Dependency injection intro

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Lin Tzu Cheng

Dependency injection introduction

Software

相依性注入
Dependency Injection
What?Where?Why?When?

程式架構
Program PrintService
建立物件
print()
呼叫方法
重點

何謂相依性(Dependency)？
PrintService ConsoleOutput
建立ConsoleOutput物件，
或建立 ConsoleOutput類別的執行個體
Depend

如果… 「輸出能不能改輸出到Text檔案裡面？」
PrintService ConsoleOutput
Depend
TextOutput
所以問題是…
Print方法須修改，並且重新編譯、測試，
導致維護成本高。
耦合性(Coupling)過高

降低耦合度…STEP 1 加入Interface
PrintService
IWriter
TextOutput
ConsoleOutput

降低耦合度…STEP 2 相依注入
請給我IWrite的東西
我只吃IWrite的東西
反正我執行就對了
PrintService
IWriter

從主程式實際注入功能…
由他人依照Iwriter實作各種Output
INJECTION!!!
PrintService以後只知道IWrite介面
耦合性(Coupling)降低

「寫程式時，請想像最後維護此程式的人，是個
有暴力傾向的精神病患，而且他知道你住哪裡。」
- Damian Conway 《Perl Best Practices》
請記得…

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