Flyweight pattern là gì? Khi nào nên dùng?

Flyweight giảm memory usage bằng cách chia sẻ state chung (intrinsic state) giữa nhiều objects tương tự — chỉ lưu state riêng (extrinsic state) trong object cụ thể.

Ví dụ game: render 10,000 cây trong rừng — thay vì mỗi Tree object lưu texture/mesh riêng, tạo TreeType flyweight lưu shared data, Tree chỉ lưu position/scale. Trong JavaScript: string interning, Symbol, React key reconciliation có elements của Flyweight.

Dùng khi: app cần số lượng rất lớn objects tương đồng và memory là bottleneck. Không dùng khi: số lượng object ít; khi overhead quản lý flyweight > memory saved. Flyweight tăng code complexity đáng kể — chỉ dùng khi profiling chứng minh memory là vấn đề thực sự.

Flyweight reduces memory usage by sharing common state (intrinsic state) among many similar objects — only the unique state (extrinsic state) is stored per object.

• Game example: rendering 10,000 trees in a forest — instead of each Tree object storing its own texture and mesh, a TreeType flyweight holds the shared data while Tree only stores position and scale.
• In JavaScript: string interning, Symbols, and React key reconciliation contain elements of Flyweight.
• Use it when an app needs a very large number of similar objects and memory is a bottleneck, or when most of an object's state can be externalized.
• Avoid it when the object count is small, when flyweight management overhead exceeds memory savings, or when the extrinsic state is more complex than the benefit justifies.
• Flyweight adds significant code complexity — only apply it when profiling confirms memory is genuinely the problem.