重构——一个小例子

菜鸟区域，老鸟绕路！

原代码，这是一个可以借阅影片的小程序，你可以想象成某个大型系统，我想代码应该都能很容易看懂：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Reconsitution
{
    public class Movie
    {
        /// <summary>
        /// 常规的
        /// </summary>
        public const int Regular = 0;
        /// <summary>
        /// 新发布
        /// </summary>
        public const int NewRelease = 1;
        /// <summary>
        /// 儿童版
        /// </summary>
        public const int Childrens = 2;

        public string Title { get; private set; }
        public int PriceCode { get; set; }

        public Movie(string title, int priceCode)
        {
            Title = title;
            PriceCode = priceCode;
        }
    }

    public class Rental
    {
        public Movie @Movie { get; private set; }
        public int DaysRented { get; private set; }

        public Rental(Movie movie, int daysRented)
        {
            @Movie = movie;
            DaysRented = daysRented;
        }
    }

    public class Customer
    {
        public string Name { get; private set; }

        public List<Rental> Rentals
        {
            get { return _rentals; }
        }
        private readonly List<Rental> _rentals = new List<Rental>();

        public Customer(string name)
        {
            Name = name;
        }

        public void AddRental(Rental rental)
        {
            _rentals.Add(rental);
        }

        public string Statement()
        {
            double totalAmount = 0;
            int frequentRenterPoints = 0;
            string result = "Rental Record for " + Name + "\n";

            foreach (Rental rental in _rentals)
            {
                double thisAmount = 0;

                switch (rental.Movie.PriceCode)
                {
                    case Movie.Regular:
                        thisAmount += 2;
                        if (rental.DaysRented > 2)
                            thisAmount += (rental.DaysRented - 2)*1.5;
                        break;
                    case Movie.NewRelease:
                        thisAmount += rental.DaysRented*3;
                        break;
                    case Movie.Childrens:
                        thisAmount += 1.5;
                        if (rental.DaysRented > 3)
                            thisAmount += (rental.DaysRented - 3) * 1.5;
                        break;
                }

                frequentRenterPoints ++;
                if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
                    frequentRenterPoints ++;

                result += "\t" + rental.Movie.Title + "\t" + thisAmount + "\n";
                totalAmount += thisAmount;
            }

            result += "Amount owed is " + totalAmount + "\n";
            result += "You earned " + frequentRenterPoints + " frequent renter points";
            return result;
        }
    }
}

Customer customer = new Customer("Dennis Ding");
customer.AddRental(new Rental(new Movie("Water World", Movie.Childrens), 5));
customer.AddRental(new Rental(new Movie("Roman Holiday", Movie.Regular), 3));
customer.AddRental(new Rental(new Movie("The First Blood", Movie.NewRelease), 6));
TextBox1.Text = customer.Statement();

一、拆解主体方法

简要预览代码，我们很直观的发现 Customer 中的方法 Statement 过长，不便于阅读，于是我们首先便考虑解体方法 Statement，我们可以这样考虑，为什么要重构，重构再于便于后期维护和功能拓展，对于Statement方法，生成一个文字版的影片借阅结果单，如果我们版本更新，要生成一个Html的结果单，或者直接返回数据Json，形成接口供第三方调用呢，我们不得不重新写一个叫HtmlStatement 或者 JsonStatement的方法，因为我们把细节到计算的整个生成逻辑多放在了这一个方法中了！这是一种很不合理的设计！

public string Statement()
{
    double totalAmount = 0;
    int frequentRenterPoints = 0;
    string result = "Rental Record for " + Name + "\n";

    foreach (Rental rental in _rentals)
    {
        var thisAmount = AmountFor(rental);

        frequentRenterPoints ++;
        if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
            frequentRenterPoints ++;

        result += "\t" + rental.Movie.Title + "\t" + thisAmount + "\n";
        totalAmount += thisAmount;
    }

    result += "Amount owed is " + totalAmount + "\n";
    result += "You earned " + frequentRenterPoints + " frequent renter points";
    return result;
}

private static double AmountFor(Rental rental)
{
    double thisAmount = 0;

    switch (rental.Movie.PriceCode)
    {
        case Movie.Regular:
            thisAmount += 2;
            if (rental.DaysRented > 2)
                thisAmount += (rental.DaysRented - 2)*1.5;
            break;
        case Movie.NewRelease:
            thisAmount += rental.DaysRented*3;
            break;
        case Movie.Childrens:
            thisAmount += 1.5;
            if (rental.DaysRented > 3)
                thisAmount += (rental.DaysRented - 3)*1.5;
            break;
    }
    return thisAmount;
}

