设计模式是可重用的编程解决方案，用于解决常见问题，尤其有利于面向对象编程。创建型：工厂方法（创建对象）、抽象工厂（创建相关对象）；结构型：适配器（转换接口）、装饰器（动态添加功能）；行为型：观察者（一对多依赖通知）、策略（封装算法，可互换）。

PHP面向对象编程：设计模式全面解析

引言

设计模式是经过反复验证的、可重用的解决方案，用于解决常见编程问题。在面向对象编程（OOP）中，它们有助于编写可维护、可扩展和稳定的代码。本文将探讨PHP中的常见设计模式，并通过实战案例展示其应用。

创建型模式

• 工厂方法：创建一个对象，而不指定其确切的类。

  interface VehicleFactory {
    public function createVehicle(string $type): Vehicle;
  }
  
  class CarFactory implements VehicleFactory {
    public function createVehicle(string $type): Vehicle {
        return new Car();
    }
  }
  
  $factory = new CarFactory();
  $car = $factory->createVehicle('car');

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• 抽象工厂：提供一个接口，供创建相关或依赖对象家族之用。

  interface ShapeFactory {
    public function createShape(string $type): Shape;
  }
  
  class CircleFactory implements ShapeFactory {
    public function createShape(string $type): Shape {
        return new Circle();
    }
  }
  
  $factory = new CircleFactory();
  $shape = $factory->createShape('circle');

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结构型模式

• 适配器：将一个类的接口转换成另一个期望的接口。

  class OldSystem {
    public function getOldValue(): string {
        return 'old value';
    }
  }
  
  class NewSystem {
    public function getNewValue(): string {
        return 'new value';
    }
  }
  
  class Adapter implements OldSystem {
    private $newSystem;
  
    public function __construct(NewSystem $newSystem) {
        $this->newSystem = $newSystem;
    }
  
    public function getOldValue(): string {
        return $this->newSystem->getNewValue();
    }
  }
  
  $adapter = new Adapter(new NewSystem());
  $value = $adapter->getOldValue();

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• 装饰器：动态地将一个对象的功能添加到另一个对象。

  class Shape {
    public function draw(): void {
        echo 'drawing shape' . PHP_EOL;
    }
  }
  
  class Circle extends Shape {
    public function draw(): void {
        parent::draw();
        echo 'drawing circle' . PHP_EOL;
    }
  }
  
  class ColorDecorator extends Shape {
    private $shape;
    private $color;
  
    public function __construct(Shape $shape, string $color) {
        $this->shape = $shape;
        $this->color = $color;
    }
  
    public function draw(): void {
        $this->shape->draw();
        echo 'drawing ' . $this->color . ' shape' . PHP_EOL;
    }
  }
  
  $shape = new Circle();
  $redShape = new ColorDecorator($shape, 'red');
  $redShape->draw();

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行为型模式

• 观察者：定义对象之间的一种一对多依赖关系，当一个对象发生变化时，所有依赖的对象都会得到通知。

  class Subject {
    private $observers = [];
    private $state;
  
    public function getState(): string {
        return $this->state;
    }
  
    public function setState(string $state): void {
        $this->state = $state;
        $this->notifyObservers();
    }
  
    public function attach(Observer $observer): void {
        $this->observers[] = $observer;
    }
  
    public function detach(Observer $observer): void {
        $index = array_search($observer, $this->observers);
        if ($index !== false) {
            unset($this->observers[$index]);
        }
    }
  
    public function notifyObservers(): void {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }
  }
  
  class Observer {
    public function update(Subject $subject): void {
        echo 'observer notified, new state: ' . $subject->getState() . PHP_EOL;
    }
  }
  
  $subject = new Subject();
  $observer1 = new Observer();
  $observer2 = new Observer();
  $subject->attach($observer1);
  $subject->attach($observer2);
  $subject->setState('new state');

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• 策略：定义一系列算法，并分别封装它们，使得它们可以互换，独立于使用它们的客户端。

  interface PaymentStrategy {
    public function pay(float $amount): void;
  }
  
  class CreditCardStrategy implements PaymentStrategy {
    public function pay(float $amount): void {
        echo 'paying with credit card: ' . $amount . PHP_EOL;
    }
  }
  
  class PayPalStrategy implements PaymentStrategy {
    public function pay(float $amount): void {
        echo 'paying with PayPal: ' . $amount . PHP_EOL;
    }
  }
  
  class Order {
    private $paymentStrategy;
  
    public function __construct(PaymentStrategy $paymentStrategy) {
        $this->paymentStrategy = $paymentStrategy;
    }
  
    public function pay(float $amount): void {
        $this->paymentStrategy->pay($amount);
    }
  }
  
  $order = new Order(new CreditCardStrategy());
  $order->pay(100);

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