状态机：choice 与 fork

简介 / Introduction

状态机（State Machine）常用于描述对象在其生命周期内的状态流转。除了普通的状态节点，还依赖两种重要的伪状态（Pseudostate）：

choice — 条件分支，类似于 if/else，根据守卫条件选择下一个状态。
fork — 并行分支，同时从一点分叉出多个并发执行路径。

本文用同一个业务场景（订单处理）分别演示 PlantUML 与 Mermaid 的写法，并做横向对比。

PlantUML 版

@startuml
hide empty description

state "新订单" as New
state "已支付" as Paid
state "已发货" as Shipped
state "已签收" as Received
state "已取消" as Cancelled
state "退款中" as Refunding

[*] --> New
New --> Paid : paymentOk
New --> Cancelled : paymentFail
Paid --> Shipped : ship
Paid --> Refunding : refundRequest

state fork_state <<fork>>
Paid --> fork_state
fork_state --> Shipped
fork_state --> Refunding

state join_state <<join>>
Shipped --> join_state
Refunding --> join_state
join_state --> Received

Received --> [*]
Cancelled --> [*]
@enduml

PlantUML 使用 <<fork>> 与 <<join>> 关键字在状态图上标注伪状态，fork 之后多个箭头并行发出。

Mermaid 版

stateDiagram-v2
    [*] --> New
    New --> Paid : paymentOk
    New --> Cancelled : paymentFail
    Paid --> Shipped : ship
    Paid --> Refunding : refundRequest

    Paid --> Fork : ship
    Fork --> Shipped
    Fork --> Refunding

    Shipped --> Received
    Refunding --> Received
    Received --> [*]
    Cancelled --> [*]

Mermaid 使用 Fork 与 Join 关键字作为特殊状态节点，实现与 PlantUML 相同的效果。

对比 / Side-by-side

特性	PlantUML	Mermaid
分支伪状态	`state X <<choice>>`	`X --> [*] : guard`（内联守卫）
fork 语法	`<<fork>>` + `<<join>>`	`Fork` / `Join` 节点
并行箭头	多个 `--> Target` 从 fork 发出	从 `Fork` 节点分出多条路径
语法风格	类图/活动图风格	YAML 风格简洁语法

小结 / Wrap-up

choice 与 fork 是状态机建模的核心伪状态。PlantUML 通过 <<choice>>、<<fork>>、<<join>> 标注实现，语义清晰但语法略繁；Mermaid 用内置的 Fork/Join 节点实现，语法更简洁但不支持显式 choice 伪状态，守卫直接写在转移线上。选择哪个工具取决于项目对语义精确度和语法简洁性的取舍。