1、FSM簡介

1.1 有限狀態(tài)機的定義

有限狀態(tài)機（Finite State Machine，FSM）是一種數(shù)學模型，用于描述系統(tǒng)在不同狀態(tài)下的行為和轉移條件。

狀態(tài)機有三個組成部分：狀態(tài)（State）、事件（Event）、動作（Action），事件（轉移條件）觸發(fā)狀態(tài)的轉移和動作的執(zhí)行。動作的執(zhí)行不是必須的，可以只轉移狀態(tài)，不指定任何動作。總體而言，狀態(tài)機是一種用以表示有限個狀態(tài)以及這些狀態(tài)之間的轉移和動作的執(zhí)行等行為的數(shù)學模型。

狀態(tài)機可以用公式?State(S) , Event(E) -> Actions (A), State(S’)表示，即在處于狀態(tài)S的情況下，接收到了事件E，使得狀態(tài)轉移到了S’，同時伴隨著動作A的執(zhí)行。

Event（事件）是指觸發(fā)狀態(tài)轉換的輸入信號或條件。它可以是任何類型的輸入，例如傳感器數(shù)據(jù)、用戶輸入、網(wǎng)絡消息等。在編程中，Event通常是一個枚舉類型，每個枚舉值代表一個特定的事件。

State（狀態(tài)）是指系統(tǒng)在某一時刻所處的狀態(tài)，它是系統(tǒng)的一種抽象描述。在有限狀態(tài)機中，狀態(tài)是由一組狀態(tài)變量來描述的，這些狀態(tài)變量的取值決定了系統(tǒng)的狀態(tài)。狀態(tài)可以是離散的，也可以是連續(xù)的。在有限狀態(tài)機中，狀態(tài)通常用一個圓圈來表示，圓圈內部寫上狀態(tài)的名稱。例如，一個簡單的有限狀態(tài)機可以有兩個狀態(tài)：開和關，它們可以用以下方式表示：

Action（動作）是指在狀態(tài)轉移時執(zhí)行的操作或動作。當有限狀態(tài)機從一個狀態(tài)轉移到另一個狀態(tài)時，可以執(zhí)行一個或多個action來改變系統(tǒng)的狀態(tài)或執(zhí)行某些操作。例如，當有限狀態(tài)機從“待機”狀態(tài)轉移到“運行”狀態(tài)時，可以執(zhí)行一個action來啟動系統(tǒng)。在實際應用中，action可以是任何有效的代碼，例如函數(shù)調用、變量賦值、打印輸出等。

FSM 通常用于編程中，用于實現(xiàn)狀態(tài)轉移和控制流程。

注意：

在任何時刻，FSM 只能處于一種狀態(tài)。

1.2 Go中的FSM

通過上面關于有限狀態(tài)機的定義，我們大概知道了狀態(tài)機是個什么東西，那么Golang中是怎么實現(xiàn)的呢。不用慌，已經(jīng)有大佬實現(xiàn)好了，只管用就好了。

安裝：

go get github.com/looplab/fsm@v1.0.1

接下來一起看看github.com/looplab/fsm?是如何使用的。

2、github.com/looplab/fsm 如何使用

注意：

不同版本的 fsm 使用方式，可能不太一樣，最好是看下?NewFSM?函數(shù)的注釋，看下具體的細節(jié)。 本篇文章以：github.com/looplab/fsm@v1.0.1?為例。

2.1 fsm 基礎使用

這里把官方的例子改了下，感覺官方的例子不是很清晰。代碼如下：

package mainimport ("context""fmt""github.com/looplab/fsm"
)type Door struct {Name  stringFSM *fsm.FSM
}func NewDoor(name string) *Door {d := &Door{Name: name,}d.FSM = fsm.NewFSM("closed",fsm.Events{{Name: "open", Src: []string{"closed"}, Dst: "open"},{Name: "close", Src: []string{"open"}, Dst: "closed"},},fsm.Callbacks{"enter_state": func(_ context.Context, e *fsm.Event) { d.enterState(e) },},)return d
}func (d *Door) enterState(e *fsm.Event) {fmt.Printf("The door's name:%s , current state:%s\n", d.Name, e.Dst)
}func main() {door := NewDoor("測試")fmt.Printf("fsm current state: %s \n", door.FSM.Current())err := door.FSM.Event(context.Background(), "open")if err != nil {fmt.Println(err)}fmt.Printf("fsm current state: %s \n", door.FSM.Current())err = door.FSM.Event(context.Background(), "close")if err != nil {fmt.Println(err)}fmt.Printf("fsm current state: %s \n", door.FSM.Current())
}

執(zhí)行結果：

fsm current state: closed 
The door's name:測試 , current state:open
fsm current state: open 
The door's name:測試 , current state:closed
fsm current state: closed

這里就通過Event改變FSM中的狀態(tài)。轉移公式為：Src，Event -> Dst，d.enterState。大意就是接受到了輸入Event，狀態(tài)機的State由Src->Dst，并且執(zhí)行了Action：d.enterState。

