The easiest state machine in Swift

Here's another article of the serie "The easiest <something>". Previous ones on Core Data (The easiest Core Data) and Future and Promises (The easiest promises in Swift).

It was a cold Sunday afternoon when I decided to bring to Swift my ADBStateMachine implemented in Objective-C right about 5 years ago. Actually, it was... today, 16/12/2018.

Stateful 🦜

Here it is Stateful, a minimalistic, thread-safe, non-boilerplate and super easy to use state machine.

Stateful is available through CocoaPods and under the MIT license.

What is a state machine

I'm pretty much assuming the reader knows what a state machine is. It's is a mathematical model of computation, an abstract concept whereby the machine can have different states, but at a given time fulfills only one of them.

state-machine-example

  • State machines have an initial state
  • Other states can be reached via transitions from previous states
  • Transitions are performed in reaction to events

Example

Let's see how to user Stateful with a concrete example.

First, you should define the events and statuses you need. From the image above:

enum EventType {
    case click
    case success
    case failure
    case retry
}

enum StateType {
    case idle
    case fetching
    case error
}

Create a state machine with the initial state (you might want to retain it in a property)

let stateMachine = StateMachine<StateType, EventType>(initialState: StateType.idle)

StateMachine will use the main queue to execute the transition pre and post blocks but you can optionally provide a custom one.

let dispatchQueue = DispatchQueue(label: "com.albertodebortoli.someSerialCallbackQueue")
let stateMachine = StateMachine<StateType, EventType>(initialState: StateType.idle, callbackQueue: dispatchQueue)

Create transitions and add them to the state machine (the state machine will automatically recognize the new states).

let t1 = Transition<StateType, EventType>(with: .click,
                                          fromState: .idle,
                                          toState: .fetching)
                    
let t2 = Transition<StateType, EventType>(with: .success,
                                          from: .fetching,
                                          to: .idle)

let t3 = Transition<StateType, EventType>(with: .failure,
                                          fromState: .fetching,
                                          toState: .error,
                                          preBlock: {
                        print("Going to move from \(StateType.fetching) to \(StateType.error)!")
                                       }, postBlock: {
                        print("Just moved from \(StateType.fetching) to \(StateType.error)!")
    })
                  
let t4 = Transition<StateType, EventType>(with: .retry,
                                          fromState: .error,
                                          toState: .fetching)

stateMachine.add(transition: t1)
stateMachine.add(transition: t2)
stateMachine.add(transition: t3)
stateMachine.add(transition: t4)

Process events like so

stateMachine.process(event: .start)
stateMachine.process(event: .pause, callback: { result in
    switch result {
    case .success:
        print("Event 'pause' was processed")
    case .failure:
        print("Event 'pause' cannot currently be processed.")
    }
})

Logging

You can optionally enable logging to print extra state change information on the console

stateMachine.enableLogging = true

Example:

[Stateful 🦜]: Processing event 'start' from 'idle'
[Stateful 🦜]: Processed pre condition for event 'start' from 'idle' to 'started'
[Stateful 🦜]: Processed state change from 'idle' to 'started'
[Stateful 🦜]: Processed post condition for event 'start' from 'idle' to 'started'
[Stateful 🦜]: Processing event 'stop' from 'started'
[Stateful 🦜]: Processed pre condition for event 'stop' from 'started' to 'idle'
[Stateful 🦜]: Processed state change from 'started' to 'idle'
[Stateful 🦜]: Processed post condition for event 'stop' from 'started' to 'idle'

Conclusions

State machines come handy even to these days when the world seems to have moved to a more stateless way of doing things. Whenever you need to explicitly surface statuses, transition to a different state as a reaction of an event, and possibly leter recover state, a finite state machine might come handy.

Good examples that come to mind are:

  • manage overall application state
  • ease deeplinking implementation
  • ease UI testing

🦜🦜🦜

Special thanks to Matteo Comisso for contributing to Stateful adding support for Generics.