模式的定义

装饰模式的定义：

Attach additional responsibilities to an object dynamically keeping the same interface.Decorators provide a flexible alternative to subclassing for extending functionality.

动态给一个对象添加一些额外的职责。就增加功能来说，装饰模式相比生成子类更为灵活。

类型

结构类

模式的使用场景

• 需要扩展一个类的功能，或给一个类增加附加功能
• 需要动态的给一个对象增加功能，这些功能可以再动态的撤销
• 需要为一批的兄弟类进行改装或加装功能

优点

• 装饰类和被装饰类可以独立发展，而不会相互耦合。换名话说，Component类无须知道Decorator类，Decorator类是从外部来扩展Component类的功能，而Decorator也不用知道具体的构件。
• 装饰模式是一个继承关系的一个替代方案。我们看装饰类Decorator，不管装饰多少层，返回对象还是Component，实现的还是is-a关系。
• 装饰模式可以动态的扩展一个实现类的功能

缺点

多层的装饰是比较复杂，要尽量减少装饰类的数量，以便降低系统的复杂度

UML类图

角色介绍

• Component类—抽象构件
  Component是一个接口或者是抽象类

• ConcreteComponent 类—-具体构件
  ConcreteComponent 是最核心，最原始，最基本的接口或抽象类的实现，你要装饰的就是它

• Decorator —-装饰角色
  一般是一个抽象类，实现接口或者抽象方法，它的属性里必然有一个private 变量指向Component抽象构件

• ConcreteDecorator1 ConcreteDecorator2 —具体装饰角色
  ConcreteDecorator1 ConcreteDecorator2 —具体装饰角色，把构件装饰成其它的东西。

模式的通用源码

Component类：

public abstract class Component {

    public abstract void operate();

}

ConcreteComponent 类：

public class ConcreteComponent extends Component {

    @Override
    public void operate() {
        // TODO Auto-generated method stub
        System.out.println("ConcreteComponent---operate()");
    }
}

Decorator 类：

public class Decorator extends Component {

    private Component component;

    public Decorator(Component component) {
        super();
        this.component = component;
    }

    @Override
    public void operate() {
        // TODO Auto-generated method stub
        component.operate();
    }

}

ConcreteDecorator1 类：

public class ConcreteDecorator1 extends Decorator {

    public ConcreteDecorator1(Component component) {
        super(component);
        // TODO Auto-generated constructor stub
    }

    private void method1(){
        System.out.println("ConcreteDecorator1---method1()");
    }

    @Override
    public void operate() {
        // TODO Auto-generated method stub
        System.out.println("ConcreteDecorator1---operate()");
        super.operate();
        method1();
    }

}

ConcreteDecorator2 类：

public class ConcreteDecorator2 extends Decorator {

    public ConcreteDecorator2(Component component) {
        super(component);
        // TODO Auto-generated constructor stub
    }

    private void method2(){
        System.out.println("ConcreteDecorator2---method2()");
    }

    @Override
    public void operate() {
        // TODO Auto-generated method stub
        System.out.println("ConcreteDecorator2---operate()");
        super.operate();
        method2();
    }

}

Client 类：

public class Client {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Component component = new ConcreteComponent();

        component = new ConcreteDecorator1(component);
        component.operate();
        System.out.println("-------------------------");
        component = new ConcreteDecorator2(component);
        component.operate();
    }
}

输出结果

ConcreteDecorator1---operate()
ConcreteComponent---operate()
ConcreteDecorator1---method1()
-------------------------
ConcreteDecorator2---operate()
ConcreteDecorator1---operate()
ConcreteComponent---operate()
ConcreteDecorator1---method1()
ConcreteDecorator2---method2()

Android源码中的模式实现

样例：ToastingBurstFacadeDecorator

./packages/apps/Camera2/src/com/android/camera/burst/ToastingBurstFacadeDecorator.java

uml图：

简单分析：

//注释也明确说明这是一个装饰模式--A simple decorator
/**
 * A simple decorator for a {@link BurstFacade} that shows toasts for when a
 * burst starts or stops.
 * <p>
 * This class can simply be removed once we have proper UI for this.
 */
 //类ToastingBurstFacadeDecorator 装饰的类就是BurstFacade，这也是is-a的实现
public class ToastingBurstFacadeDecorator implements BurstFacade {

     //这就是BurstFacade类，要被装饰的对象
    /** Shows burst-related toasts to the user. */
    public static class BurstToaster {
        private final Context mContext;

        public BurstToaster(Context context) {
            mContext = context;
        }

        public void showToastBurstStarted() {
            Toast.makeText(mContext, MSG_BURST_STARTED, Toast.LENGTH_SHORT).show();
        }

        public void showToastBurstStopped() {
            Toast.makeText(mContext, MSG_BURST_STOPPED, Toast.LENGTH_SHORT).show();
        }
    }
    ......
    ......            
    //这就是have-a的实现
    private final BurstFacade mBurstFacade;
    private final BurstToaster mToaster;

    /**
     * Initialize the toasting burst facade decorator.
     *
     * @param facadeToDecorate the facade to decorate.
     * @param toaster the toaster to use to show toasts about the burst status.
     */
    public ToastingBurstFacadeDecorator(BurstFacade facadeToDecorate, BurstToaster toaster) {
        mBurstFacade = facadeToDecorate;
        mToaster = toaster;
    }

    @Override
    public void startBurst(CaptureSession.CaptureSessionCreator captureSessionCreator,
            DeviceOrientation deviceOrientation, Facing cameraFacing, int imageOrientationDegrees) {
        mToaster.showToastBurstStarted();
        mBurstFacade.startBurst(captureSessionCreator, deviceOrientation, cameraFacing,
                imageOrientationDegrees);
    }


    @Override
    public boolean stopBurst() {
        boolean burstStopped = mBurstFacade.stopBurst();

        // Only show the toast if a burst was actually stopped.
        if (burstStopped) {
            mToaster.showToastBurstStopped();
        }
        return burstStopped;
    }

    @Override
    public void initialize(SurfaceTexture surfaceTexture) {
        mBurstFacade.initialize(surfaceTexture);
    }

    @Override
    public void release() {
        mBurstFacade.release();
    }

    @Override
    public Surface getInputSurface() {
        return mBurstFacade.getInputSurface();
    }

    @Override
    public void setBurstTaker(BurstTaker burstTaker) {
        mBurstFacade.setBurstTaker(burstTaker);
    }
}

杂谈

我们平时经常使用的是继承类和实现接口，这是一种is-a形式，
而我们另外也经常使用一个将对象以属性的形式包含到一个类中的have-a形式。

装饰模式就是一种have-a和is-a模式的二个结合。

至于装饰模式是怎么做到这二者的结合，类图就给了我们详细的解释。

参考资料

(1).设计模式之禅—第17章 装饰模式
(2)装饰模式
https://github.com/simple-android-framework/android_design_patterns_analysis/tree/master/decorator/tiny-times