訪問者模式 Visitor Pattern

目的#

定義能逐一施行於物件結構裡各個元素的操作，讓你不必修改作用對象的類別介面，就能定義新的操作；將操作集中到物件結構上，以讓它們能獨立變化，但仍以多型方式進行。

適用#

物件結構含有許多介面各異的類別，希望能依據物件的實體類別來執行對應的動作時。需要對整個物件結構體系執行各種互不相干的操作，不會造成干擾。物件結構的類別不常變動，但施於其上的操作卻常有增減時。

結構及成員#

Collaborations: Client先建立一個ConcreteVisitor物件，再於巡訪物件結構的過程中，將Visitor物件餵給每一個遇到的Element。 Element被巡訪到時，會呼叫Visitor相對應的操作，如有必要，也會把自己列入參數傳送過去讓Visitor存取。

Visitor: like the NodeVisitor替每一種ConcreteElement制訂對應的Visit()方法。Visitor藉由方法的名字和簽名（Sinature）（多型）區分是哪個ConcreteElement送過來的訊息。將訊息轉傳給ConcreteVisitor。ConcreteVisitor: like the TypeCheckingVisitor, SpellingVisitor具體實作出Visitor。演算法可從ConcreteVisitor得知目前的Context，在整個巡訪過程中均可在此儲存局部狀態以累積資訊。Element: like the Glyph, Node宣告以Visitor物件當參數的Accept()介面。ConcreteElement: like the Row, Column, Text, Image, etc.具體實作出Accept()方法。Client:可巡訪整群Element物件。可提供高階介面讓Visitor巡訪Element。可以是Composite，也可以是容器（List、Set）。

影響結果#

好處#

容易增添新的操作，新增一個Visitor即可新增操作。每一種Visitor都將相關操作集中起來，將無關的摒除在外。處理不同的類別階層, Iterator只能巡訪Item或Item的子類別，而Visitor只要把相關類別寫入方法，就沒有巡訪限制。累積狀態。Visitor可在拜訪物件結構各個元素時順便累積狀態。

壞處#

難以增添新的ConcreteElement類別。Visitor可能會迫使ConcreteElement公開一些可存取的Element內部狀態，破壞封裝。

實作#

雙重分派（Double Dispatch）？#

在單一分派程式語言，呼叫哪個Overload方法，由方法呼叫者和方法參數共同在執行期間依靜態類別決定。如：以下程式碼所有的ride()出來都會是「騎馬」，因為他們的靜態類別都是Horse。

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public class Mozi {
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    public void ride(Horse h){ System.out.println("騎馬"); }
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    public void ride(WhiteHorse wh){ System.out.println("騎白馬"); }
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    public void ride(BlackHorse bh){ System.out.println("騎黑馬"); }
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    public static void main(String[] args) {
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        Horse wh = new WhiteHorse();
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        Horse bh = new BlackHorse();
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        Mozi mozi = new Mozi();
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        mozi.ride(wh);
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        mozi.ride(bh);
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    }
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}

但JAVA可以透過Override實作動態分派

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public class Horse {
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    public void ride(){ System.out.println("騎馬"); }
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}
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public class WhiteHorse extends Horse {
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    public void ride(){ System.out.println("騎白馬"); }
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}
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public class BlackHorse extends Horse {
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    public void ride(){ System.out.println("騎黑馬"); }
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}
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public class Mozi {
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    public static void main(String[] args) {
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        Horse wh = new WhiteHorse();
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        Horse bh = new BlackHorse();
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        wh.ride();
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        bh.ride();
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    }
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}

Visitor模式將真正會啟用的操作，依Visitor的型別和作用對象的型別決定，不用把靜態操作繫結在Element介面裡。

誰負責巡訪物件結構？#

有三種角色可以幫Visitor一一拜訪物件結構裡的每一個元素：物件結構、Visitor物件、Iterator物件。

Example: Liquor#

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//Interface: Visitable
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package liquor;
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interface Visitable {
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  public double accept(Visitor visitor);
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}

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//Class: Liquor
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package liquor;
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class Liquor implements Visitable {
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  private double price;
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  Liquor(double item) {
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    price = item;
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  }
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  public double accept(Visitor visitor) {
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    return visitor.visit(this);
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  }
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  public double getPrice() {
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    return price;
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  }
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}

