行为型设计模式-状态 State

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

• Context 上下文

  可视为状态机，其内部持有State引用

  可通过改变State的值，来达到改变Context行为的目的

• 抽象 State

• 具体 State

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。

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();

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.