设计模式（2）建造者模式

模式介绍

Builder模式将对象的构造与其表示分离，以便相同的构造过程可以创建不同的表示。

一般思想是，在实例化对象时发生的事情的顺序是相同的，但是这些步骤的实际细节根据具体实现是什么而改变。

示例

比如我们现在做三明治，不同的三明治，做法步骤总归是一样的。所以我们通过同一个组装线进行处理。

Director类：生产线

/// <summary>
/// The Director
/// </summary>
class AssemblyLine
{
    // Builder uses a complex series of steps
    // 
    public void Assemble(SandwichBuilder sandwichBuilder)
    {
        sandwichBuilder.AddBread();
        sandwichBuilder.AddMeats();
        sandwichBuilder.AddCheese();
        sandwichBuilder.AddVeggies();
        sandwichBuilder.AddCondiments();
    }
}

Product类：三明治

/// <summary>
/// The Product class
/// </summary>
class Sandwich
{
    private string _sandwichType;
    private Dictionary<string, string> _ingredients = new Dictionary<string, string>();

    // Constructor
    public Sandwich(string sandwichType)
    {
        this._sandwichType = sandwichType;
    }

    // Indexer
    public string this[string key]
    {
        get { return _ingredients[key]; }
        set { _ingredients[key] = value; }
    }

    public void Show()
    {
        Console.WriteLine("
---------------------------");
        Console.WriteLine("Sandwich: {0}", _sandwichType);
        Console.WriteLine(" Bread: {0}", _ingredients["bread"]);
        Console.WriteLine(" Meat: {0}", _ingredients["meat"]);
        Console.WriteLine(" Cheese: {0}", _ingredients["cheese"]);
        Console.WriteLine(" Veggies: {0}", _ingredients["veggies"]);
        Console.WriteLine(" Condiments: {0}", _ingredients["condiments"]);
    }
}

Builder类：三明治Builder类，抽象类

/// <summary>
/// The Builder abstract class
/// </summary>
abstract class SandwichBuilder
{
    protected Sandwich sandwich;

    // Gets sandwich instance
    public Sandwich Sandwich
    {
        get { return sandwich; }
    }

    // Abstract build methods
    public abstract void AddBread();
    public abstract void AddMeats();
    public abstract void AddCheese();
    public abstract void AddVeggies();
    public abstract void AddCondiments();
}

具体的Builder类

/// <summary>
/// A ConcreteBuilder class
/// </summary>
class TurkeyClub : SandwichBuilder
{
    public TurkeyClub()
    {
        sandwich = new Sandwich("Turkey Club");
    }

    public override void AddBread()
    {
        sandwich["bread"] = "12-Grain";
    }

    public override void AddMeats()
    {
        sandwich["meat"] = "Turkey";
    }

    public override void AddCheese()
    {
        sandwich["cheese"] = "Swiss";
    }

    public override void AddVeggies()
    {
        sandwich["veggies"] = "Lettuce, Tomato";
    }

    public override void AddCondiments()
    {
        sandwich["condiments"] = "Mayo";
    }
}


/// <summary>
/// A ConcreteBuilder class
/// </summary>
class BLT : SandwichBuilder
{
    public BLT()
    {
        sandwich = new Sandwich("BLT");
    }

    public override void AddBread()
    {
        sandwich["bread"] = "Wheat";
    }

    public override void AddMeats()
    {
        sandwich["meat"] = "Bacon";
    }

    public override void AddCheese()
    {
        sandwich["cheese"] = "None";
    }

    public override void AddVeggies()
    {
        sandwich["veggies"] = "Lettuce, Tomato";
    }

    public override void AddCondiments()
    {
        sandwich["condiments"] = "Mayo, Mustard";
    }
}

/// <summary>
/// A ConcreteBuilder class
/// </summary>
class HamAndCheese : SandwichBuilder
{
    public HamAndCheese()
    {
        sandwich = new Sandwich("Ham and Cheese");
    }

    public override void AddBread()
    {
        sandwich["bread"] = "White";
    }

    public override void AddMeats()
    {
        sandwich["meat"] = "Ham";
    }

    public override void AddCheese()
    {
        sandwich["cheese"] = "American";
    }

    public override void AddVeggies()
    {
        sandwich["veggies"] = "None";
    }

    public override void AddCondiments()
    {
        sandwich["condiments"] = "Mayo";
    }
}

客户端调用：

static void Main(string[] args)
{
    SandwichBuilder builder;

    // Create shop with sandwich assembly line
    AssemblyLine shop = new AssemblyLine();

    // Construct and display sandwiches
    builder = new HamAndCheese();
    shop.Assemble(builder);
    builder.Sandwich.Show();

    builder = new BLT();
    shop.Assemble(builder);
    builder.Sandwich.Show();

    builder = new TurkeyClub();
    shop.Assemble(builder);
    builder.Sandwich.Show();

    // Wait for user
    Console.ReadKey();
}

总结

Builder模式允许我们使用相同的步骤构建相关的对象集，但将这些步骤的实现留给子类。

源代码

https://github.com/exceptionnotfound/DesignPatterns/tree/master/Builder

原文

https://www.exceptionnotfound.net/builder-the-daily-design-pattern/