android設計模式之build模式

當我們要複雜對象的創建過程和這個對象的表示（展示）分離開來，一般就會想到build模式。
在android開發中，AlertDialog就是一個很好的例子。初次使用AlertDialog的同學可能會很困惑。爲什麼AlertDialog不能直接通過new的方式創建出來，要通過new AlertDialog.Builder(this).create()的方式創建。首先我們簡單的舉個例子：

public class Person
{
   private final String lastName;
   private final String firstName;
   private final String middleName;
   private final String salutation;
   private final String suffix;
   private final String streetAddress;
   private final String city;
   private final String state;
   private final boolean isFemale;
   private final boolean isEmployed;
   private final boolean isHomewOwner;

   public Person(
      final String newLastName,
      final String newFirstName,
      final String newMiddleName,
      final String newSalutation,
      final String newSuffix,
      final String newStreetAddress,
      final String newCity,
      final String newState,
      final boolean newIsFemale,
      final boolean newIsEmployed,
      final boolean newIsHomeOwner)
   {
      this.lastName = newLastName;
      this.firstName = newFirstName;
      this.middleName = newMiddleName;
      this.salutation = newSalutation;
      this.suffix = newSuffix;
      this.streetAddress = newStreetAddress;
      this.city = newCity;
      this.state = newState;
      this.isFemale = newIsFemale;
      this.isEmployed = newIsEmployed;
      this.isHomewOwner = newIsHomeOwner;
   }
}

看這個persion類，不使用build模式，如果想創建對象，只能通過構造函數依次填入參數。但是在這種參數過多的情況下，閱讀起來會很困難。還有如果有些參數是可選的時候，我們需要爲這個類創建多個構造函數。然後我們看看使用build模式之後：

public class Person
{
   private final String lastName;
   private final String firstName;
   private final String middleName;
   private final String salutation;
   private final String suffix;
   private final String streetAddress;
   private final String city;
   private final String state;
   private final boolean isFemale;
   private final boolean isEmployed;
   private final boolean isHomewOwner;

   public Person(
      final String newLastName,
      final String newFirstName,
      final String newMiddleName,
      final String newSalutation,
      final String newSuffix,
      final String newStreetAddress,
      final String newCity,
      final String newState,
      final boolean newIsFemale,
      final boolean newIsEmployed,
      final boolean newIsHomeOwner)
   {
      this.lastName = newLastName;
      this.firstName = newFirstName;
      this.middleName = newMiddleName;
      this.salutation = newSalutation;
      this.suffix = newSuffix;
      this.streetAddress = newStreetAddress;
      this.city = newCity;
      this.state = newState;
      this.isFemale = newIsFemale;
      this.isEmployed = newIsEmployed;
      this.isHomewOwner = newIsHomeOwner;
   }

   public static class PersonBuilder
   {
      private String nestedLastName;
      private String nestedFirstName;
      private String nestedMiddleName;
      private String nestedSalutation;
      private String nestedSuffix;
      private String nestedStreetAddress;
      private String nestedCity;
      private String nestedState;
      private boolean nestedIsFemale;
      private boolean nestedIsEmployed;
      private boolean nestedIsHomeOwner;

      public PersonBuilder(
         final String newFirstName,
         final String newCity,
         final String newState) 
      {
         this.nestedFirstName = newFirstName;
         this.nestedCity = newCity;
         this.nestedState = newState;
      }

      public PersonBuilder lastName(String newLastName)
      {
         this.nestedLastName = newLastName;
         return this;
      }

      public PersonBuilder firstName(String newFirstName)
      {
         this.nestedFirstName = newFirstName;
         return this;
      }

      public PersonBuilder middleName(String newMiddleName)
      {
         this.nestedMiddleName = newMiddleName;
         return this;
      }

      public PersonBuilder salutation(String newSalutation)
      {
         this.nestedSalutation = newSalutation;
         return this;
      }

      public PersonBuilder suffix(String newSuffix)
      {
         this.nestedSuffix = newSuffix;
         return this;
      }

      public PersonBuilder streetAddress(String newStreetAddress)
      {
         this.nestedStreetAddress = newStreetAddress;
         return this;
      }

      public PersonBuilder city(String newCity)
      {
         this.nestedCity = newCity;
         return this;
      }

      public PersonBuilder state(String newState)
      {
         this.nestedState = newState;
         return this;
      }

      public PersonBuilder isFemale(boolean newIsFemale)
      {
         this.nestedIsFemale = newIsFemale;
         return this;
      }

      public PersonBuilder isEmployed(boolean newIsEmployed)
      {
         this.nestedIsEmployed = newIsEmployed;
         return this;
      }

      public PersonBuilder isHomeOwner(boolean newIsHomeOwner)
      {
         this.nestedIsHomeOwner = newIsHomeOwner;
         return this;
      }

      public Person createPerson()
      {
         return new Person(
            nestedLastName, nestedFirstName, nestedMiddleName,
            nestedSalutation, nestedSuffix,
            nestedStreetAddress, nestedCity, nestedState,
            nestedIsFemale, nestedIsEmployed, nestedIsHomeOwner);
      }
   }
}

每個方法後面都return了this，這樣就可以鏈式調用，就和我們使用dialog時候一樣new Person.PersonBuilder(..).set().createPerson().
到這裏還有人還疑問，爲什麼需要這麼多此一舉，爲什麼不直接在Person裏面添加set方法。
如果採用javabean的寫法 先new一個對象然後通過set方法設置屬性，會導致這個對象的不確定性，無法保證對象的統一性。後期可以隨意set修改。但是build模式的使用必然會增加代碼的以及類的數量，具體是不是用build模式還要根據實際情況來看。