add support for WeakRefs

docs/computeds.md

@@ -236,3 +236,7 @@ It is recommended to set this one to `true` on very expensive computed values. I
 ### `keepAlive`
 
 This avoids suspending computed values when they are not being observed by anything (see the above explanation). Can potentially create memory leaks, similar to the ones discussed for [reactions](reactions.md#always-dispose-of-reactions).
+
+### `weak`
+
+Intended for use with `keepAlive`. When `true`, MobX will use a `WeakRef` (_if_ you're not targeting something old that doesn't support `WeakRef`s) when add the `computed` to any `observables`. If your reference to the `computed` is garbage collected, the `computed` will be too (instead of `observable`s holding references and preventing garbage collection)

packages/mobx/__tests__/v5/base/weakset.ts

@@ -0,0 +1,128 @@
+import {
+    IObservableValue,
+    autorun,
+    computed,
+    observable,
+    onBecomeObserved,
+    onBecomeUnobserved,
+    runInAction
+} from "../../../src/mobx"
+const gc = require("expose-gc/function")
+
+let events: string[] = []
+beforeEach(() => {
+    events = []
+})
+
+function nextFrame() {
+    return new Promise(accept => setTimeout(accept, 1))
+}
+
+async function gc_cycle() {
+    await nextFrame()
+    gc()
+    await nextFrame()
+}
+
+test("observables should not hold a reference to weak reactions", async () => {
+    let x = 0
+    const o = observable.box(10)
+
+    ;(() => {
+        const au = autorun(
+            () => {
+                x += o.get()
+            },
+            { weak: true }
+        )
+
+        o.set(5)
+        expect(x).toEqual(15)
+    })()
+
+    await gc_cycle()
+    expect((o as any).observers_.size).toEqual(0)
+
+    o.set(20)
+    expect(x).toEqual(15)
+})
+
+test("observables should hold a reference to reactions", async () => {
+    let x = 0
+    const o = observable.box(10)
+    ;(() => {
+        autorun(() => {
+            x += o.get()
+        }, {})
+
+        o.set(5)
+    })()
+
+    await gc_cycle()
+    expect((o as any).observers_.size).toEqual(1)
+
+    o.set(20)
+    expect(x).toEqual(35)
+})
+
+test("observables should not hold a reference to weak computeds", async () => {
+    const o = observable.box(10)
+    let wref
+    ;(() => {
+        const kac = computed(
+            () => {
+                return o.get()
+            },
+            { keepAlive: true, weak: true }
+        )
+        wref = new WeakRef(kac)
+        kac.get()
+    })()
+
+    expect(wref.deref()).not.toEqual(null)
+    await gc_cycle()
+    expect(wref.deref() == null).toBeTruthy()
+    expect((o as any).observers_.size).toEqual(0)
+})
+
+test("observables should hold a reference to computeds", async () => {
+    const o = observable.box(10)
+    let wref
+    ;(() => {
+        const kac = computed(
+            () => {
+                return o.get()
+            },
+            { keepAlive: true }
+        )
+        kac.get()
+        wref = new WeakRef(kac)
+    })()
+
+    expect(wref.deref() != null).toBeTruthy()
+    await nextFrame()
+    gc()
+    await nextFrame()
+    expect(wref.deref() != null).toBeTruthy()
+    expect((o as any).observers_.size).toEqual(1)
+})
+
+test("garbage collection should trigger onBOU", async () => {
+    const o = observable.box(10)
+
+    onBecomeObserved(o, () => events.push(`o observed`))
+    onBecomeUnobserved(o, () => events.push(`o unobserved`))
+
+    ;(() => {
+        autorun(
+            () => {
+                o.get()
+            },
+            { weak: true }
+        )
+    })()
+
+    expect(events).toEqual(["o observed"])
+    await gc_cycle()
+    expect(events).toEqual(["o observed", "o unobserved"])
+})

packages/mobx/package.json

@@ -44,7 +44,8 @@
         "@babel/preset-typescript": "^7.9.0",
         "@babel/runtime": "^7.9.2",
         "conditional-type-checks": "^1.0.5",
-        "flow-bin": "^0.123.0"
+        "flow-bin": "^0.123.0",
+        "expose-gc": "^1.0.0"
     },
     "keywords": [
         "mobx",

packages/mobx/src/api/autorun.ts

@@ -25,6 +25,12 @@ export interface IAutorunOptions {
     requiresObservable?: boolean
     scheduler?: (callback: () => void) => any
     onError?: (error: any) => void
+    /**
+     * Observees will not prevent this Reaction from being garbage collected and disposed - you'll need to keep a reference to it somewhere
+     *
+     * This is an advanced feature that, in 99.99% of cases you won't need.
+     */
+    weak?: boolean
 }
 
