TransitionKit 2.2.1

TransitionKit 2.2.1

LangLanguage Obj-CObjective C
License Apache 2
ReleasedLast Release Jun 2015

Maintained by Blake Watters, Blake Watters.

  • By
  • Blake Watters

A simple, elegantly designed block based API for implementing State Machines in Objective-C

TransitionKit is a small Cocoa library that provides an API for implementing a state machine in Objective C. It is full-featured, completely documented, and very thoroughly unit tested. State machines are a great way to manage complexity in your application and TransitionKit provides you with an elegant API for implementing state machines in your next iOS or Mac OS X application.


  • Supports an arbitrary number of States and Events.
  • States and Events support a thorough set of block based callbacks for responding to state transitions.
  • States, Events, and State Machines are NSCopying and NSCoding compliant, enabling easy integration with archiving and copying in your custom classes.
  • Strongly enforced. The state machine includes numerous runtime checks for misconfigurations, making it easy to debug and trust your state machines.
  • Transitions support the inclusion of arbitrary user data via a userInfo dictionary, making it easy to broadcast metadata across callbacks.
  • Completely Documented. The entire library is marked up with Appledoc.
  • Thorougly unit tested. You know it works and can make changes with confidence.
  • Lightweight. TransitionKit has no dependencies beyond the Foundation library and works on iOS and Mac OS X.


Simple Example

The following example is a simple state machine that models the state of a Message in an Inbox.

TKStateMachine *inboxStateMachine = [TKStateMachine new];

TKState *unread = [TKState stateWithName:@"Unread"];
[unread setDidEnterStateBlock:^(TKState *state, TKTransition *transition) {
    [self incrementUnreadCount];
TKState *read = [TKState stateWithName:@"Read"];
[read setDidExitStateBlock:^(TKState *state, TKTransition *transition) {
    [self decrementUnreadCount];
TKState *deleted = [TKState stateWithName:@"Deleted"];
[deleted setDidEnterStateBlock:^(TKState *state, TKTransition *transition) {
    [self moveMessageToTrash];

[inboxStateMachine addStates:@[ unread, read, deleted ]];
inboxStateMachine.initialState = unread;

TKEvent *viewMessage = [TKEvent eventWithName:@"View Message" transitioningFromStates:@[ unread ] toState:read];
TKEvent *deleteMessage = [TKEvent eventWithName:@"Delete Message" transitioningFromStates:@[ read, unread ] toState:deleted];
TKEvent *markAsUnread = [TKEvent eventWithName:@"Mark as Unread" transitioningFromStates:@[ read, deleted ] toState:unread];

[inboxStateMachine addEvents:@[ viewMessage, deleteMessage, markAsUnread ]];

// Activate the state machine
[inboxStateMachine activate];

[inboxStateMachine isInState:@"Unread"]; // YES, the initial state

// Fire some events
NSDictionary *userInfo = nil;
NSError *error = nil;
BOOL success = [inboxStateMachine fireEvent:@"View Message" userInfo:userInfo error:&error]; // YES
success = [inboxStateMachine fireEvent:@"Delete Message" userInfo:userInfo error:&error]; // YES
success = [inboxStateMachine fireEvent:@"Mark as Unread" userInfo:userInfo error:&error]; // YES

success = [inboxStateMachine canFireEvent:@"Mark as Unread"]; // NO

// Error. Cannot mark an Unread message as Unread
success = [inboxStateMachine fireEvent:@"Mark as Unread" userInfo:nil error:&error]; // NO

// error is an TKInvalidTransitionError with a descriptive error message and failure reason

Unit Tests

TransitionKit is tested using the Kiwi BDD library. In order to run the tests, you must do the following:

  1. Install the dependencies via CocoaPods: pod install
  2. Open the workspace: open TransitionKit.xcworkspace
  3. Run the specs via the Product menu > Test


Blake Watters


TransitionKit is available under the Apache 2 License. See the LICENSE file for more info.