flow v2

[hellocashmere]

P.S: I used #657 as a basis.

In the basic version, I did not like that Flow = tele.HandlerFunc:

var email, password string
err := flowBus.Flow(
	"/start",
	flow.New().
                OnEachStep(loggingEachStep).
		Next(flow.NewStep(sendUserMessage("Enter email:")).Assign(TextAssigner(&email))).
		Next(flow.NewStep(sendUserMessage("Enter password:")).Assign(TextAssigner(&password))).
		Then(registerUser).
		Catch(flowFailed),
)

I would like to see the steps described inside the tele.HandlerFunc:

bot.Handle("/signup", func(c tele.Context) error {
		ctx := context.Background()

		f := bus.Flow("signup", 1*time.Minute)

		var username string

		f.OnText(func(state *teleflow.State) error {
			return state.Context.EditOrSend("Enter username:")
		}).Assign(func(state *teleflow.State) error {
			username = state.Context.Message().Text
			return nil
		})

		f.OnText(func(state *teleflow.State) error {
			return state.Context.EditOrSend("Enter password:")
		}).Ok(func(state *teleflow.State) error {
			log.Println("username:", username)
			log.Println("password:", state.Context.Message().Text)

			return nil
		})

		return bus.Start(ctx, c, f)
	})

Why is this option better in my opinion?
In all my bots, I have a common Handlers structure that contains methods for registration, deposits, etc:

type Handler struct {
	userService    *service.UserService
	orderService   *service.OrderService
	depositService *service.DepositService
	
	bus flow.Bus
}

func (h *Handler) SignUp() tele.HandlerFunc {
	return func(c tele.Context) error {
		// ...
	}
}

func (h *Handler) DepositByUserID() tele.HandlerFunc {
	return func(c tele.Context) error {
		// ...
	}
}

Therefore, I think my option is preferable, I want to thank @shindakioku for his code.