add args to Transition

[okhowang]

related #16

[qmuntal]

Can you specify why you need the arguments in the Transition? They are normally already passed in the function call-backs.

[okhowang]

I want write log once state changed
for debugging state machine.
And args is the detail info of transition.

[qmuntal]

I'm hesitant to add the Arguments property to Transition. It does make sense but there are already many callbacks whose parameters are a transition and the arguments, so it would be confusing to have the arguments in both place.

Yet your use case is valid, so I will think on a way to cover it. Other proposals are also welcomed.

[okhowang]

@qmuntal is any better way here?

[okhowang]

@qmuntal is any better way here after 2 years?

[daniel-sullivan]

@okhowang
This feature: #92 added the args to the external state handler. You can use a locally-scoped variable rather than an "external" state then add your log to this callback. e.g:

var smState stateless.State
var sm = stateless.NewStateMachineWithExternalStorageAndArgs(func(ctx context.Context) (stateless.State, []any, error) {
		return smState, nil, nil
	}, func(ctx context.Context, state stateless.State, args ...any) error {
		log.Info(fmt.Sprintf("now transitioning %s -> %s (args: %v)", smState, state, args))
		smState = state
		return nil
	}, stateless.FiringQueued
)

[okhowang]

stateMutator in stateless.NewStateMachineWithExternalStorageAndArgs is equal to OnTransitioning.
but there is no alternative for OnTransitioned which behaviour different with OnTransitioning, especially for sub state

for example

	var smState stateless.State = "init"
	var sm = stateless.NewStateMachineWithExternalStorageAndArgs(func(ctx context.Context) (stateless.State, []any, error) {
		return smState, nil, nil
	}, func(ctx context.Context, state stateless.State, args ...any) error {
		log.Printf("state transitioning %s -> %s (args: %v)", smState, state, args)
		smState = state
		return nil
	}, stateless.FiringQueued,
	)
	sm.Configure("init").
		Permit("start", "running").
		Permit("startThenStop", "running")
	sm.Configure("running").
		InitialTransition("sub-running").
		Permit("stop", "stopped").
		OnEntryFrom("startThenStop", func(ctx context.Context, args ...any) error {
			return sm.Fire("stop")
		})
	sm.Configure("sub-running").SubstateOf("running")
	sm.OnTransitioning(func(ctx context.Context, transition stateless.Transition) {
		log.Printf("on transitioning %s -> %s", transition.Source, transition.Destination)
	})
	sm.OnTransitioned(func(ctx context.Context, transition stateless.Transition) {
		log.Printf("on transitioned %s -> %s", transition.Source, transition.Destination)
	})
	_ = sm.Fire("startThenStop")

will output

2025/02/05 11:48:09 on transitioning init -> running
2025/02/05 11:48:09 state transitioning init -> running (args: [])
2025/02/05 11:48:09 on transitioning running -> sub-running
2025/02/05 11:48:09 state transitioning running -> sub-running (args: [])
2025/02/05 11:48:09 on transitioned init -> sub-running
2025/02/05 11:48:09 on transitioning sub-running -> stopped
2025/02/05 11:48:09 state transitioning sub-running -> stopped (args: [])
2025/02/05 11:48:09 on transitioned sub-running -> stopped