重构手法之处理概括关系【4】

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10 Form Template Method（塑造模板函数）

概要

你有一些子类，其中相应的某些函数以相同的顺序执行类似的操作，但各个操作的细节不同。

将这些操作分别放进独立的函数中，并保持它们都有相同的签名，于是原函数也就变得相同了，然后将原函数上移至基类。

动机

继承是避免重复行为的一个强大工具。无论何时，只要你看见两个子类之中有类似的函数，就可以把它们提升到基类。但是如果这些函数并不完全相同该这么办?仍有必要尽量避免重复，但又必须保持这些函数之间的实质差异。

常见的一种情况是：两个函数以相同的顺序执行大致相近的操作，但是各操作不完全相同。这种情况下我们可以将执行的序列移至基类，并借助多态保证各操作仍得以保持差异性。这样的函数被称为Template Method（模板函数）。

范例

Customer类中有两个用于打印的函数。Statment()函数用于ASCII码打印报表，HtmlStatement()函数则以HTML格式输出报表：

class Customer
{
    public string Name { get; }

    private List<Rental> _rentals = new List<Rental>();

    public List<Rental> GetRentals()
    {
        return _rentals;
    }
    public Customer(string name)
    {
        Name = name;
    }
    public string Statement()
    {
        string result = "Rental Record for " + Name + "
";

        foreach (var rental in _rentals)
        {
            result += "	" + rental.Title + "	" + rental.GetCharge().ToString() + "
";
        }
        //add footer lines
        result += "Amount owed is " + GetTotalCharge() + "
";
        result += "You earned " + GetTotalFrequentRenterPoints() + " frequent renter points";
        return result;
    }
    public string HtmlStatement()
    {
        string result = "<h1>Rentals for <em>" + Name + "</em></h1>
";

        foreach (var rental in _rentals)
        {
            result += rental.Title + ": " + rental.GetCharge().ToString() + "<br/>
";
        }
        //add footer lines
        result += "<p>You owe  <em>" + GetTotalCharge() + "<em/></p>
";
        result += "On this rental you earned <em>" + GetTotalFrequentRenterPoints() + "</em> frequent renter points";
        return result;
    }
    public double GetTotalCharge()
    {
        return _rentals.Sum(rental => rental.GetCharge());
    }

    public int GetTotalFrequentRenterPoints()
    {
        return _rentals.Sum(rental => rental.GetFrequentRenterPoints());
    }
}

class Rental
{
    public string Title { get; set; }
    public double GetCharge()
    {
        return 1.5;
    }
    public int GetFrequentRenterPoints()
    {
        return 3;
    }
}

使用Form Template Method之前，需要对上述两个函数做一些整理，使它们成为同一个基类下的子类函数。为了这一目的，使用函数对象针对“报表打印”创建一个独立的策略继承体系：

class Statement{}

class TextStatement : Statement{}

class HtmlStatement : Statement{}

现在，通过Move Method，将两个负责输出报表的函数分别搬移到对应的子类中：

class Customer
{
    public string Name { get; }

    private List<Rental> _rentals = new List<Rental>();

    public List<Rental> GetRentals()
    {
        return _rentals;
    }
    public Customer(string name)
    {
        Name = name;
    }
    /// <summary>
    /// 以ASCII码打印报表
    /// </summary>
    /// <returns></returns>
    public string Statement()
    {
        return new TextStatement().Value(this);
    }
    /// <summary>
    /// 以HTML格式打印报表
    /// </summary>
    /// <returns></returns>
    public string HtmlStatement()
    {
        return new HtmlStatement().Value(this);
    }
    public double GetTotalCharge()
    {
        return _rentals.Sum(rental => rental.GetCharge());
    }

    public int GetTotalFrequentRenterPoints()
    {
        return _rentals.Sum(rental => rental.GetFrequentRenterPoints());
    }
}
class Statement
{

}

class TextStatement : Statement
{
    /// <summary>
    /// 以ASCII码打印报表
    /// </summary>
    /// <returns></returns>
    public string Value(Customer customer)
    {
        string result = "Rental Record for " + customer.Name + "
";
        var rentals = customer.GetRentals();
        foreach (var rental in rentals)
        {
            result += "	" + rental.Title + "	" + rental.GetCharge().ToString() + "
";
        }
        //add footer lines
        result += "Amount owed is " + customer.GetTotalCharge() + "
";
        result += "You earned " + customer.GetTotalFrequentRenterPoints() + " frequent renter points";
        return result;
    }
}

class HtmlStatement : Statement
{
    /// <summary>
    /// 以HTML格式打印报表
    /// </summary>
    /// <returns></returns>
    public string Value(Customer customer)
    {
        string result = "<h1>Rental Record for <em>" + customer.Name + "</em></h1>
";
        var rentals = customer.GetRentals();
        foreach (var rental in rentals)
        {
            result += rental.Title + ": " + rental.GetCharge().ToString() + "<br/>
";
        }
        //add footer lines
        result += "<p>You owe  <em>" + customer.GetTotalCharge() + "<em/></p>
";
        result += "On this rental you earned <em>" + customer.GetTotalFrequentRenterPoints() + "</em> frequent renter points";
        return result;
    }
}

class Rental
{
    public string Title { get; set; }
    public double GetCharge()
    {
        return 1.5;
    }
    public int GetFrequentRenterPoints()
    {
        return 3;
    }
}