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//Class: Necessity
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package liquor;
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class Necessity implements Visitable {
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  private double price;
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  Necessity(double item) {
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    price = item;
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  }
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  public double accept(Visitor visitor) {
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    return visitor.visit(this);
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  }
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  public double getPrice() {
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    return price;
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  }
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}

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//Class: Tobaco
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package liquor;
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class Tobacco implements Visitable {
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  private double price;
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  Tobacco(double item) {
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    price = item;
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  }
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  public double accept(Visitor visitor) {
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    return visitor.visit(this);
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  }
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  public double getPrice() {
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    return price;
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  }
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}

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//Interface: Visitor
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package liquor;
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interface Visitor {
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  public double visit(Liquor liquorItem);
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  public double visit(Tobacco tobaccoItem);
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  public double visit(Necessity necessityItem);
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}

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//Class: TaxVisitor
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package liquor;
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import java.text.DecimalFormat;
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class TaxVisitor implements Visitor {
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  DecimalFormat df = new DecimalFormat("#.##");
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  public TaxVisitor() {
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  }
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  public double visit(Liquor liquorItem) {
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    System.out.println("Liquor Item: Price with Tax");
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    return Double.parseDouble(df.format((liquorItem.getPrice() * .18) + liquorItem.getPrice()));
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  }
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  public double visit(Tobacco tobaccoItem) {
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    System.out.println("Tobacco Item: Price with Tax");
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    return Double.parseDouble(df.format((tobaccoItem.getPrice() * .32) + tobaccoItem.getPrice()));
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  }
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  public double visit(Necessity necessityItem) {
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    System.out.println("Necessity Item: Price with Tax");
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    return Double.parseDouble(df.format(necessityItem.getPrice()));
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  }
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}

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//Class: TaxHolidayVisitor
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package liquor;
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import java.text.DecimalFormat;
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class TaxHolidayVisitor implements Visitor {
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    DecimalFormat df = new DecimalFormat("#.##");
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  public TaxHolidayVisitor() {
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  }
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  public double visit(Liquor liquorItem) {
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    System.out.println("Liquor Item: Price with Tax");
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    return Double.parseDouble(df.format((liquorItem.getPrice() * .10) + liquorItem.getPrice()));
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  }
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  public double visit(Tobacco tobaccoItem) {
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    System.out.println("Tobacco Item: Price with Tax");
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    return Double.parseDouble(df.format((tobaccoItem.getPrice() * .30) + tobaccoItem.getPrice()));
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  }
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  public double visit(Necessity necessityItem) {
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    System.out.println("Necessity Item: Price with Tax");
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    return Double.parseDouble(df.format(necessityItem.getPrice()));
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  }
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}

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//Class: VisitorTest
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package liquor;
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public class VisitorTest {
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  public static void main(String[] args) {
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    TaxVisitor taxCalc = new TaxVisitor();
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    TaxHolidayVisitor taxHolidayCalc = new TaxHolidayVisitor();
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    Necessity milk = new Necessity(3.47);
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    Liquor vodka = new Liquor(11.99);
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    Tobacco cigars = new Tobacco(19.99);
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    System.out.println(milk.accept(taxCalc) + "\n");
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    System.out.println(vodka.accept(taxCalc) + "\n");
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    System.out.println(cigars.accept(taxCalc) + "\n");
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    System.out.println("TAX HOLIDAY PRICES\n");
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    System.out.println(milk.accept(taxHolidayCalc) + "\n");
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    System.out.println(vodka.accept(taxHolidayCalc) + "\n");
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    System.out.println(cigars.accept(taxHolidayCalc) + "\n");
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  }
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}

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Result:
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Necessity Item: Price with Tax
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3.47
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Liquor Item: Price with Tax
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14.15
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Tobacco Item: Price with Tax
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26.39
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TAX HOLIDAY PRICES
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Necessity Item: Price with Tax
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3.47
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Liquor Item: Price with Tax
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13.19
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Tobacco Item: Price with Tax
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25.99

目的#

適用#

結構及成員#

影響結果#

好處#

壞處#

實作#

雙重分派（Double Dispatch）？#

誰負責巡訪物件結構？#

Example: Liquor#

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