# What I Learned from "Working Effectively with Legacy Code"


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The page image is mind map made by myself. (Apologize you need to enlarge the picture if you want to take a look)

## How to entering a legacy system?
### Common pain points
* Hard find where to add new features
* Mental overload when reading the code
* Hard to identify the reported issue belongs to bug or feature
* Hard to find the root cause when error happened
* No tests existed
### The project structure is lost
* Sketch the concept first, don't jump into the code directly
* Figure out the responsibility of the system
### The intent of the functions or methods are ambiguous
* Give the comment tag with corresponded responsibility to the lines of code
### Scratch Refactoring
* Even there is not testing, it is still recommended to create a new branch to do refactor
* By extracting the method and moving variables, you can avoid to checking the code again
* If the refactored code can work correctly then you can make sure you understand the code correctly

## How to deal with the system with too many API calls?
### What are the pain points?
* Hard to writing tests
* We often think there only few of self business logics so no need to write the tests
* If we writing the tests would cause the side effects
### Steps for the problems
1. Write a brief description for the code
2. Separate the code responsibility
3. Draw the design with class diagram
### Strategies for interacting with API call
#### Wrapping
* A higher level abstraction interface to the application
* Let us can fake the method during test. (OOP support virtual method)
#### Responsibility-based extraction
* You want to completely separate out the dependency from API calls

## The difference between high level test and low level test
### High level test
* Information for telling you the system currently break the change
### Low level test
* Information for telling you why the system break the change

## Avoid reorder and breaking apart expression while refactoring
* Instead, do extraction from high level piece and break dependency at the same time