SwiftState

Elegant state machine for Swift.

Example

enum MyState: StateType {
    case state0, state1, state2
}

// setup state machine
let machine = StateMachine<MyState, NoEvent>(state: .state0) { machine in
    
    machine.addRoute(.state0 => .state1)
    machine.addRoute(.any => .state2) { context in print("Any => 2, msg=\(context.userInfo)") }
    machine.addRoute(.state2 => .any) { context in print("2 => Any, msg=\(context.userInfo)") }
    
    // add handler (`context = (event, fromState, toState, userInfo)`)
    machine.addHandler(.state0 => .state1) { context in
        print("0 => 1")
    }
    
    // add errorHandler
    machine.addErrorHandler { event, fromState, toState, userInfo in
        print("[ERROR] \(fromState) => \(toState)")
    }
}

// initial
XCTAssertEqual(machine.state, MyState.state0)

// tryState 0 => 1 => 2 => 1 => 0

machine <- .state1
XCTAssertEqual(machine.state, MyState.state1)

machine <- (.state2, "Hello")
XCTAssertEqual(machine.state, MyState.state2)

machine <- (.state1, "Bye")
XCTAssertEqual(machine.state, MyState.state1)

machine <- .state0  // fail: no 1 => 0
XCTAssertEqual(machine.state, MyState.state1)

This will print:

0 => 1
Any => 2, msg=Optional("Hello")
2 => Any, msg=Optional("Bye")
[ERROR] state1 => state0

Transition by Event

Use <-! operator to try transition by Event rather than specifying target State.

enum MyEvent: EventType {
    case event0, event1
}

let machine = StateMachine<MyState, MyEvent>(state: .state0) { machine in
    
    // add 0 => 1 => 2
    machine.addRoutes(event: .event0, transitions: [
        .state0 => .state1,
        .state1 => .state2,
    ])
    
    // add event handler
    machine.addHandler(event: .event0) { context in
        print(".event0 triggered!")
    }
}

// initial
XCTAssertEqual(machine.state, MyState.state0)

// tryEvent
machine <-! .event0
XCTAssertEqual(machine.state, MyState.state1)

// tryEvent
machine <-! .event0
XCTAssertEqual(machine.state, MyState.state2)

// tryEvent (fails)
machine <-! .event0
XCTAssertEqual(machine.state, MyState.state2, "event0 doesn't have 2 => Any")

If there is no Event-based transition, use built-in NoEvent instead.

State & Event enums with associated values

Above examples use arrow-style routing which are easy to understand, but it lacks in ability to handle state & event enums with associated values. In such cases, use either of the following functions to apply closure-style routing:

• machine.addRouteMapping(routeMapping)
  • RouteMapping: (_ event: E?, _ fromState: S, _ userInfo: Any?) -> S?
• machine.addStateRouteMapping(stateRouteMapping)
  • StateRouteMapping: (_ fromState: S, _ userInfo: Any?) -> [S]?

For example:

enum StrState: StateType {
    case str(String) ...
}
enum StrEvent: EventType {
    case str(String) ...
}

let machine = Machine<StrState, StrEvent>(state: .str("initial")) { machine in
    
    machine.addRouteMapping { event, fromState, userInfo -> StrState? in
        // no route for no-event
        guard let event = event else { return nil }
        
        switch (event, fromState) {
            case (.str("gogogo"), .str("initial")):
                return .str("Phase 1")
            case (.str("gogogo"), .str("Phase 1")):
                return .str("Phase 2")
            case (.str("finish"), .str("Phase 2")):
                return .str("end")
            default:
                return nil
        }
    }
    
}

// initial
XCTAssertEqual(machine.state, StrState.str("initial"))

// tryEvent (fails)
machine <-! .str("go?")
XCTAssertEqual(machine.state, StrState.str("initial"), "No change.")

// tryEvent
machine <-! .str("gogogo")
XCTAssertEqual(machine.state, StrState.str("Phase 1"))

// tryEvent (fails)
machine <-! .str("finish")
XCTAssertEqual(machine.state, StrState.str("Phase 1"), "No change.")

// tryEvent
machine <-! .str("gogogo")
XCTAssertEqual(machine.state, StrState.str("Phase 2"))

// tryEvent (fails)
machine <-! .str("gogogo")
XCTAssertEqual(machine.state, StrState.str("Phase 2"), "No change.")

// tryEvent
machine <-! .str("finish")
XCTAssertEqual(machine.state, StrState.str("end"))

This behaves very similar to JavaScript's safe state-container rackt/Redux, where RouteMapping can be interpretted as Redux.Reducer.

For more examples, please see XCTest cases.

Features

• Easy Swift syntax
  • Transition: .state0 => .state1, [.state0, .state1] => .state2
  • Try state: machine <- .state1
  • Try state + messaging: machine <- (.state1, "GoGoGo")
  • Try event: machine <-! .event1
• Highly flexible transition routing

  • Using Condition

  • Using .any state

    • Entry handling: .any => .someState
    • Exit handling: .someState => .any
    • Blacklisting: .any => .any + Condition

  • Using .any event

  • Route Mapping (closure-based routing): #36

• Success/Error handlers with order: UInt8 (more flexible than before/after handlers)
• Removable routes and handlers using Disposable
• Route Chaining: .state0 => .state1 => .state2
• Hierarchical State Machine: #10

Terms

Term	Type	Description
State	StateType (protocol)	Mostly enum, describing each state e.g. .state0.
Event	EventType (protocol)	Name for route-group. Transition can be fired via Event instead of explicitly targeting next State.
State Machine	Machine	State transition manager which can register Route/RouteMapping and Handler separately for variety of transitions.
Transition	Transition	From- and to- states represented as .state1 => .state2. Also, .any can be used to represent any state.
Route	Route	Transition + Condition.
Condition	Context -> Bool	Closure for validating transition. If condition returns false, transition will fail and associated handlers will not be invoked.
Route Mapping	(event: E?, fromState: S, userInfo: Any?) -> S?	Another way of defining routes using closure instead of transition arrows (=>). This is useful when state & event are enum with associated values. Return value (S?) means preferred-toState, where passing nil means no routes available. See #36 for more info.
State Route Mapping	(fromState: S, userInfo: Any?) -> [S]?	Another way of defining routes using closure instead of transition arrows (=>). This is useful when state is enum with associated values. Return value ([S]?) means multiple toStates from single fromState (synonym for multiple routing e.g. .state0 => [.state1, .state2]). See #36 for more info.
Handler	Context -> Void	Transition callback invoked when state has been changed successfully.
Context	(event: E?, fromState: S, toState: S, userInfo: Any?)	Closure argument for Condition & Handler.
Chain	TransitionChain / RouteChain	Group of continuous routes represented as .state1 => .state2 => .state3

Related Articles

1. Swiftで有限オートマトン(ステートマシン)を作る - Qiita (Japanese)
2. Swift+有限オートマトンでPromiseを拡張する - Qiita (Japanese)

Licence

MIT