1. 可抽換元件設計模式

2. 相依
 名詞：dependency
 動詞：depend(s) on
 形容詞：dependent on
類別UserAppService相依於
UserRepository及Logger兩個類
別
public class UserAppService
{
public void AddUser(UserInfo user)
{
UserRepository repository = new UserRepository();
repository.Add(user);
Logger log = new Logger();
Dictionary<string, string> messages = new Dictionary<string, string>();
messages.Add("Action", "建立使用者");
messages.Add("Detail", string.Format("UserId: {0}, UserName: {1}", user.UserId,
user.UserName));
messages.Add("LogTime", DateTime.Now.ToString());
log.Write(messages);
}
}

3. 耦合
 Coupling，以程度區分為
loose-coupling / loosely-coupled / 鬆散耦合：
相依於介面(interface)
tight-coupling / tightly-coupled / 緊密耦合：
相依於實作(implementation)

4. 需求案例
驚!某甲要求導入log機制，且不得使用third-
party元件(如log4net, NLog, Enterprise library
的Logging Application Block, etc.)

5. 一塊蛋糕
public class TextLogger
{
public void Write(Dictionary<string, string> messages)
{
StringBuilder sb = new StringBuilder();
messages.ToList().ForEach(c => sb.AppendLine(string.Format("{0}:{1}", c.Key, c.Value)));
sb.AppendLine();
File.AppendAllText(string.Format("{0}.txt", DateTime.Now.ToString("yyyyMMdd")), sb.ToString());
}
}
public class UserAppService
{
public void AddUser(UserInfo user)
{
UserRepository repository = new UserRepository();
repository.Add(user);
TextLogger log = new TextLogger();
Dictionary<string, string> messages = new Dictionary<string, string>();
messages.Add("Action", "建立使用者");
messages.Add("Detail", string.Format("UserId: {0}, UserName: {1}", user.UserId, user.UserName));
messages.Add("LogTime", DateTime.Now.ToString());
log.Write(messages);
}
}

6. WHAT IF…
 某甲：文字檔太粗糙，請log成XML格式的檔
案
 某乙：沒問題！

7. 仍然是一塊蛋糕
public class XmlLogger
{
public void Write(Dictionary<string, string> messages)
{
XElement element = new XElement(
new XElement("Record",
new XElement("Action", messages["Action"]),
new XElement("Detail", messages["Detail"]),
new XElement("LogTime", messages["LogTime"])
)
);
File.AppendAllText(string.Format("{0}_{1}.xml", DateTime.Now.ToString("yyyyMMdd"), Guid.NewGuid()),
element.ToString());
}
}
public class UserAppService
{
public void AddUser(UserInfo user)
{
…
XmlLogger log = new XmlLogger();
…
}
}

8. WHAT IF AGAIN…
 某甲：長官說要改成寫進資料庫唷， ^.<
 某乙：…(OS: what the…（╯－＿－）╯╧╧ )

9. 翻桌前思考一下
 耦 合 度 太 高 。 系 統 提 供 新 的 log 機 制 時 ，
UserAppService類別就必須修改實作內容以
符合需求(TextLogger->XmlLogger)
 彈性不足，無法任意抽換實作機制

10. 解決方案
 Plugin Pattern
 Provider Pattern (.NET Framework內建)
 IoC Pattern

11. PLUGIN PATTERN
 Links classes during configuration rather
than compilation. – Patterns of Enterprise
Application Architecture [P of EAA], p.499
透過反射(reflection)機制於執行期(run time)
由設定檔(configuration)取得實際要執行的物
件 TextLogger / XmlLogger
/ …

12. PLUGIN PATTERN特色
 會有一個實作Factory Method Pattern的類別，
用於生成實際要執行之物件(plugin object)
 設定檔內會有實際生成物件的組件名稱及型
別，如放置在App.config或Web.config的
appSettings中
 實際生成之物件必定實作一通用介面

