Agile Design

Symptoms of poor design

• Rigidity
• Fragility
• Immobility
• Viscosity
• Needless Complexity
• Needless Repetition
• Opacity

Principles

• SRP
• OCP
• LSP
• DIP
• ISP

What's agile design?

• Always multiply your estimates by three
• Requirements always change
• Writing software would be a lot easier if it weren't for customers
  

Since the requirements will always change, it is necessary to do some extra work before implementation , so this will leave space when you later add more function by , for this part you can also follow OCP prinicple.