装饰器设计模式 - 不扩展对象行为

Question

我正在学习有关设计模式的更多信息，并且我正在尝试实现装饰器模式。我想扩展一个计算机对象以在每个新组件的描述末尾添加一个字符串。这是 Unity/C# 改编自 Design Patterns For Dummies 一书中的 java 示例。

public class Computer {
    public Computer(){}

    public string Description(){
        return "Computer";
    }
}

这是每个组件将从中继承的装饰器组件：

public abstract class ComponentDecorator : Computer {
    new public abstract string Description();
}

这里有两个组件类 Monitor 和 Disk 可以装饰计算机类。

public class Monitor : ComponentDecorator {
    Computer computer;

    public Monitor(Computer c){
        this.computer = c;
    }

    public override string Description(){
        return computer.Description () + " and a Monitor";
    }

}

public class Disk : ComponentDecorator {
    Computer computer;

    public Disk(Computer c){
        this.computer = c;
    }

    public override string Description(){
        return computer.Description () + " and a Disk";
    }
}

下面是Start方法：

   void Start(){

        Computer computer = new Computer ();

        computer = new Disk (computer);
        computer = new Monitor (computer);

        print("You have a " + computer.Description () + ".");
    }

我的预期输出是：“您有一台计算机、一个监视器和一个磁盘。”

实际输出是：“你有一台电脑。”

计算机现在不应该像调用 Monitor 描述方法一样调用描述方法吗？如何修改它以获得预期的输出？

Answer 1

您可以通过让您正在装饰的类和装饰器类具有相同的抽象（接口或抽象类）来实现装饰器。下面的代码显示了使用抽象类。我在这里添加的 Device 类是装饰器类和装饰类的抽象。

public abstract class Device
{
    public abstract string Description();
}


public class Computer : Device
{
    public Computer() { }

    public override string Description()
    {
        return "Computer";
    }
}


public class Monitor : Device
{
    Device device;
    public Monitor(Device c)
    {            this.device = c;
    }

    public override string Description()
    {
        return device.Description() + " and a Monitor";
    }

}

public class Disk : Device
{
    Device device;

    public Disk(Device c)
    {
        this.device = c;
    }

    public override string Description()
    {
        return device.Description() + " and a Disk";
    }
}

您不需要装饰器类的抽象，除非您希望所有装饰器都实现任何特定方法。

Answer 2

您的实现有缺失点。 http://www.dofactory.com/net/decorator-design-pattern 有同样好的解释和 UML 图。我这样实现你的案例：

public abstract class ComputerParts
{
    public abstract string Description();
}

public class Computer : ComputerParts
{
    public Computer()
        {
        }

    public override string Description()
        {
            return "Computer";
        }
}

public abstract class ComponentDecorator : ComputerParts
{
    public abstract void ExtraMethod();
}

public class Disk : ComponentDecorator
{
    ComputerParts computerParts;

    public Disk(ComputerParts c)
        {
            this.computerParts = c;
        }

    public override string Description()
        {
            return computerParts.Description() + " and a Disk";
        }

    public override void ExtraMethod()
        {
            throw new NotImplementedException();
        }
}

public class Monitor : ComponentDecorator
{
    ComputerParts computerParts;

    public Monitor(ComputerParts c)
        {
            this.computerParts = c;
        }

    public override string Description()
        {
            return computerParts.Description() + " and a Monitor";
        }

    public override void ExtraMethod()
        {
            throw new NotImplementedException();
        }
}