13. PLUGIN PATTERN實作(1)
 建立一通用介面
public interface ILogger
{
void Write(Dictionary<string, string> messages);
}

14. PLUGIN PATTERN實作(2)
 建立TextLogger類別(plugin object)，並實作
ILogger介面
public class TextLogger : ILogger
{
public void Write(Dictionary<string, string> messages)
{
StringBuilder sb = new StringBuilder();
messages.ToList().ForEach(c => sb.AppendLine(string.Format("{0}:{1}", c.Key, c.Value)));
sb.AppendLine();
File.AppendAllText(string.Format("{0}.txt", DateTime.Now.ToString("yyyyMMdd")),
sb.ToString());
}
}

15. PLUGIN PATTERN實作(3)
 建立XmlLogger類別(plugin object) ，並實作
ILogger介面
public class XmlLogger : ILogger
{
public void Write(Dictionary<string, string> messages)
{
XElement element = new XElement(
new XElement("Record",
new XElement("Action", messages["Action"]),
new XElement("Detail", messages["Detail"]),
new XElement("LogTime", messages["LogTime"])
)
);
File.AppendAllText(string.Format("{0}_{1}.xml", DateTime.Now.ToString("yyyyMMdd"),
Guid.NewGuid()), element.ToString());
}
}

16. PLUGIN PATTERN實作(4)
 建立一實作Factory Method Pattern的類別
public class LoggerFactory
{
private static ILogger _logger;
public static ILogger CreateLogger()
{
if (_logger == null)
{
string assemblyName = ConfigurationManager.AppSettings["AssemblyName"];
string classType = ConfigurationManager.AppSettings["ClassType"];
Assembly assembly = Assembly.Load(assemblyName);
_logger = assembly.CreateInstance(classType) as ILogger;
}
return _logger;
}
}

17.  建立設定檔
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="AssemblyName" value="LoggerSample"/>
<add key="ClassType" value="LoggerSample.PluginPattern.TextLogger"/>
</appSettings>
</configuration>
PLUGIN PATTERN實作(5)
Fully qualified name

18.  修改相依TextLogger/XmlLogger/…類別的程
式碼
public class UserAppService
{
public void AddUser(UserInfo user)
{
UserRepository repository = new UserRepository();
repository.Add(user);
//TextLogger log = new TextLogger();
ILogger log = LoggerFactory.CreateLogger();
Dictionary<string, string> messages = new Dictionary<string, string>();
messages.Add("Action", "建立使用者");
messages.Add("Detail", string.Format("UserId: {0}, UserName: {1}", user.UserId,
user.UserName));
messages.Add("LogTime", DateTime.Now.ToString());
log.Write(messages);
}
}
PLUGIN PATTERN實作(6)
相依實作轉為相依介面，耦合度降低

19.  若要切換log機制，僅須修改設定檔
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="AssemblyName" value="LoggerSample"/>
<add key="ClassType" value="LoggerSample.PluginPattern.XmlLogger"/>
</appSettings>
</configuration>
PLUGIN PATTERN實作(7)

20. PLUGIN PATTERN兩三事
 一組介面，提供多種實作
 通用介面(ILogger)與實際要生成的物件(plugin
object)通常會存在於不同的組件中
 提供新的plugin object時，不需修改原始程式
碼(UserAppService)，僅需提供新的plugin組
件並修改設定檔
 plugin object實作內容若有異動，僅需patch其
組件

21. PROVIDER PATTERN
 .NET Framework內建的設計模式
 提供一組介面，多種實作切換的功能

22. PROVIDER PATTERN特色
 會有一個實作Factory Method Pattern的類別，
用於生成實際要執行之物件
 會有一個繼承ConfigurationSection類別的類
別且設定檔內會存放可於執行期生成之物件
的組件名稱及型別
 實際生成之物件必定繼承ProviderBase類別
並實作一通用介面

