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1.动机

• 在软件系统中，经常面临着“一系列相互依赖的对象”的创建工作；同时，由于需求的变化，往往存在更多系列对象的创建工作
• 如何应对这种变化？如何绕过常规的对象创建方法(new)，提供一 种“封装机制”来避免客户程序和这种“多系列具体对象创建工作”的紧耦合？

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2.模式定义

• 定义：提供一个接口，让该接口负责创建一系列“相关或者相互依赖的对象”，无需指定它们具体的类
  • 个人感觉抽象工厂叫家族工厂可能更形象:P
• 结构
  

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3.要点总结

• 如果没有应对“多系列对象构建”的需求变化，则没有必要使用Abstract Factory模式，这时候使用简单的工厂完全可以
• “系列对象”指的是在某一特定系列下的对象之间有相互依赖、 或作用的关系。不同系列的对象之间不能相互依赖
• Abstract Factory模式主要在于应对“新系列”的需求变动
  • 缺点：难以应对“新对象”的需求变动

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4.代码感受

1.代码一

class EmployeeDAO
{
public:
    vector<EmployeeDO> GetEmployees()
    {
        SqlConnection* connection = new SqlConnection();
        connection->ConnectionString = "...";

        SqlCommand* command = new SqlCommand();
        command->CommandText = "...";
        command->SetConnection(connection);

        SqlDataReader* reader = command->ExecuteReader();
        while (reader->Read())
        {}
    }
};

2.代码二 – 工厂方法

// 数据库访问有关的基类
class IDBConnection
{};
class IDBConnectionFactory
{
public:
    virtual IDBConnection* CreateDBConnection() = 0;
};

class IDBCommand
{};
class IDBCommandFactory
{
public:
    virtual IDBCommand* CreateDBCommand() = 0;
};

class IDataReader
{};
class IDataReaderFactory
{
public:
    virtual IDataReader* CreateDataReader() = 0;
};

//支持SQL Server
class SqlConnection: public IDBConnection
{};
class SqlConnectionFactory:public IDBConnectionFactory
{};

class SqlCommand: public IDBCommand
{};
class SqlCommandFactory:public IDBCommandFactory
{};

class SqlDataReader: public IDataReader
{};
class SqlDataReaderFactory:public IDataReaderFactory
{};

//支持Oracle
class OracleConnection: public IDBConnection
{};
class OracleCommand: public IDBCommand
{};
class OracleDataReader: public IDataReader
{};

class EmployeeDAO
{
    IDBConnectionFactory* dbConnectionFactory;
    IDBCommandFactory* dbCommandFactory;
    IDataReaderFactory* dataReaderFactory;
public:
    vector<EmployeeDO> GetEmployees()
    {
        IDBConnection* connection =
            dbConnectionFactory->CreateDBConnection();
        connection->ConnectionString("...");

        IDBCommand* command =
            dbCommandFactory->CreateDBCommand();
        command->CommandText("...");
        command->SetConnection(connection); // 关联性

        IDBDataReader* reader = command->ExecuteReader(); // 关联性
        while (reader->Read())
        {}
    }
};

3.代码三 – 抽象工厂

//数据库访问有关的基类
class IDBConnection
{};
class IDBCommand
{};
class IDataReader
{};

class IDBFactory
{
public:
    virtual IDBConnection* CreateDBConnection() = 0;
    virtual IDBCommand* CreateDBCommand() = 0;
    virtual IDataReader* CreateDataReader() = 0;
};

//支持SQL Server
class SqlConnection: public IDBConnection
{};
class SqlCommand: public IDBCommand
{};
class SqlDataReader: public IDataReader
{};

class SqlDBFactory:public IDBFactory
{
public:
    virtual IDBConnection* CreateDBConnection()
    {}
    virtual IDBCommand* CreateDBCommand()
    {}
    virtual IDataReader* CreateDataReader()
    {}
 
};

//支持Oracle
class OracleConnection: public IDBConnection
{};
class OracleCommand: public IDBCommand
{};
class OracleDataReader: public IDataReader
{};

class EmployeeDAO
{
    IDBFactory* dbFactory;
public:
    vector<EmployeeDO> GetEmployees()
    {
        IDBConnection* connection =
            dbFactory->CreateDBConnection();
        connection->ConnectionString("...");

        IDBCommand* command =
            dbFactory->CreateDBCommand();
        command->CommandText("...");
        command->SetConnection(connection);

        IDBDataReader* reader = command->ExecuteReader();
        while (reader->Read())
        {}
    }
};