Flyweight

Intent

Share common object state to reduce memory usage when many similar objects are needed.

Problem

Large collections can waste memory by duplicating identical intrinsic data.

Solution

Separate shared intrinsic state from per-use extrinsic state and reuse flyweights through a factory.

TypeScript Implementation

MarkerIconFactory reuses map marker icons by type.

npm run flyweight

Trade-offs

• Useful for large object counts.
• Premature use can overcomplicate code.

Practical Perspective

Structural patterns are about shaping relationships between objects so systems can evolve without rewriting every caller.

For Flyweight, the important question is not “can I draw the UML diagram?” but “what dependency or decision becomes easier to change after I introduce this pattern?” In production code, the pattern should make ownership clearer, reduce accidental coupling, and give tests a natural seam.

Real-World Use Cases

• Third-party API boundaries where Flyweight keeps responsibilities separated.
• Legacy migration layers where Flyweight keeps responsibilities separated.
• UI component composition where Flyweight keeps responsibilities separated.
• Cross-cutting wrappers such as caching, logging, or access checks where Flyweight keeps responsibilities separated.

Decision Questions

• Which interface should client code depend on?
• Where should translation, composition, or access control live?
• Does this abstraction reduce coupling or just rename it?
• Use it only when many objects share measurable immutable state.
• Keep intrinsic shared state immutable and extrinsic state outside the flyweight.

Design Checklist

• Start with the client code: define the interface you want callers to depend on.
• Keep concrete classes small and named after one responsibility.
• Make creation, selection, delegation, or notification rules explicit instead of hidden in conditionals.
• Prefer composition roots for wiring objects together.
• Document the reason for using the pattern so future contributors do not cargo-cult it.

Common Mistakes

• Adding the pattern before the code has a real variation point.
• Creating abstractions that only rename concrete classes.
• Hiding important runtime behavior so debugging becomes harder.
• Letting examples stay toy-sized without showing where the pattern boundary sits in real code.
• Forgetting tests for negative paths, invalid states, or fallback behavior.

Testing Guidance

• Test through the public abstraction, not private implementation details.
• Use fakes or test doubles for collaborators so the pattern seam is verified.
• Add one integration-style test proving the objects are wired correctly.
• Cover edge cases that motivated the pattern: missing strategy, rejected state transition, failed handler, invalid factory family, stale proxy cache, or similar.
• Keep tests named after behavior and business outcome rather than pattern terminology.

Refactoring Signals

• The pattern is useful when adding a new variation no longer requires editing stable caller code.
• It is probably overdesigned when every new class has only one trivial method and no independent reason to exist.
• If contributors cannot explain the runtime flow quickly, simplify the wiring or improve names.
• If tests must mock too many layers, the abstraction boundary is likely in the wrong place.