大话设计模式读书笔记7——工厂方法模式

工厂方法模式

工厂方法模式（Factory Method）:定义一个用于创建对象的接口，让子类决定实例化哪一个类。工厂方法是一个类的实例化延迟到其子类。

工厂方法模式UML结构图

工厂方法模式结构代码

    /// <summary>
    /// 声明‘Product‘ 抽象类
    /// </summary>
    abstract class Product
    {
    }

    /// <summary>
    /// 声明 ‘ConcreteProductA‘ 类，继承Product
    /// </summary>
    class ConcreteProductA : Product
    {
    }

    /// <summary>
    ///  声明 ‘ConcreteProductB 类，继承Product
    /// </summary>
    class ConcreteProductB : Product
    {
    }

    /// <summary>
    /// 声明 ‘Creator‘ 抽象类
    /// </summary>
    abstract class Creator
    {
        //工厂方法
        public abstract Product FactoryMethod();
    }

    /// <summary>
    /// 声明 ‘ConcreteCreatorA‘ 类，继承Creator类
    /// </summary>
    class ConcreteCreatorA : Creator
    {
        //实现父类工厂方法返回类ConcreteProductA实例
        public override Product FactoryMethod()
        {
            return new ConcreteProductA();
        }
    }

    /// <summary>
    ///  声明 ‘ConcreteCreatorB‘ 类，继承Creator类
    /// </summary>
    class ConcreteCreatorB : Creator
    {
        //实现父类工厂方法返回类ConcreteProductB实例
        public override Product FactoryMethod()
        {
            return new ConcreteProductB();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Creator[] creators = new Creator[2];
            creators[0] = new ConcreteCreatorA();
            creators[1] = new ConcreteCreatorB();
            foreach (Creator creator in creators)
            {
                Product product = creator.FactoryMethod();
                Console.WriteLine("Created {0}",product.GetType().Name);
            }
            Console.ReadKey();
        }
    }

执行结果：

工厂模式实例代码

     /// <summary>
    /// 运算类
    /// </summary>
   public class Operation
    {
        public double NumA { get; set; }

        public double NumB { get; set; }

        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }

    /// <summary>
    /// 加法类
    /// </summary>
   public class OperationAdd : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA + NumB;
            return result;
        }
    }

    /// <summary>
    /// 减法类
    /// </summary>
    public class OperationSub : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA - NumB;
            return result;
        }
    }

    /// <summary>
    /// 工厂接口
    /// </summary>
    public interface IFactory
    {
        Operation FactoryMethod();
    }

    /// <summary>
    /// 加法工厂
    /// </summary>
    public class AddFactory : IFactory
    {
        public Operation FactoryMethod()
        {
            return new OperationAdd();
        }
    }

    /// <summary>
    /// 减法工厂
    /// </summary>
    public class SubFactory : IFactory
    {
        public Operation FactoryMethod()
        {
            return new OperationSub();
        }
    }

   class Program
    {
        static void Main(string[] args)
        {
            IFactory[] fac = new IFactory[2];
            fac[0] = new AddFactory();
            fac[1] = new SubFactory();

            Operation oper;
            oper = fac[0].FactoryMethod();
            oper.NumA=10;
            oper.NumB=5;
            Console.WriteLine("Called By {0}", oper.GetType().Name);
            Console.WriteLine("Result={0}", oper.GetResult().ToString());

            oper = fac[1].FactoryMethod();
            oper.NumA = 10;
            oper.NumB = 5;
            Console.WriteLine("Called By {0}", oper.GetType().Name);
            Console.WriteLine("Result={0}", oper.GetResult().ToString());

            Console.ReadKey();
        }
    }

运行结果：

简单工厂模式和工厂方法模式的区别

简单工厂模式的最大优点在于工厂类中包含了必要的逻辑判断，根据客户端的选择条件动态实例化相关的类，对于客户端来说，去除了与具体产品的依赖。

工厂方法模式实现时，客户端需要决定实例化哪一个工厂来实现运算类，选择判断的问题还是存在的，也就是说，工厂方法把简单工厂的内部逻辑判断移到了客户端代码来进行。如想要加功能，本来是改工厂类的，而现在是修改客户端。