装饰者模式

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The decorator pattern can be used to extend (decorate) the functionality of a certain object statically, or in some cases at run-time, independently of other instances of the same class, provided some groundwork is done at design time. This is achieved by designing a new Decorator class that wraps the original class. This wrapping could be achieved by the following sequence of steps:

  1. Subclass the original Component class into a Decorator class (see UML diagram);
  2. In the Decorator class, add a Component pointer as a field;
  3. In the Decorator class, pass a Component to the Decorator constructor to initialize the Component pointer;
  4. In the Decorator class, forward all Component methods to the Component pointer;
  5. In the ConcreteDecorator class, override any Component method(s) whose behavior needs to be modified.

“装饰者模式”：在不改变原类和继承的前提下动态地扩展一个对象的功能；它是通过__包裹（Wrap）__方式来实现的，即创建一个包含_目标对象引用_的装饰器（函数或类），然后装饰器生成目标对象的_包裹对象_，该包裹对象就如目标对象的克隆体一样，它拥有和目标对象一样的接口（方法）但这些接口都是加强版的，最后返回包裹对象。

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// PHP示例代码，来自网络

abstract class Component {
  protected $data;
  protected $value;
  abstract public function getData();
  abstract public function getValue();
}

class ConcreteComponent extends Component {
  public function __construct() {
    $this->value = 1000;
    $this->data = "Concrete Component:\t{$this->value}\n";
  }

  public function getData() {
    return $this->data;
  }

  public function getValue() {
    return $this->value;
  }
}

abstract class Decorator extends Component {}

class ConcreteDecorator1 extends Decorator {  // 该类返回包裹对象
  public function __construct(Component $data) {  // Component $data就是目标对象的引用
    $this->value = 500;
    $this->data = $data;
  }

  // 加强版的接口
  public function getData() {
    return $this->data->getData() . "Concrete Decorator 1:\t{$this->value}\n";
  }

  public function getValue() {
    return $this->value + $this->data->getValue();
  }
}

class ConcreteDecorator2 extends Decorator {
  public function __construct(Component $data) {
    $this->value = 500;
    $this->data = $data;
  }

  public function getData() {
    return $this->data->getData() . "Concrete Decorator 2:\t{$this->value}\n";
  }

  public function getValue() {
    return $this->value + $this->data->getValue();
  }
}

class Client {
  private $component;

  public function __construct() {
    $this->component = new ConcreteComponent();
    $this->component = $this->wrapComponent($this->component);
    echo $this->component->getData();
    echo "Client:\t\t\t";
    echo $this->component->getValue();
  }

  private function wrapComponent(Component $component) {
    $component1 = new ConcreteDecorator1($component);
    $component2 = new ConcreteDecorator2($component1);
    return $component2;
  }
}

$client = new Client();

// Result: 
// Concrete Component:   1000
// Concrete Decorator 1: 500
// Concrete Decorator 2: 500
// Client:               2000