Throw error when recursion is detected in transition (#47)

* Use Xcode 12.5.1

* Throw error when recursion is detected in transition

* Add test for recursion detected error

Author: tinder-cfuller

Swift/Sources/StateMachine/StateMachine.swift

@@ -27,6 +27,11 @@ open class StateMachine<State: StateMachineHashable, Event: StateMachineHashable
         public struct Invalid: Error, Equatable {}
     }
 
+    public enum StateMachineError: Error {
+
+        case recursionDetected
+    }
+
     private struct Observer {
 
         weak var object: AnyObject?
@@ -51,6 +56,8 @@ open class StateMachine<State: StateMachineHashable, Event: StateMachineHashable
     private let states: States
     private var observers: [Observer] = []
 
+    private var isNotifying: Bool = false
+
     public init(@DefinitionBuilder build: () -> Definition) {
         let definition: Definition = build()
         state = definition.initialState.state
@@ -86,6 +93,8 @@ open class StateMachine<State: StateMachineHashable, Event: StateMachineHashable
 
     @discardableResult
     public func transition(_ event: Event) throws -> Transition.Valid {
+        guard !isNotifying
+        else { throw StateMachineError.recursionDetected }
         let result: Transition.Result
         defer { notify(result) }
         do {
@@ -111,6 +120,8 @@ open class StateMachine<State: StateMachineHashable, Event: StateMachineHashable
     }
 
     private func notify(_ result: Transition.Result) {
+        isNotifying = true
+        defer { isNotifying = false }
         var observers: [Observer] = []
         for observer in self.observers {
             guard observer.object != nil

Swift/Tests/StateMachineTests/StateMachineTests.swift

@@ -169,6 +169,28 @@ final class StateMachineTests: XCTestCase, StateMachineBuilder {
         // Then
         expect(transitionCount).to(equal(2))
     }
+
+    func testRecursionDetectedError() throws {
+
+        var error: TestStateMachine.StateMachineError? = nil
+
+        // Given
+        let stateMachine: TestStateMachine = givenState(is: .stateOne)
+
+        stateMachine.startObserving(self) { [unowned stateMachine] _ in
+            do {
+                try stateMachine.transition(.eventOne)
+            } catch let e as TestStateMachine.StateMachineError {
+                error = e
+            } catch {}
+        }
+
+        // When
+        try stateMachine.transition(.eventOne)
+
+        // Then
+        expect(error).to(equal(.recursionDetected))
+    }
 }
 
 final class Logger {