App Lifecycle

An app is not a long-running process. The hub wakes it on an event, a schedule, a UI render, install/update/uninstall, or an HTTP endpoint hit, runs one method, and sleeps.

Callbacks

• installed() — first install only.
• updated() — every time the user presses Done on an already-installed app.
• uninstalled() — on removal; subscriptions and schedules are auto-cleaned, so use it only for external cleanup.
• appButtonHandler(String name) — a button input was pressed.
• hubStartupHandler() — auto-called on hub startup (no subscription needed).

The reinitialize idiom (mandatory)

• On the first-ever Done, only installed() runs — not updated(). An app that creates its subscriptions solely in updated() silently does nothing until the second Done. This is the single most common app bug.
• The fix every app follows: installed() calls updated(); updated() calls unsubscribe() then re-subscribes.

def installed() { updated() }
def updated()   { unsubscribe(); initialize() }
def initialize(){ subscribe(motionSensor, "motion", "motionHandler") }

• unsubscribe() at the top of updated()/initialize() prevents duplicate subscriptions when the user changes a selected device. The same applies to schedules — unschedule() before re-scheduling, or runIn/schedule stack silently unless overwrite is left at its default.

Handlers that skip initialize()

• hubStartupHandler() runs on hub startup without routing through installed()/updated()/initialize(). Any state.* value that initialize() sets up may be null when it fires, and reading it NPEs on boot (cannot invoke method keySet() on null object).
• The same trap catches any entry point the platform invokes before the first Done or after a state reset — hubStartupHandler, a subscribed event, a scheduled job.
• Do not initialize state.* collections in initialize() alone — a Map and a List both start null (the example seeds each). Put the setup in a small helper, call it from every entry point that reads them, or null-guard at the read site (rules/groovy-gotchas.md).

private ensureState() {
    if (state.activeSince == null) state.activeSince = [:]
    if (state.stuck == null)       state.stuck = []
}
def initialize()        { ensureState(); /* subscribe … */ }
private reconcile()     { ensureState(); /* reached from hubStartupHandler(), which skips initialize() */ }
def hubStartupHandler() { reconcile() }

• Test it: a spec that drives the startup/handler path on a freshly-loaded, never-initialized instance and asserts no NPE catches this deterministically — it fails before the guard, passes after (skills/test).

Mutating settings from code

• app.updateSetting(name, value) writes a setting; app.removeSetting(name) / clearSetting(name) drop one. All undocumented platform methods.
• removeSetting is deferred — the in-memory settings map still returns the old value for the rest of the current execution, so it is cleanup for the next wake, never a same-render crash-guard (rules/groovy-gotchas.md).

Subscriptions & scheduling

• Handler method names are passed as bare strings: subscribe(dev, "switch", "switchHandler"), runIn(300, "checkState"). A typo'd or missing handler name fails quietly — see rules/groovy-gotchas.md.
• Handlers take one evt param: evt.name, evt.value, evt.device.
• Prefer runIn/runInMillis/runOnce/schedule (7-field Quartz cron) over any busy-wait. Parent/child app communication goes through exposed methods, never shared state — see skills/_reference/endpoints.md only for hub-side APIs, not for cross-app calls.