23. PROVIDER PATTERN實作(1)
 建立一通用介面
public interface ILoggerProvider
{
void Write(Dictionary<string, string> messages);
}

24. PROVIDER PATTERN實作(2)
 建 立 TextLoggerProvider 類 別 ， 繼 承
ProviderBase類別並實作ILoggerProvider介
面public class TextLoggerProvider : ProviderBase, ILoggerProvider
{
public void Write(Dictionary<string, string> messages)
{
StringBuilder sb = new StringBuilder();
messages.ToList().ForEach(c => sb.AppendLine(string.Format("{0}:{1}", c.Key, c.Value)));
sb.AppendLine();
File.AppendAllText(string.Format("{0}.txt", DateTime.Now.ToString("yyyyMMdd")),
sb.ToString());
}
}

25. PROVIDER PATTERN實作(3)
 建 立 XmlLoggerProvider 類 別 ， 繼 承
ProviderBase類別並實作ILoggerProvider介
面public class XmlLoggerProvider : ProviderBase, ILoggerProvider
{
public void Write(Dictionary<string, string> messages)
{
XElement element = new XElement(
new XElement("Record",
new XElement("Action", messages["Action"]),
new XElement("Detail", messages["Detail"]),
new XElement("LogTime", messages["LogTime"])
)
);
File.AppendAllText(string.Format("{0}_{1}.xml", DateTime.Now.ToString("yyyyMMdd"),
Guid.NewGuid()), element.ToString());
}
}

26. PROVIDER PATTERN實作(4)
 建 立 LoggerProviderSection 類 別 並 繼 承
ConfigurationSection類別
public class LoggerProviderSection : ConfigurationSection
{
public const string SectionName = "loggerProvider";
[ConfigurationProperty("providers", IsDefaultCollection = true)]
public ProviderSettingsCollection Providers
{
get
{
return (ProviderSettingsCollection)base["providers"];
}
}
[ConfigurationProperty("defaultProvider")]
public string DefaultProvider
{
get
{
return (string)base["defaultProvider"];
}
set
{
base["defaultProvider"] = value;
}
}
}

27. PROVIDER PATTERN實作(5)
 建立一實作Factory Method Pattern的類別
public class LoggerProviderFactory
{
private static ProviderBase _provider;
public static ILoggerProvider CreateLoggerProvider()
{
if (_provider == null)
{
LoggerProviderSection section =
(LoggerProviderSection)ConfigurationManager.GetSection(LoggerProviderSection.SectionName);
ProviderSettings settings = section.Providers[section.DefaultProvider];
_provider = Activator.CreateInstance(Type.GetType(settings.Type)) as ProviderBase;
_provider.Initialize(settings.Name, settings.Parameters);
}
return _provider as ILoggerProvider;
}
}

28.  建立設定檔，設定預設
provider(defaultProvider)<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="loggerProvider" type="LoggerSample.ProviderPattern.LoggerProviderSection,
LoggerSample" />
</configSections>
<loggerProvider defaultProvider="xml">
<providers>
<add name="text" type="LoggerSample.ProviderPattern.TextLoggerProvider, LoggerSample" />
<add name="xml" type="LoggerSample.ProviderPattern.XmlLoggerProvider, LoggerSample" />
</providers>
</loggerProvider>
</configuration>
PROVIDER PATTERN實作(6)
Fully qualified name Assembly name

29. IOC PATTERN
 To be continued…

30. 參考
 Patterns of Enterprise Application Architecture
 Microsoft .NET: Architecting Applications for the Enterprise
 Design Patterns: Elements of Reusable Object-Oriented Software
 Provider Pattern
 撰寫自己的 Configuration 區段 Part 1：不要再賴在 appSettings 的屋簷
下了，寫個自己的 Configuration 區段吧~
 Provider Model Design Pattern and Specification, Part 1
 ProviderBase Class
 Designing loosely coupled components in .NET - Provider Pattern