Design Pattern-Decorator

Design Pattern

  ↳Structural
    ↳Decorator

Main idea:

Attach additional responsibilities to an object dynamically.
Decorators provide a flexible alternative to subclassing for extending functionality.
动态的 给一个对象添加一些额外的职责，比生成子类更加灵活

装饰一个已有的对象，override或添加额外的功能，同时又不改变其结构。Dynamically decorate and remove. 看到的一个最合适的比喻：孙悟空有72变，当孙悟空变身成一座庙(Decorator)的时候，他还是孙悟空（被装饰的类本身），但是他拥有额外的庙的功能（additional responsibilities).

Example：

Component Class

public abstract class BoardComponent
{
    protected BoardComponent parent;

    public BoardComponent(){//}    
    public abstract void Operation();
    public abstract void Add(BoardComponent child);
    public abstract void Remove(BoardComponent child);
    public abstract void Update();
    public void SetParent(BoardComponent parent){//}
}

Component Concrete Class

public class Square extends BoardComponent
{
    public Square(){//}

    @Override
    public void Update() {
        //Square update
    }

    @Override
    public void Operation()
    {
        //Square operation
    }

    @Override
    public void Add(BoardComponent child){//}

    public ArrayList<BoardComponent> getChildren(){//}

    @Override
    public void Remove(BoardComponent child){//}
}

Decorator Class

单出一个decorator类是遵循了Open/Close原则，可以拓展装饰类，只有一个装饰类的话可以省略这一步，直接写装饰concrete类。

public abstract class Decorator extends Square{
    protected Square square;
    public BoardComponent parent;

    public Decorator(Square square) {
        this.square = square;
    }

    @Override
    public void Operation() {
        square.Operation();
    }

    @Override
    public void Update() {
        square.Update();
    }

    @Override
    public void Add(BoardComponent child) {
        square.Add(child);
    }

    @Override
    public void Remove(BoardComponent child) {
        square.Remove(child);
    }
}

Decorator Concrete Class

public class Shield extends Decorator{
    public Shield(Square square) {
        super(square);
    }

    @Override
    public void Operation() {
        //Shield operation
    }

    @Override
    public void Update() {
        //Shield update
    }

    @Override
    public void Add(BoardComponent child) {//}

    @Override
    public void Remove(BoardComponent child) {//}
}

使用Shield下的Square的成员：shield.square

Conclusion

Use Decorator when:
- To add responsibilities to individual objects dynamically and transparently, without affecting other objects
- For responsibilities that can be withdrawn. (e.g. logic gates)
- When extension by subclassing is impractical:
  - When you have a large number of independent extensions possible and subclassing would create an unmanageable number of small classes without much code in them
  - When a class definition may be hidden or unavailable for subclassing (e.g. final in Java)