Implemented the PreStart handler.

cmd/test/main.go

@@ -84,8 +84,10 @@ var beh = tg.NewBehaviour().
 	WithInitFunc(func(c *tg.Context) {
 		// The session initialization.
 		c.Session.Value = &UserData{}
-		c.ChangeScreen("start")
 
+	}). // On any message update before the bot created session.
+	WithPreStartFunc(func(c *tg.Context){
+		c.Send("Please, use the /start command to start the bot")
 	}).WithScreens(
 	tg.NewScreen("start").
 		WithText(
@@ -141,6 +143,11 @@ var beh = tg.NewBehaviour().
 			),
 		),
 ).WithCommands(
+	tg.NewCommand("start").
+		Desc("start the bot").
+		ActionFunc(func(c *tg.Context){
+			c.ChangeScreen("start")
+		}),
 	tg.NewCommand("hello").
 		Desc("sends the 'Hello, World!' message back").
 		ActionFunc(func(c *tg.Context) {

tg/beh.go

@@ -5,6 +5,7 @@ package tg
 
 // The type describes behaviour for the bot in personal chats.
 type Behaviour struct {
+	PreStart *action
 	Init      *action
 	Screens   ScreenMap
 	Keyboards KeyboardMap
@@ -27,12 +28,27 @@ func (b *Behaviour) WithInit(a Action) *Behaviour {
 	return b
 }
 
+// Alias to WithInit to simplify behaviour definitions.
 func (b *Behaviour) WithInitFunc(
 	fn ActionFunc,
 ) *Behaviour {
 	return b.WithInit(fn)
 }
 
+// Defines pre-start action.
+// E. g. when the user has not type the "/start" command.
+// Mostly used to send the "/start" command back
+// with some warning.
+func (b *Behaviour) WithPreStart(a Action) *Behaviour {
+	b.PreStart = newAction(a)
+	return b
+}
+
+// Alias for WithPreStart to be used with function inside.
+func (b *Behaviour) WithPreStartFunc(fn ActionFunc) *Behaviour {
+	return b.WithPreStart(fn)
+}
+
 // The function sets screens.
 func (b *Behaviour) WithScreens(
 	screens ...*Screen,

tg/bot.go

@@ -168,6 +168,9 @@ func (bot *Bot) handlePrivate(updates chan *Update) {
 		session, sessionOk := bot.sessions[sid]
 		chn, chnOk := chans[sid]
 		if sessionOk {
+			// Creating new goroutine for 
+			// the session that exists
+			// but has none.
 			if !chnOk {
 				ctx := &context{
 					Bot:     bot,
@@ -178,21 +181,19 @@ func (bot *Bot) handlePrivate(updates chan *Update) {
 				chans[sid] = chn
 				go ctx.handleUpdateChan(chn)
 			}
-		} else {
-			if u.Message != nil && u.Message.Command() == "start" {
-				if !sessionOk {
-					bot.sessions.Add(sid)
-				}
-				lsession := bot.sessions[sid]
-				ctx := &context{
-					Bot:     bot,
-					Session: lsession,
-					updates: make(chan *Update),
-				}
-				chn := make(chan *Update)
-				chans[sid] = chn
-				go ctx.handleUpdateChan(chn)
+		} else if u.Message != nil {
+			// Create session on any message
+			// if we have no one.
+			bot.sessions.Add(sid)
+			lsession := bot.sessions[sid]
+			ctx := &context{
+				Bot:     bot,
+				Session: lsession,
+				updates: make(chan *Update),
 			}
+			chn := make(chan *Update)
+			chans[sid] = chn
+			go ctx.handleUpdateChan(chn)
 		}
 
 		chn, ok := chans[sid]

tg/private.go

@@ -23,6 +23,8 @@ type context struct {
 func (c *context) handleUpdateChan(updates chan *Update) {
 	beh := c.Bot.behaviour
 
+	session := c.Session
+	preStart := beh.PreStart
 	if beh.Init != nil {
 		c.run(beh.Init, nil)
 	}
@@ -31,14 +33,34 @@ func (c *context) handleUpdateChan(updates chan *Update) {
 		screen := c.curScreen
 		// The part is added to implement custom update handling.
 		if u.Message != nil {
-			if u.Message.IsCommand() && !c.readingUpdate {
+			if !session.Started {
+				if u.Message.IsCommand() &&
+						u.Message.Command() == "start" {
+					// Special treatment for the "/start"
+					// command.
+					session.Started = true
+					cmdName := CommandName("start")
+					cmd, ok := beh.Commands[cmdName]
+					if ok {
+						act = cmd.Action
+					} else {
+						// Some usage.
+					}
+				} else {
+					// Prestart handling.
+					act = preStart
+				}
+			} else if u.Message.IsCommand() {
+				// Command handling.
 				cmdName := CommandName(u.Message.Command())
 				cmd, ok := beh.Commands[cmdName]
 				if ok {
 					act = cmd.Action
 				} else {
+					// Some usage.
 				}
 			} else {
+				// Simple messages handling.
 				kbd := screen.Keyboard
 				if kbd == nil {
 					if c.readingUpdate {
@@ -69,7 +91,7 @@ func (c *context) handleUpdateChan(updates chan *Update) {
 					act = btn.Action
 				}
 			}
-		} else if u.CallbackQuery != nil {
+		} else if u.CallbackQuery != nil && session.Started {
 			cb := tgbotapi.NewCallback(
 				u.CallbackQuery.ID,
 				u.CallbackQuery.Data,

tg/session.go

@@ -14,6 +14,9 @@ func (si SessionId) ToApi() int64 {
 type Session struct {
 	// Id of the chat of the user.
 	Id SessionId
+	// True if the session started.
+	// (got the '/start' command.
+	Started bool
 	// Custom value for each user.
 	Value any
 }