SMStateMachine

A GCD backed Objective-C state machine. Released as open source under the MIT license.

Step 1

A state is a class that conforms to the SMState protocol. There are four class methods that handle the state transitions.

@interface MyInitialState : NSObject <SMState>
@end

@implementation MyInitialState

+ (void)willExitWithTransition:(SMTransition *)transition {
	NSLog(@"%s", __func__);
}

+ (void)willEnterWithTransition:(SMTransition *)transition {
	NSLog(@"%s", __func__);
}

+ (void)didEnterWithTransition:(SMTransition *)transition {
	NSLog(@"%s", __func__);
}

+ (SMState)didFireEvent:(id<SMEvent>)event stateContext:(id<SMState>)stateContext {
	MyInitialState *context = (MyInitialState *)stateContext;
	if (SMEvent(event) == SMEvent(MyStartEvent)) {
		return SMState(MyNextEvent);
	}
	return nil;
}
@end

Step 2

Create an event to trigger a transition.

@interface MyStartEvent : NSObject <SMEvent>
@property (nonatomic, copy, readonly) NSString *message;
- (instancetype)initWithMessage:(NSString *)message;
@end

@implementation MyStartEvent
- (instancetype)initWithMessage:(NSString *)message {
	self = [super init];
	if (self) {
		_message = [message copy];
	}
	return self;
}
@end

Step 3

Create an array of valid transitions that may occur in the state machine. This is for runtime validation of transitions.

NSArray *transitions = @[SMTransition(MyInitialState, MyNextState),
                         SMTransition(MyNextState, MyThirdState),
                         SMTransition(MyThirdState, MyNextState)];

Step 4

Create the state machine. Feed it the array of valid transitions and the initial state that the state machine should start in.

SMStateMachine *stateMachine = [SMStateMachine stateMachineWithTransitions:transitions 
                                                              initialState:SMState(MyInitialState)];

Step 5

Fire an event.

MyStartEvent *event = [[MyStartEvent alloc] initWithMessage:@"Hello, World!"];
[stateMachine fireEvent:event];

It's possible to fire events from within the willEnterWithTransition: and didEnterWithTransition: methods to trigger transitions.

Full Example

See Example.m for a full example.

memfrag / SMStateMachine