这时候，我们发现，方法 AmountFor 并没有使用到和 Customer 相关的数据，但却放在 Customer 中，于是我们先考虑吧方法移到 Rental 中，

    public class Rental
    {
        public Movie Movie { get; private set; }
        public int DaysRented { get; private set; }

        public Rental(Movie movie, int daysRented)
        {
            Movie = movie;
            DaysRented = daysRented;
        }

        public double AmountFor()
        {
            double thisAmount = 0;

            switch (this.Movie.PriceCode)
            {
                case Movie.Regular:
                    thisAmount += 2;
                    if (DaysRented > 2)
                        thisAmount += (DaysRented - 2) * 1.5;
                    break;
                case Movie.NewRelease:
                    thisAmount += DaysRented * 3;
                    break;
                case Movie.Childrens:
                    thisAmount += 1.5;
                    if (DaysRented > 3)
                        thisAmount += (DaysRented - 3) * 1.5;
                    break;
            }
            return thisAmount;
        }
    }

原 Statement 方法变成 ：

public string Statement()
{
    double totalAmount = 0;
    int frequentRenterPoints = 0;
    string result = "Rental Record for " + Name + "\n";

    foreach (Rental rental in _rentals)
    {
        var thisAmount = rental.AmountFor();

        frequentRenterPoints ++;
        if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
            frequentRenterPoints ++;

        result += "\t" + rental.Movie.Title + "\t" + thisAmount + "\n";
        totalAmount += thisAmount;
    }

    result += "Amount owed is " + totalAmount + "\n";
    result += "You earned " + frequentRenterPoints + " frequent renter points";
    return result;
}

（为了让方法符合实际功能，我们可以把方法 AmountFor 命名为 GetCharge）

在方法Statement中，此时我们看到这样的代码

var thisAmount = rental.GetCharge();

//……被省略的代码
result += "\t" + rental.Movie.Title + "\t" + thisAmount + "\n";
totalAmount += thisAmount;

由于 rental.GetCharge() 被使用两次，为了避免二次计算，代码中使用了一个临时变量 thisAmount，于是乎就产生了一个《重构与性能》的话题，这是一个很大的话题，所以我暂时也没深入学习，在重构中，我们可以去掉这样的临时变量（一两个不可怕，但是在一个项目中过度的临时变量使得代码无比混乱），实际上我们去掉这样的临时变量，可以尝试从 Rental 类中寻求优化方法。至于《重构与性能》后面介绍！

代码如下：

//……被省略的代码

result += "\t" + rental.Movie.Title + "\t" + rental.GetCharge() + "\n";

totalAmount += rental.GetCharge();

再往后，Statement方法中，有这样的代码：

frequentRenterPoints ++;
if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
    frequentRenterPoints ++;

frequentRenterPoints 是一个在方法中定义的临时变量（阿飘），这小部分代码的作用也不过是计算积分（现在的各种商店都有积分），这两句代码只是积分的计算方式，和实际生成逻辑没有关联，于是我们考虑把它拆出来。

public string Statement()
{
    double totalAmount = 0;
    int frequentRenterPoints = 0;
    string result = "Rental Record for " + Name + "\n";

    foreach (Rental rental in _rentals)
    {
        frequentRenterPoints += GetFrequentRenterPoints(rental);

        result += "\t" + rental.Movie.Title + "\t" + rental.GetCharge() + "\n";
        totalAmount += rental.GetCharge();
    }

    result += "Amount owed is " + totalAmount + "\n";
    result += "You earned " + frequentRenterPoints + " frequent renter points";
    return result;
}

private int GetFrequentRenterPoints(Rental rental)
{
    if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
        return 2;
    return 1;
}

接下来，我们接着处理“阿飘”的问题，对于临时变量，能少用则少用，这是某某牛人给出的重构名言一则。

代码中 totalAmount 的计算可以单独封装为：

private double GetTotalAmount()
{
    return _rentals.Sum(rental => rental.GetCharge());
}

