diff --git a/packages/dataverse/src/prism/prism.ts b/packages/dataverse/src/prism/prism.ts index a8d6801..856547b 100644 --- a/packages/dataverse/src/prism/prism.ts +++ b/packages/dataverse/src/prism/prism.ts @@ -225,29 +225,50 @@ class PrismInstance implements Prism { listener: (v: V) => void, immediate: boolean = false, ): VoidFn { + // the prism will call this function every time it goes from fresh to stale const dependent = () => { + // schedule the listener to be called on the next tick, unless + // we're already on a tick, in which case it'll be called on the current tick. ticker.onThisOrNextTick(refresh) } - let lastValue = emptyObject + // let's cache the last value so we don't call the listener if the value hasn't changed + let lastValue = + // use an empty object as the initial value so that the listener is called on the first tick. + // if we were to use, say, undefined, and this.getValue() also returned undefined, the listener + // would never be called. + emptyObject + // this function will be _scheduled_ to be called on the currently running, or next tick, + // after the prism has gone from fresh to stale. const refresh = () => { const newValue = this.getValue() + // if the value hasn't changed, don't call the listener if (newValue === lastValue) return + // the value has changed - cache it lastValue = newValue + + // and let the listener know listener(newValue) } + // add the dependent to the prism's list of dependents (which will make it go hot) this._addDependent(dependent) + // if the caller wants the listener to be called immediately, call it now if (immediate) { lastValue = this.getValue() listener(lastValue as $IntentionalAny as V) } + // the unsubscribe function const unsubscribe = () => { + // remove the dependent from the prism's list of dependents (and if it was the last dependent, the prism will go cold) this._removeDependent(dependent) + // in case we're scheduled for a tick, cancel that + ticker.offThisOrNextTick(refresh) + ticker.offNextTick(refresh) } return unsubscribe