面对两个子类中的相似函数，我可以开始实施Form Template Method了。本重构的关键在于：运用Extract Method将两个函数的不同部分提炼出来，从而将相似的代码和变动的代码分开。每次提炼后，就建立一个签名相同但本体不同的函数。

class TextStatement : Statement
{

    public string HeaderString(Customer customer)
    {
        return "Rental Record for " + customer.Name + "
";
    }

    public string EachRentalsString(Rental rental)
    {
        return "	" + rental.Title + "	" + rental.GetCharge().ToString() + "
";
    }

    public string FooterString(Customer customer)
    {
        return "Amount owed is " + customer.GetTotalCharge() + "
" +
         "You earned " + customer.GetTotalFrequentRenterPoints() + " frequent renter points";
    }
    /// <summary>
    /// 以ASCII码打印报表
    /// </summary>
    /// <returns></returns>
    public string Value(Customer customer)
    {
        string result = HeaderString(customer);
        var rentals = customer.GetRentals();
        foreach (var rental in rentals)
        {
            result += EachRentalsString(rental);
        }
        //add footer lines
        result += FooterString(customer);
        return result;
    }
}

class HtmlStatement : Statement
{

    public string HeaderString(Customer customer)
    {
        return "<h1>Rental Record for <em>" + customer.Name + "</em></h1>
";
    }

    public string EachRentalsString(Rental rental)
    {
        return rental.Title + ": " + rental.GetCharge().ToString() + "<br/>
";
    }

    public string FooterString(Customer customer)
    {
        return "<p>You owe  <em>" + customer.GetTotalCharge() + "<em/></p>
" +
               "On this rental you earned <em>" + customer.GetTotalFrequentRenterPoints() + "</em> frequent renter points";
    }
    /// <summary>
    /// 以HTML格式打印报表
    /// </summary>
    /// <returns></returns>
    public string Value(Customer customer)
    {
        string result = HeaderString(customer);
        var rentals = customer.GetRentals();
        foreach (var rental in rentals)
        {
            result += EachRentalsString(rental);
        }
        //add footer lines
        result += FooterString(customer);
        return result;
    }
}

所有这些都修改完毕之后，两个Value()函数看上去已经非常相似了，因此可以使用Pull Up Method将它们提升到基类中。提升完毕后，需要在基类中把子函数声明为抽象函数。

public abstract class Statement
{
    public abstract string HeaderString(Customer customer);
    public abstract string EachRentalsString(Rental rental);
    public abstract string FooterString(Customer customer);

    public string Value(Customer customer)
    {
        string result = HeaderString(customer);
        var rentals = customer.GetRentals();
        foreach (var rental in rentals)
        {
            result += EachRentalsString(rental);
        }
        //add footer lines
        result += FooterString(customer);
        return result;
    }
}

class TextStatement : Statement
{

    public override string HeaderString(Customer customer)
    {
        return "Rental Record for " + customer.Name + "
";
    }

    public override string EachRentalsString(Rental rental)
    {
        return "	" + rental.Title + "	" + rental.GetCharge().ToString() + "
";
    }

    public override string FooterString(Customer customer)
    {
        return "Amount owed is " + customer.GetTotalCharge() + "
" +
         "You earned " + customer.GetTotalFrequentRenterPoints() + " frequent renter points";
    }

}

class HtmlStatement : Statement
{

    public override string HeaderString(Customer customer)
    {
        return "<h1>Rental Record for <em>" + customer.Name + "</em></h1>
";
    }

    public override string EachRentalsString(Rental rental)
    {
        return rental.Title + ": " + rental.GetCharge().ToString() + "<br/>
";
    }

    public override string FooterString(Customer customer)
    {
        return "<p>You owe  <em>" + customer.GetTotalCharge() + "<em/></p>
" +
               "On this rental you earned <em>" + customer.GetTotalFrequentRenterPoints() + "</em> frequent renter points";
    }

}

完成本重构后，处理其他种类的报表就容易多了：只需为Statement再建一个子类，并在其中覆写3个抽象函数即可。

小结

模板方法模式是基于继承的代码复用技术，它体现了面向对象的诸多重要思想，是一种使用较为频繁的模式。模板方法模式被广泛应用于框架设计中，以确保通过父类来控制处理流程的逻辑顺序。

To Be Continued……