到此时，Statement方法已经比较简洁了，我们Copy再稍加修改就可以提供HtmlStatement和JsonStatement了，因为方法里面的计算逻辑已经被我们提取出来！

二、拆分后的方法调整

下一步，对于刚刚拆解的方法GetCharge，主体为一个switch，而switch却依赖对象 Movie的一个成员PriceCode，既然这样，为什么不把计算方式迁移到Movie类呢：

public double GetCharge()
{
    double thisAmount = 0;

    switch (this.Movie.PriceCode)
    {
        case Movie.Regular:
            thisAmount += 2;
            if (DaysRented > 2)
                thisAmount += (DaysRented - 2) * 1.5;
            break;
        case Movie.NewRelease:
            thisAmount += DaysRented * 3;
            break;
        case Movie.Childrens:
            thisAmount += 1.5;
            if (DaysRented > 3)
                thisAmount += (DaysRented - 3) * 1.5;
            break;
    }
    return thisAmount;
}

于是我们尝试在Movie中新建：

        public double GetCharge(int daysRented)
        {
            double thisAmount = 0;

            switch (PriceCode)
            {
                case Movie.Regular:
                    thisAmount += 2;
                    if (daysRented > 2)
                        thisAmount += (daysRented - 2) * 1.5;
                    break;
                case Movie.NewRelease:
                    thisAmount += daysRented * 3;
                    break;
                case Movie.Childrens:
                    thisAmount += 1.5;
                    if (daysRented > 3)
                        thisAmount += (daysRented - 3) * 1.5;
                    break;
            }
            return thisAmount;
        }

原方法则可用：

        public double GetCharge(int daysRented)
        {
            return Movie.GetCharge(DaysRented);
        }

然后看Customer中方法：

private int GetFrequentRenterPoints(Rental rental)
{
    if ((rental.Movie.PriceCode == Movie.NewRelease) && rental.DaysRented > 1)
        return 2;
    return 1;
}

我们可以迁移到Rental中：

public int GetFrequentRenterPoints()
{
    if ((Movie.PriceCode == Movie.NewRelease) && DaysRented > 1)
        return 2;
    return 1;
}

接着发现和上面同样使用了Movie的成员，尝试移动到Movie中：

public int GetFrequentRenterPoints(int daysRented)
{
    if ((PriceCode == Movie.NewRelease) && daysRented > 1)
        return 2;
    return 1;
}

Rental中可以这样：

public int GetFrequentRenterPoints()
{
    return Movie.GetFrequentRenterPoints(DaysRented);
}

和之前的GetTotalAmount方法一样，我们也可以有个GetTotalFrequentRenterPoints:

private double GetTotalFrequentRenterPoints()
{
    return _rentals.Sum(rental => rental.GetFrequentRenterPoints());
}

于是Statement就表示为：

public string Statement()
{
    string result = "Rental Record for " + Name + "\n";

    foreach (Rental rental in _rentals)
    {
        result += "\t" + rental.Movie.Title + "\t" + rental.GetCharge() + "\n";
    }

    result += "Amount owed is " + GetTotalAmount() + "\n";
    result += "You earned " + GetTotalFrequentRenterPoints() + " frequent renter points";
    return result;
}

三、大的变化——继承

例子中，我们的Movie类分成三个类型

/// <summary>
/// 常规的
/// </summary>
public const int Regular = 0;
/// <summary>
/// 新发布
/// </summary>
public const int NewRelease = 1;
/// <summary>
/// 儿童版
/// </summary>
public const int Childrens = 2;

从而出现了方法 GetCharge 中计算的条件判断，当我们做同一件事使用不同的方式时候，我们想到了继承！对不同的电影类型，有不同的价格，我们认为价格是一个基类，每种电影的价格为子类，最终代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Reconsitution
{
    public class Movie
    {
        #region 常量电影分类

        /// <summary>
        /// 常规的
        /// </summary>
        public const int Regular = 0;
        /// <summary>
        /// 新发布
        /// </summary>
        public const int NewRelease = 1;
        /// <summary>
        /// 儿童版
        /// </summary>
        public const int Childrens = 2;

