Behavioural Patterns

Source: Gang of Four — Design Patterns (1994)

Pain Signals — You Are in This Branch When:

• A method keeps growing if/switch branches for each new case or plan
• The same algorithm is implemented differently in multiple places
• A pipeline of steps needs to be reordered, skipped, or extended without touching the core
• An object behaves completely differently depending on what state it is in
• Multiple objects need to react when one changes, but coupling them directly is fragile
• Actions need to be deferred, logged, retried, or undone
• You need to add new operations to a stable structure without modifying it

Chain of Responsibility

Passes a request through a sequence of handlers. Each handler decides to process it, stop it, or pass it forward.