利用Stub來打破關連性 (3) @ David Ko的學習之旅

利用Stub來打破關連性 (3)

Source: The art of unit testing with examples in .NET, Roy Osherove
http://www.amazon.com/Art-Unit-Testing-Examples-Net/dp/1933988274/ref=sr_1_1?ie=UTF8&s=books&qid=1267086529&sr=1-1
Chapter 3 Using stubs to break dependencies

接下來作者提出另一種作法, 使用factory paatern. 這樣做的好處是
(1) 你的受測程式便不用管FileExtension Manager是怎樣產生, 或是要用哪一個
(2) 每個class只有單一責任(SRP), LogAnalyzer只負責log, FileExtensionManager只負責怎麼驗證檔案, Factory則負責產生FileExtensionManager的object.

所以會讓程式更簡單, 可讀性更高, 責任更清楚, 將來更容易修改和維護.

方法 3. Factory pattern
// 受測程式
public class LogAnalyzer {
private IExtensionManager manager;

// 設定指向真的File Extension Manager
public LogAnalyzer() {

    // 利用factory pattern來決定真正的Extension Manager
    manager = ExtensionManagerFactory.Create();
}

public bool IsValidLogFileName(string fileName) {
    return manager.IsValid(fileName);
}
}

[TestFixture]
public class LogAnnalyzerTests {

[Test]
public void IsValidFileName_NameShorterThan6() {

    // 設定stub的內容
    StubExtensionManager myFakeManager =
      new StubExtensionManager();
    myFakeManager.ShouldExtensionBeValid = true;

    // 要做測試才將stub object放到factory object中
    ExtensionManagerFactory.SetManager(myFakeManager);

    LogAnalyzer log = new LogAnalyzer();

    // 進行測試
    bool result = log.IsValidLogFileName( "short.ext" );

    // 檢查測試結果
    Assert.IsFalse( result, "File name with less than 5 chars should have failed the method." );
}

// File Extension Manager的stub
internal class StubExtensionManager : IExtensionManager {
    public bool ShouldExtensionBeValid;

    public bool IsValid(string fileName) {
      //實作僅供測試使用的功能
      return ShouldExtensionBeValid;
    }
}
}

// Factory pattern的實作
class ExtensionManagerFactory {
private IExtensionManager customManager = null;

public IExtensionManager Create() {
    if (customManager != null )
      return customManager;

    // 沒有特殊狀況時, 預設就傳為真正的Extension manager
    return new FileExtensionManager();
}

public void SetManager(IExtensionManager mgr) {
    customManager = mgr;
}
}