# Patterns/observer

> Observer Pattern (Callbacks) pattern for C development

- Skill: `majiayu000/patterns-observer` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/patterns-observer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/patterns-observer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/patterns-observer

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# Observer Pattern (Callbacks)

Objects register interest in events and get notified when they occur. In C, implemented as callback function pointers with context.

## ikigai Application

**Streaming chunks:** LLM client notifies REPL as response chunks arrive:
```c
typedef void (*chunk_callback_t)(void *ctx, const char *chunk, size_t len);
res_t ik_llm_stream(client, messages, on_chunk, user_ctx);
```

**Terminal resize:** Signal handler notifies REPL of SIGWINCH.

**Future uses:**
- Database change notifications
- Tool execution progress
- Background task completion

**Convention:** Always pair callback function pointer with `void *user_ctx` parameter.