGameControllerStateBackup) {
! ! ! self.state = GSGameControllerStateStoppingBackup;


! ! } else if (oldState != GSGameControllerStateError) {
! ! ! self.state = GSGameControllerStateStopped;
! ! }
! } else if (_state == GSGameControllerStateStoppingPrimary) {
! ! [self stopServicingBackups];
! ! [_server stop];
! ! // TODO: actually monitor the stop instead of just setting state to GSGameControllerStateStopped
! } else if (_state == GSGameControllerStateStoppingBackup) {
! ! [self stopMonitoringPrimary];
! ! [_clientToPrimary stop];
! ! // TODO: actually monitor the stop instead of just setting state to GSGameControllerStateStopped
! } else if (_state == GSGameControllerStateStopped) {
! ! if (oldState == GSGameControllerStatePrimary) {
! ! ! [self tearDownPrimary];
! ! } else ! if (oldState == GSGameControllerStateStoppingPrimary) {
! ! ! [self tearDownBackup];
! ! } else if (oldState == GSGameControllerStateStoppingBackup) {
! ! }
! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidStop:)];
! } else if (_state == GSGameControllerStateError) {
! ! if (oldState == GSGameControllerStateTryingToFindPrimary) {
! ! } else if (oldState == GSGameControllerStateTryingToBecomePrimary) {
! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStart:)];
! ! } else if (oldState == GSGameControllerStateTryingToConnectToPrimary) {


! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStart:)];
! ! } else if (oldState == GSGameControllerStatePrimary) {
! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?
! ! } else if (oldState == GSGameControllerStateStopping) {
! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStop:)];
! ! } else if (oldState == GSGameControllerStateStoppingPrimary) {
! ! } else if (oldState == GSGameControllerStateStoppingBackup) {
! ! }
! ! /* else ??? */ [self tellDelegate:_delegate
performSelectorWithSelf:@selector(gameControllerErrorOccurred:)];
! }
}

First Refactoring
{
self.state = GSGameControllerStateStopping;
}

- (void) setState: (GSGameControllerState)newState
{
! GSGameControllerState oldState = _state;
! _state = newState;

} else if (_state == GSGameControllerStateStopping) {
if (oldState == GSGameControllerStatePrimary) {
self.state = GSGameControllerStateStoppingPrimary;
} else if (oldState == GSGameControllerStateBackup) {
self.state = GSGameControllerStateStoppingBackup;
} else if (oldState != GSGameControllerStateError) {
self.state = GSGameControllerStateStopped;
}
}

} else if (_state == GSGameControllerStateStoppingPrimary) {
[self stopServicingBackups];
[_server stop];
self.state = GSGameControllerStateStopped;
}

} else if (_state == GSGameControllerStateStopped) {
[self tearDownPrimary];
} else if (oldState == GSGameControllerStateStoppingPrimary) {
[self tearDownBackup];
} else if (oldState == GSGameControllerStateStoppingBackup) {
}
[self tellDelegate:_delegate
performSelectorWithSelf:
@selector(gameControllerDidStop:)];
}

State Refactoring

{
[self.state stop: self];
}

Stopping a primary
@implementation GSGameControllerStatePrimary

- (void) enter: (GSGameController *)gc
{
! [gc installServerTargets];
}

- (void) error: (GSGameController *)gc
{
! [gc tearDownPrimary];
! [super error: gc];
}

- (void) incrementIntegerForKey: (id)aKey context: (GSGameController *)gc
{
! [gc primaryIncrementIntegerForKey: aKey];
}

- (void) stop: (GSGameController *)gc
{
! [self transition: gc to: [GSGameControllerStateStoppingPrimary state]];
}

@end

Only I know how
@implementation GSGameControllerStateStoppingPrimary

{
! [gc stopServicingBackups];
}

{
}

- (void) primaryDidStop: (GSGameController *)gc
{
! [self transition: gc to: [GSGameControllerStateStopped state]];
}

@end

Transition to stopped
@implementation GSGameControllerStateStoppingPrimary

{
! [gc stopServicingBackups];
}

{
}

- (void) primaryDidStop: (GSGameController *)gc
{
! [self transition: gc to: [GSGameControllerStateStopped state]];
}

@end

Done
@implementation GSGameControllerStateStopped

{
[gc tellDelegate:gc.delegate
}

@end

Same result on stop
} else if (_state == GSGameControllerStateStopped) {
} else if (oldState == GSGameControllerStateStoppingPrimary) {
} else if (oldState == GSGameControllerStateStoppingBackup) {
}
[self tellDelegate:_delegate
}

Result
• GSGameController only knows its own
operations
• Not states or transitions
• Separation of concerns
• Each state is a black box
• Doesn’t know details of other states

Conclusion

The State Design Pattern
made complex code easier
to understand and modify

Questions?

The State Design Pattern
made complex code easier
to understand and modify

Refactoring to the State Design Pattern

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