Class Design

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Designing classes

How to write classes in a way taht they are easily understandable, maintainable, and reusable

Software changes

• Software is not like a novel that is written once and then remains unchanged

• Software is extended, corrected, maintained, ported, adapted …

• There are only two options for software

  • It is dead
  • It is changing
• Software that cannot be maintained will be thrown away

Code quality

• Two important concepts for quality of code:
  • Coupling
  • Cohesion

Coupling | 耦合

• Coupling refers to links between separate units of a program
• If two classes depend closely on many details of each other, they are said to be tightly coupled
• We aim for loosely coupled

Loose coupling

Loose coupling makes it possible to:
* understand one class without reading others
* change one class without affecting others
* Thus: improve maintainability

tech. to achieve loose coupling

1. call-back — 发消息定义一个接口，另一方可以通过这个接口注册一个回调函数，当发消息时调用其回调函数从而通知自己这件事情发生了
2. message mechanism — 实现中央消息机制，把事情发给中央，中央再发布给注册者

Cohesion | 内聚

• Cohesion refers to the number and diversity of tasks that diversity of tasks that a single unit is responsible for
• If each unit is responsible for one single logical task, we say it has high cohesion
• Cohesion applies to classes and methods
• We aim for high cohesion

让一个单元只复杂一件事情，小到变量，大到模块···

High cohesion

High cohesion makes it easier to:
* understand a class or method
* use descriptive names
* reuse classes or methods

Code duplication | 代码重复

• is an indicator of bad design
• makes maintenance harder
• can lead to introduction of errors during maintenance

Responsibility-driven design | 责任驱动设计

• Question: where should we add a new method(which class)?
• Each class should be responsible for manipulating its own data
• The class that owns the data should be responsible for processing it
• RDD leads to low coupling and high cohesion

Localizing changes | 局部化变化

• One aim of reducing coupling and responsibility-driven design is to localize changes
• When a change is needed, as few classes as possible should be affected

可扩展性（extensibility） — 代码不需要修改（or 很少的修改）就可以满足未来的需求

Refactoring | 重构

Code refactoring 是什么

在不改变软件的外部行为的条件下，通过修改代码改变软件内部结构，将效率低下和过于复杂的代码转换为更高效、更简单的代码

• Advantages:
  • 提高代码质量
  • 优化软件产品架构与性能
  • 减少项目的技术债，避免项目重写
• Disadvantages:
  • 增加工作负担
  • 可能出现一些业务上的漏洞
  • 可能代码过于精炼导致代码可读性变差

重构 != 重写

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