简介
跟状态机密切相关。有限状态机 FSM 包含 状态、事件、动作三个元素。
当发生一个事件时，引发老状态变成新状态，并执行一个动作。
状态和行为间一般是有限制的，如某些行为只能再某些状态下进行，某些状态只能触发某些行为。
简单的状态间转换可使用 if else。
更有条理的可以用查表法：二维表中纵向是状态，横向是事件，value 则是新状态和动作。做成二维数组配置，事件发生时结合当前状态，组成[老状态][事件]索引去查二维表，得出 value 是新状态和动作，如果没有 value 则不变换
再复杂一些之后，可使用状态模式。
角色
类图
如图，Context/Machine 中可以 changeState 来改变当前的状态，其中 doThis 和 doThat 签名与 State 一致，这样看起来就像改变状态的同时，改变了 Context 的行为。
类图
代码
class Context{ private $state;
public function __construct(State $state) { $this->transitionTo($state); }
public function transitionTo(State $state): void { echo "Context: Transition to " . get_class($state) . ".\n"; $this->state = $state; $this->state->setContext($this); }
public function request1(): void { $this->state->handle1(); }
public function request2(): void { $this->state->handle2(); }}abstract class State{ protected $context;
public function setContext(Context $context) { $this->context = $context; }
abstract public function handle1(): void;
abstract public function handle2(): void;}
class ConcreteStateA extends State{ public function handle1(): void { echo "ConcreteStateA handles request1.\n"; echo "ConcreteStateA wants to change the state of the context.\n"; $this->context->transitionTo(new ConcreteStateB()); }
public function handle2(): void { echo "ConcreteStateA handles request2.\n"; }}
class ConcreteStateB extends State{ public function handle1(): void { echo "ConcreteStateB handles request1.\n"; }
public function handle2(): void { echo "ConcreteStateB handles request2.\n"; echo "ConcreteStateB wants to change the state of the context.\n"; $this->context->transitionTo(new ConcreteStateA()); }}
$context = new Context(new ConcreteStateA());$context->request1();$context->request2();
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output:
Context: Transition to ConcreteStateA.ConcreteStateA handles request1.ConcreteStateA wants to change the state of the context.Context: Transition to ConcreteStateB.ConcreteStateB handles request2.ConcreteStateB wants to change the state of the context.Context: Transition to ConcreteStateA.
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