Creational Patterns

2019/05/19

介紹

Creational Design Patterns主要是一些產生物件的作法，不同的pattern要解決不同的問題。

Creational pattern
Creational Patterns要解決什麼問題(上)?
- 直接在需要產生物件的時候把物件給new出來
- 用Reflection的方式來產生物件
- 用Simple Factory pattern
Creational Patterns要解決什麼問題(中)?
- 用clone（Prototype pattern）的方式來產生物件。
- 用Singleton pattern
  - 尋找Force實驗6：Singleton篇
- 用Object Pool pattern
Creational Patterns要解決什麼問題(下)?
- 用Factory Method pattern
  - Simple Factory Method
  - Polymorphic Factory Method
  - Class Factory Method (Static Factory)
- 用Abstract Factory pattern
  - Design Pattern 比對(2) Factory Method和 Abstract Factory的異同
  - 設計模式 - 工廠方法及抽象工廠
- 用Builder pattern

public class DbContext

    public string Server { get; set; }

    public string ConnectionStr { get; set; }

    public void Connect()

        System.Diagnostics.Trace.WriteLine($"Connect to {this.Server}");

public enum DbEnum

    DataMart = 1,

    History,

    Online

Static factory (Class Factory Method)

public class StcDbFactory

    public static DbContext Create(DbEnum dbEnum)

        switch (dbEnum)

            case DbEnum.DataMart:

                return new DbContext()

                    Server = "DataMart",

                    ConnectionStr = "DataMart connection string"

};

            case DbEnum.History:

                return new DbContext()

                    Server = "History",

                    ConnectionStr = "History connection string"

};

            case DbEnum.Online:

                return new DbContext()

                    Server = "Online",

                    ConnectionStr = "Online connection string"

};

            default:

                throw new Exception("No mapping database settings!");

    public static DbContext CreateDataMart()

        return new DbContext()

            Server = "DataMart",

            ConnectionStr = "DataMart connection string"

};

    public static DbContext CreateHistory()

        return new DbContext()

            Server = "History",

            ConnectionStr = "History connection string"

};

    public static DbContext CreateOnline()

        return new DbContext()

            Server = "Online",

            ConnectionStr = "Online connection string"

};

使用方式

//1. 由參數決定建立哪種連線

var dmDbcontext =  StcDbFactory.Create(DbEnum.DataMart);

dmDbcontext.Connect();

//2. 直接呼叫對應的方法

var olDbcontext =  StcDbFactory.CreateOnline();

olDbcontext.Connect();

Abstract factory，把Factory分割成多個子類別，而Factory本身就變成一個Abstract class

public abstract class AbsDbFactory

    public abstract DbContext Create();

public class DataMartDbFactory : AbsDbFactory

    public override DbContext Create()

        return new DbContext()

            Server = "DataMart",

            ConnectionStr = "DataMart connection string"

};

public class HistoryDbFactory : AbsDbFactory

    public override DbContext Create()

        return new DbContext()

            Server = "History",

            ConnectionStr = "History connection string"

};

public class OnlineDbFactory : AbsDbFactory

    public override DbContext Create()

        return new DbContext()

            Server = "Online",

            ConnectionStr = "Online connection string"

};

使用方式

var dmFactory = new DataMartDbFactory();

var dmDbcontext =  dmFactory.Create();

dmDbcontext.Connect();