Simplify Derivation.onChange()

It no longer uses `DerivationEmitter`.

packages/dataverse/src/derivations/DerivationEmitter.ts

@@ -1,64 +0,0 @@
-import type Ticker from '../Ticker'
-import Emitter from '../utils/Emitter'
-import type {default as Tappable} from '../utils/Tappable'
-import type {IDerivation} from './IDerivation'
-
-/**
- * An event emitter that emits events on changes to a derivation.
- */
-export default class DerivationEmitter<V> {
-  private _derivation: IDerivation<V>
-  private _ticker: Ticker
-  private _emitter: Emitter<V>
-  private _lastValue: undefined | V
-  private _lastValueRecorded: boolean
-  private _hadTappers: boolean
-
-  /**
-   * @param derivation - The derivation to emit events for.
-   * @param ticker - The ticker to use to batch events.
-   */
-  constructor(derivation: IDerivation<V>, ticker: Ticker) {
-    this._derivation = derivation
-    this._ticker = ticker
-    this._emitter = new Emitter()
-    this._emitter.onNumberOfTappersChange(() => {
-      this._reactToNumberOfTappersChange()
-    })
-    this._hadTappers = false
-    this._lastValueRecorded = false
-    this._lastValue = undefined
-    return this
-  }
-
-  private _possiblyMarkAsStale = () => {
-    this._ticker.onThisOrNextTick(this._refresh)
-  }
-
-  private _reactToNumberOfTappersChange() {
-    const hasTappers = this._emitter.hasTappers()
-    if (hasTappers !== this._hadTappers) {
-      this._hadTappers = hasTappers
-      if (hasTappers) {
-        this._derivation.addDependent(this._possiblyMarkAsStale)
-      } else {
-        this._derivation.removeDependent(this._possiblyMarkAsStale)
-      }
-    }
-  }
-
-  /**
-   * The tappable associated with the emitter. You can use it to tap (subscribe to) the underlying derivation.
-   */
-  tappable(): Tappable<V> {
-    return this._emitter.tappable
-  }
-
-  private _refresh = () => {
-    const newValue = this._derivation.getValue()
-    if (newValue === this._lastValue && this._lastValueRecorded === true) return
-    this._lastValue = newValue
-    this._lastValueRecorded = true
-    this._emitter.emit(newValue)
-  }
-}

packages/dataverse/src/derivations/prism/prism.ts

@@ -1,7 +1,6 @@
 import type Ticker from '../../Ticker'
 import type {$IntentionalAny, VoidFn} from '../../types'
 import Stack from '../../utils/Stack'
-import DerivationEmitter from '../DerivationEmitter'
 import type {IDerivation} from '../IDerivation'
 import {isDerivation} from '../IDerivation'
 import {
@@ -200,6 +199,8 @@ class HotHandle<V> {
   }
 }
 
+const emptyObject = {}
+
 class PrismDerivation<V> implements IDerivation<V> {
   /**
    * Whether the object is a derivation.
@@ -227,12 +228,31 @@ class PrismDerivation<V> implements IDerivation<V> {
     listener: (v: V) => void,
     immediate: boolean = false,
   ): VoidFn {
-    const unsubscribe = new DerivationEmitter(this, ticker)
+    const dependent = () => {
-      .tappable()
+      ticker.onThisOrNextTick(refresh)
-      .tap(listener)
-    if (immediate) {
-      listener(this.getValue())
     }
+
+    let lastValue = emptyObject
+
+    const refresh = () => {
+      const newValue = this.getValue()
+      if (newValue === lastValue) return
+
+      lastValue = newValue
+      listener(newValue)
+    }
+
+    this.addDependent(dependent)
+
+    if (immediate) {
+      lastValue = this.getValue()
+      listener(lastValue as $IntentionalAny as V)
+    }
+
+    const unsubscribe = () => {
+      this.removeDependent(dependent)
+    }
+
     return unsubscribe
   }
 

packages/playground/src/shared/dom/Scene.tsx

@@ -183,7 +183,7 @@ export const Scene: React.FC<{project: IProject}> = ({project}) => {
     return studio.onSelectionChange((newState) => {
       setSelection(newState)
     })
-  })
+  }, [])
 
   const containerRef = useRef<HTMLDivElement>(null!)