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An Objective-C library for elegantly implementing state machines.
Objective-C Ruby
Branch: master


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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.

Installation via CocoaPods

The recommended approach for installing TransitionKit is via the CocoaPods package manager, as it provides flexible dependency management and dead simple installation. For best results, it is recommended that you install via CocoaPods >= 0.16.0 using Git >= 1.8.0 installed via Homebrew.

Install CocoaPods if not already available:

$ [sudo] gem install cocoapods
$ pod setup

Change to the directory of your Xcode project, and Create and Edit your Podfile and add TransitionKit:

$ cd /path/to/MyProject
$ touch Podfile
$ edit Podfile
platform :ios, '5.0' 
# Or platform :osx, '10.7'
pod 'TransitionKit', '~> 2.0.0'

Install into your project:

$ pod install

Open your project in Xcode from the .xcworkspace file (not the usual project file)

$ open MyProject.xcworkspace


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.

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