Stimulus Patterns

Stimulus patterns

Keep controllers small, Turbo-safe, and driven by the server. Full patterns: references/stimulus-guide.md (grounded in the Piazza controllers).

When to use

• Adding a Stimulus controller.
• Wiring elements that arrive later (Turbo Stream append, frame load) — use the xTargetConnected self-wiring callback instead of re-querying.
• Passing data/CSS from the server (Values/Classes API).
• Debugging: a controller that never connects, a this.fooTarget undefined error, or a listener/timer that double-fires after navigation.

The patterns (and their footguns)

1. Small, single-purpose controllers — many tiny ones over one big one.
2. Self-wiring via xTargetConnected(target) — fires per element as each connects, including streamed-in ones (templates/self_wiring_controller.js.tmpl).
3. Values/Classes API — push config + CSS class names from ERB (this.fooValue, this.fooClasses); keep styling in the view.
4. State machine + delegate — controller toggles DOM state; a plain helper does SDK/transport plumbing and calls back.
5. Clone a server <template> — stamp repeated markup instead of building HTML in JS.
6. Clean up in disconnect() — undo addEventListener(window/document), setInterval, observers, or native UI you rendered, or it leaks and double-fires after Turbo navigation. data-action listeners are auto-managed; manual ones aren't.

Start a controller

sed 's/__NAME__/your-name/g' templates/controller.js.tmpl > your_name_controller.js
# or templates/self_wiring_controller.js.tmpl for the streamed-in case

Register it (manifest index.js via bin/rails stimulus:manifest:update, or stimulus-loading) — an unregistered controller never connects.

Lint

scripts/lint_stimulus.sh path/to/rails-app      # or a controllers/ dir

Flags: this.xTarget/xValue/xClass used but not declared in static targets/values/classes (runtime errors); connect() that registers a listener/timer/observer with no disconnect() (leak/double-fire); a controller file missing from the index.js manifest (never connects). Heuristic regex scan (needs ruby); names its ceiling. Verified: clean on all 7 Piazza controllers, flags a synthetic broken one on every check.