Golang如何使用状态模式简化条件判断

type OrderState interface {
    Pay(order *Order)
    Ship(order *Order)
    Complete(order *Order)
    Cancel(order *Order)
}

type PendingState struct{}

func (s *PendingState) Pay(order *Order) {
    fmt.Println("订单已支付")
    order.setState(&PaidState{})
}

func (s *PendingState) Ship(order *Order) {
    fmt.Println("无法发货：订单未支付")
}

func (s *PendingState) Complete(order *Order) {
    fmt.Println("无法完成：订单未发货")
}

func (s *PendingState) Cancel(order *Order) {
    fmt.Println("订单已取消")
    order.setState(&CancelledState{})
}

type PaidState struct{}

func (s *PaidState) Pay(order *Order) {
    fmt.Println("订单已支付，无需重复操作")
}

func (s *PaidState) Ship(order *Order) {
    fmt.Println("已发货")
    order.setState(&ShippedState{})
}

type Order struct {
    state OrderState
}

func NewOrder() *Order {
    return &Order{state: &PendingState{}}
}

func (o *Order) setState(state OrderState) {
    o.state = state
}

// 委托调用
func (o *Order) Pay()    { o.state.Pay(o) }
func (o *Order) Ship()   { o.state.Ship(o) }
func (o *Order) Complete() { o.state.Complete(o) }
func (o *Order) Cancel() { o.state.Cancel(o) }

order := NewOrder()
order.Pay()     // 输出：订单已支付
order.Ship()    // 输出：已发货
order.Complete() // 输出：订单已完成
order.Cancel()  // 输出：无法取消：已完成订单