5.4.2.4.2. Default Transition

Default Transition

Default Transition is the way to move from one State to another, by this changing stable configuration of State Machine.

Default Transitions are not explicitly triggered by Events sent to State Machine Behavior, but are automaticaly triggered by Completion Event sent to State Machine Behavior by Stateflows framework after each successful Transition.

Default Transition consist of:

• Source State or Pseudostate,
• Target State or Pseudostate,
• Guard, a predicate logic (that returns boolean value) that is evaluated in order to check if Transition can happen or not,
• Effect, logic that is executed when Transition is happening.

Definition

In UML, Default Transition is represented by an arrow that connects two States or Pseudostates, but has no Trigger:

stateDiagram-v2
SomeState --> AnotherState

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If there is any logic within Transition, it is described textually using following special characters:

stateDiagram-v2
SomeState --> AnotherState : [Guard]<br/>/Effect

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Equivalent Stateflows notation of Default Transition:

Lambda style

Definition fragment (inside the builder callback):

    /* fragment of State Machine definition */
    .AddState("SomeState", b => b
        .AddDefaultTransition("AnotherState", b => b
            .AddGuard(async c => true /* bool-returning logic here */)
            .AddEffect(c => Task.CompletedTask) 
        )
    )
    .AddState("AnotherState")

Using lambda style here means that States are referred by string name and each bit of Transition logic is implemented as lambda function passed as parameters to Add* methods in Transition definition.

Typed style

    public class SomeState : IState
    { }

    public class AnotherState : IState
    { }

    public class MyDefaultTransition : IDefaultTransitionGuard, IDefaultTransitionEffect
    {
        public Task<bool> GuardAsync()
        {
            /* bool-returning logic here */
            return Task.FromResult(true);
        }

        public Task EffectAsync()
        {
            return Task.CompletedTask;
        }
    }

Definition fragment (inside the builder callback):

    /* fragment of State Machine definition */
    .AddState<SomeState>(b => b
        .AddDefaultTransition<MyDefaultTransition, AnotherState>()
    )
    .AddState<AnotherState>()

Using typed style here means that States and Transitions are classes that is referenced in typed overloads of Add* methods in State Machine definition.

Note: AddDefaultTransition<>() method accepts several type parameters. Regardless the overload variant, the order of those parameters is always the same: Transition class > Target State / Pseudostate class.

Class can be used as Default Transition if it implements at least one interface from IDefaultTransition*<TEvent> family:

• IDefaultTransition - simply represents Default Transition,
• IDefaultTransitionGuard - represents Transition with Guard logic implemented as GuardAsync() method,
• IDefaultTransitionEffect - represents Transition with Effect logic implemented as EffectAsync() method,
• IDefaultTransitionDefinition - represents Transition with additional Build() method that allows to extend its definition, for example to define Guards and Effects in Default Transition class.

Transition logic may also be split into several classes like in the example:

    public class SomeState : IState
    { }

    public class AnotherState : IState
    { }

    public class MyDefaultGuard : IDefaultTransitionGuard
    {
        public Task<bool> GuardAsync()
        {
            /* bool-returning logic here */
            return Task.FromResult(true);
        }
    }

    public class MyDefaultEffect : IDefaultTransitionEffect
    {
        public Task EffectAsync()
        {
            return Task.CompletedTask;
        }
    }

Definition fragment (inside the builder callback):

    /* fragment of State Machine definition */
    .AddState<SomeState>(b => b
        .AddDefaultTransition<AnotherState>(b => b
            .AddGuard<MyDefaultGuard>()
            .AddEffect<MyDefaultEffect>()
        )
    )
    .AddState<AnotherState>()

This technique enables reusing of Default Transition's logic.

Transition classes can constructor-inject ITransitionContext, IStateMachineContext, common behavior/execution contexts, and application services. They cannot inject a current IStateContext; use ITransitionContext.Source / .Target to access the source and target state context.

Generic Transitions

If Default Transition logic is suitable to be reused in other Transitions, Default Transition classes may be created by implementing at least one interface from ITransition* family:

• ITransitionGuard - represents Transition with Guard logic implemented as GuardAsync() method,
• ITransitionEffect - represents Transition with Effect logic implemented as EffectAsync() method,
• ITransitionDefinition - represents Transition with additional Build() method that allows to extend its definition, for example to define Guards and Effects in Transition class.

Such classes are called Generic Transitions and may be used to declare all types of Transitions.