        #endregion

        public string Title { get; private set; }

        public int PriceCode
        {
            get
            {
                return _price.GetPriceCode();
            }
            private set
            {
                switch (value)
                {
                    case Regular:
                        _price = new RegularPrice();
                        break;
                    case Childrens:
                        _price = new ChidrensPrice();
                        break;
                    case NewRelease:
                        _price = new NewReleasePrice();
                        break;
                    default:
                        throw new Exception("Incorrect Price Code!");
                }
            }
        }
        private Price _price ;

        public Movie(string title, int priceCode)
        {
            Title = title;
            PriceCode = priceCode;
        }

        public double GetCharge(int daysRented)
        {
            return _price.GetCharge(daysRented);
        }

        public int GetFrequentRenterPoints(int daysRented)
        {
            return _price.GetFrequentRenterPoints(daysRented);
        }
    }

    #region 价格类Price

    public abstract class Price
    {
        public abstract int GetPriceCode();
        public abstract double GetCharge(int daysRented);
        public abstract int GetFrequentRenterPoints(int daysRented);
    }

    public class ChidrensPrice : Price
    {
        public override int GetPriceCode()
        {
            return Movie.Childrens;
        }
        public override double GetCharge(int daysRented)
        {
            double thisAmount = 1.5;
            if (daysRented > 3)
                thisAmount += (daysRented - 3) * 1.5;
            return thisAmount;
        }
        public override int GetFrequentRenterPoints(int daysRented)
        {
            return 1;
        }
    }

    public class NewReleasePrice : Price
    {
        public override int GetPriceCode()
        {
            return Movie.NewRelease;
        }
        public override double GetCharge(int daysRented)
        {
            double thisAmount = daysRented * 3;
            return thisAmount;
        }
        public override int GetFrequentRenterPoints(int daysRented)
        {
            return (daysRented > 1) ? 2 : 1;
        }
    }

    public class RegularPrice : Price
    {
        public override int GetPriceCode()
        {
            return Movie.Regular;
        }
        public override double GetCharge(int daysRented)
        {
            double thisAmount = 2;
            if (daysRented > 2)
                thisAmount += (daysRented - 2) * 1.5;
            return thisAmount;
        }
        public override int GetFrequentRenterPoints(int daysRented)
        {
            return 1;
        }
    }

    #endregion

    public class Rental
    {
        public Movie Movie { get; private set; }
        public int DaysRented { get; private set; }

        public Rental(Movie movie, int daysRented)
        {
            Movie = movie;
            DaysRented = daysRented;
        }

        public double GetCharge()
        {
            return Movie.GetCharge(DaysRented);
        }

        public int GetFrequentRenterPoints()
        {
            return Movie.GetFrequentRenterPoints(DaysRented);
        }
    }

    public class Customer
    {
        public string Name { get; private set; }

        public List<Rental> Rentals
        {
            get { return _rentals; }
        }
        private readonly List<Rental> _rentals = new List<Rental>();

        public Customer(string name)
        {
            Name = name;
        }

        public void AddRental(Rental rental)
        {
            _rentals.Add(rental);
        }

        private double GetTotalAmount()
        {
            return _rentals.Sum(rental => rental.GetCharge());
        }
        private double GetTotalFrequentRenterPoints()
        {
            return _rentals.Sum(rental => rental.GetFrequentRenterPoints());
        }

        public string Statement()
        {
            string result = "Rental Record for " + Name + "\n";

            foreach (Rental rental in _rentals)
            {
                result += "\t" + rental.Movie.Title + "\t" + rental.GetCharge() + "\n";
            }

            result += "Amount owed is " + GetTotalAmount() + "\n";
            result += "You earned " + GetTotalFrequentRenterPoints() + " frequent renter points";
            return result;
        }

    }
}

可能有人会问了， 为什么要这样做，是不是没有必要，我想你可以这样设想：正如之前提及的，对于同一一件事，如果使用的不同的方式去解决，就应该尝试通过继承分离出来，好处在于，如果我有新的电影分类出现，将只需要再继承一个Movie产生新的子类，就能解决问题，或者某类型电影价格、积分计算方式变动，我们也只用更改相应子类的计算方式，不影响整体方式！