2.2 fsm 中 Action 何時執(zhí)行？

剛開始使用的時候，好奇d.enterState(e)是什么時候調用的，我們一起看看?NewFSM?中的注釋就清楚了。

// NewFSM constructs a FSM from events and callbacks.
//
// The events and transitions are specified as a slice of Event structs
// specified as Events. Each Event is mapped to one or more internal
// transitions from Event.Src to Event.Dst.
// Callbacks are added as a map specified as Callbacks where the key is parsed
// as the callback event as follows, and called in the same order:
//
// 1. before_<EVENT> - called before event named <EVENT>
//
// 2. before_event - called before all events
//
// 3. leave_<OLD_STATE> - called before leaving <OLD_STATE>
//
// 4. leave_state - called before leaving all states
//
// 5. enter_<NEW_STATE> - called after entering <NEW_STATE>
//
// 6. enter_state - called after entering all states
//
// 7. after_<EVENT> - called after event named <EVENT>
//
// 8. after_event - called after all events
//
// There are also two short form versions for the most commonly used callbacks.
// They are simply the name of the event or state:
//
// 1. <NEW_STATE> - called after entering <NEW_STATE>
//
// 2. <EVENT> - called after event named <EVENT>
//
// If both a shorthand version and a full version is specified it is undefined
// which version of the callback will end up in the internal map. This is due
// to the pseudo random nature of Go maps. No checking for multiple keys is
// currently performed.

從上面我們知道了，d.enterState(e)?是在called after entering all states?時執(zhí)行的。

2.2.1 完整版書寫的Callbacks執(zhí)行順序

從上面的注釋能知道完整版書寫的Callbacks的執(zhí)行順序如下:

2.2.2 簡寫版的Callbacks執(zhí)行順序

2.2.3 注意事項

雖然Callbacks的寫法有兩種，但是不能同時使用完整版和簡寫版，否則最終使用那個版本是不確定的。

2.3 較為完整的例子

package mainimport ("context""fmt""github.com/looplab/fsm"
)type Door struct {Name  stringFSM *fsm.FSM
}func NewDoor(name string) *Door {d := &Door{Name: name,}d.FSM = fsm.NewFSM("closed",fsm.Events{{Name: "open", Src: []string{"closed"}, Dst: "open"},{Name: "close", Src: []string{"open"}, Dst: "closed"},},fsm.Callbacks{"before_open": func(_ context.Context, e *fsm.Event) { d.beforeOpen(e) },"before_event": func(_ context.Context, e *fsm.Event) { d.beforeEvent(e) },"leave_closed": func(_ context.Context, e *fsm.Event) { d.leaveClosed(e) },"leave_state": func(_ context.Context, e *fsm.Event) { d.leaveState(e) },"enter_open": func(_ context.Context, e *fsm.Event) { d.enterOpen(e) },"enter_state": func(_ context.Context, e *fsm.Event) { d.enterState(e) },"after_open": func(_ context.Context, e *fsm.Event) { d.afterOpen(e) },"after_event": func(_ context.Context, e *fsm.Event) { d.afterEvent(e) },},)return d
}func (d *Door) beforeOpen(e *fsm.Event) {fmt.Printf("beforeOpen, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) beforeEvent(e *fsm.Event) {fmt.Printf("beforeEvent, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) leaveClosed(e *fsm.Event) {fmt.Printf("leaveClosed, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) leaveState(e *fsm.Event) {fmt.Printf("leaveState, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) enterOpen(e *fsm.Event) {fmt.Printf("enterOpen, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) enterState(e *fsm.Event) {fmt.Printf("enterState, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) afterOpen(e *fsm.Event) {fmt.Printf("afterOpen, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func (d *Door) afterEvent(e *fsm.Event) {fmt.Printf("afterEvent, current state:%s, Dst:%s \n", d.FSM.Current(), e.Dst)
}func main() {door := NewDoor("測試")fmt.Printf("fsm current state: %s \n", door.FSM.Current())err := door.FSM.Event(context.Background(), "open")if err != nil {fmt.Println(err)}fmt.Printf("fsm current state: %s \n", door.FSM.Current())err = door.FSM.Event(context.Background(), "close")if err != nil {fmt.Println(err)}fmt.Printf("fsm current state: %s \n", door.FSM.Current())
}

執(zhí)行結果：大家重點看current state何時發(fā)生的變化。?

fsm current state: closed 
beforeOpen, current state:closed, Dst:open 
beforeEvent, current state:closed, Dst:open 
leaveClosed, current state:closed, Dst:open 
leaveState, current state:closed, Dst:open 
enterOpen, current state:open, Dst:open 
enterState, current state:open, Dst:open 
afterOpen, current state:open, Dst:open 
afterEvent, current state:open, Dst:open 
fsm current state: open 
beforeEvent, current state:open, Dst:closed 
leaveState, current state:open, Dst:closed 
enterState, current state:closed, Dst:closed 
afterEvent, current state:closed, Dst:closed 
fsm current state: closed 

?