工厂方法模式

工厂方法模式是对简单工厂模式的改进，它为每个对象增加了一个工厂类，专门用于生成该对象。

工厂方法实现加减乘除例子如下：

1 操作类

 public class Operation
    {
        public double NumberA { get; set; }
        public double NumberB { get; set; }

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

    public class OperationAdd : Operation
    {
        public override double GetResult()
        {
            return NumberA + NumberB;
        }
    }

    public class OperationSub : Operation
    {
        public override double GetResult()
        {
            return NumberA - NumberB;
        }
    }

    public class OperationMul : Operation
    {
        public override double GetResult()
        {
            return NumberA * NumberB;
        }
    }

    public class OperationDiv : Operation
    {
        public override double GetResult()
        {
            if (NumberB == 0)
            {
                throw new Exception("除数不能为0");
            }
            return NumberA / NumberB;
        }
    }

　　2 为每一个操作类添加一个工厂对象

     /// <summary>
    /// 工厂方法模式把简单工厂的内部逻辑判断移导了客户端代码来进行，增加方法时，不需要增加条件分支，只需增加对象和工厂
    /// </summary>
    interface IFactory
    {
        Operation CreateOperation();
    }

    public class AddFactory:IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationAdd();
        }
    }

    public class SubFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationSub();
        }
    }

    public class MulFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationMul();
        }
    }

    public class DivFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationDiv();
        }
    }

　　3 在客户端使用工厂生产需要使用的对象

IFactory addFactory = new AddFactory();

Operation operation = addFactory.CreateOperation();
operation.NumberA = 1;
operation.NumberB = 2;
Console.WriteLine(operation.GetResult());

　　运行结果为：3

      工厂方法模式把判断移到了客户端，并没有解决判断实例化哪个对象的问题，但这种模式在某些场景中是有很大的好处的，例如使用SQLServer数据库，代码开发完成后，如果数据改为Oracle，只需修改实例化工厂，不需要修改其它地方，更不需要修改被操作对象的实例化过程，下面比较使用工厂方法模式和不使用时代码的区别：

        private void UseFactory()
        {
            //如果所有加法变减法，只修改一句
            IFactory addFactory = new AddFactory();

            Operation operation1 = addFactory.CreateOperation();
            operation1.NumberA = 1;
            operation1.NumberB = 2;
            Console.WriteLine(operation1.GetResult());

            Operation operation2 = addFactory.CreateOperation();
            operation2.NumberA = 5;
            operation2.NumberB = 3;
            Console.WriteLine(operation1.GetResult());

            Operation operation3 = addFactory.CreateOperation();
            operation3.NumberA = 6;
            operation3.NumberB = 1;
            Console.WriteLine(operation1.GetResult());
        }


        private void unUseFactory()
        {
            //如果所有加法变减法，需要修改所有实例化的地方，简单工厂也存在该问题
            Operation operation1 = new OperationAdd();
            operation1.NumberA = 1;
            operation1.NumberB = 2;
            Console.WriteLine(operation1.GetResult());

            Operation operation2 = new OperationAdd();
            operation2.NumberA = 5;
            operation2.NumberB = 3;
            Console.WriteLine(operation1.GetResult());

            Operation operation3 = new OperationAdd();
            operation3.NumberA = 6;
            operation3.NumberB = 1;
            Console.WriteLine(operation1.GetResult());
        }

　　

总体而言，工厂发放克服了简单工厂违反开放-封闭原则的缺点，又保持了封装对象创建过程的优点，封装对象创建的过程使得更换对象时，程序不需要做大的改动，降低了客户端与具体对象的耦合度。