 /**
@@ -59,7 +65,8 @@ export function autorun(
                 this.track(reactionRunner)
             },
             opts.onError,
-            opts.requiresObservable
+            opts.requiresObservable,
+            opts.weak
         )
     } else {
         const scheduler = createSchedulerFromOptions(opts)
@@ -80,7 +87,8 @@ export function autorun(
                 }
             },
             opts.onError,
-            opts.requiresObservable
+            opts.requiresObservable,
+            opts.weak
         )
     }
 
@@ -152,7 +160,8 @@ export function reaction<T, FireImmediately extends boolean = false>(
             }
         },
         opts.onError,
-        opts.requiresObservable
+        opts.requiresObservable,
+        opts.weak
     )
 
     function reactionRunner() {

packages/mobx/src/core/atom.ts

@@ -11,11 +11,28 @@ import {
     propagateChanged,
     reportObserved,
     startBatch,
-    Lambda
+    Lambda,
+    StrongWeakSet,
+    queueForUnobservation
 } from "../internal"
 
 export const $mobx = Symbol("mobx administration")
 
+export function createObserverStore(observee: IObservable): Set<IDerivation> {
+    if (typeof WeakRef != "undefined" && typeof FinalizationRegistry != "undefined") {
+        const store = new StrongWeakSet<IDerivation>(() => {
+            if (store.size === 0) {
+                startBatch()
+                queueForUnobservation(observee)
+                endBatch()
+            }
+        })
+        return store
+    } else {
+        return new Set<IDerivation>()
+    }
+}
+
 export interface IAtom extends IObservable {
     reportObserved(): boolean
     reportChanged()
@@ -24,7 +41,7 @@ export interface IAtom extends IObservable {
 export class Atom implements IAtom {
     isPendingUnobservation_ = false // for effective unobserving. BaseAtom has true, for extra optimization, so its onBecomeUnobserved never gets called, because it's not needed
     isBeingObserved_ = false
-    observers_ = new Set<IDerivation>()
+    observers_ = createObserverStore(this)
 
     diffValue_ = 0
     lastAccessedBy_ = 0

packages/mobx/src/core/computedvalue.ts

@@ -29,7 +29,8 @@ import {
     UPDATE,
     die,
     allowStateChangesStart,
-    allowStateChangesEnd
+    allowStateChangesEnd,
+    createObserverStore
 } from "../internal"
 
 export interface IComputedValue<T> {
@@ -45,6 +46,12 @@ export interface IComputedValueOptions<T> {
     context?: any
     requiresReaction?: boolean
     keepAlive?: boolean
+    /**
+     * Stop any observees from preventing this computed from being garbage collected
+     *
+     * This is an advanced feature and primarily intended for use with `keepAlive` computeds.
+     */
+    weak?: boolean
 }
 
 export type IComputedDidChange<T = any> = {
@@ -81,7 +88,7 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
     newObserving_ = null // during tracking it's an array with new observed observers
     isBeingObserved_ = false
     isPendingUnobservation_: boolean = false
-    observers_ = new Set<IDerivation>()
+    observers_ = createObserverStore(this)
     diffValue_ = 0
     runId_ = 0
     lastAccessedBy_ = 0
@@ -99,6 +106,7 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
     private equals_: IEqualsComparer<any>
     private requiresReaction_: boolean | undefined
     keepAlive_: boolean
+    weak_: boolean
 
     /**
      * Create a new computed value based on a function expression.
@@ -132,6 +140,7 @@ export class ComputedValue<T> implements IObservable, IComputedValue<T>, IDeriva
         this.scope_ = options.context
         this.requiresReaction_ = options.requiresReaction
         this.keepAlive_ = !!options.keepAlive
+        this.weak_ = !!options.weak
     }
 
     onBecomeStale_() {

packages/mobx/src/core/derivation.ts

@@ -58,6 +58,8 @@ export interface IDerivation extends IDepTreeNode {
      *  warn if the derivation has no dependencies after creation/update
      */
     requiresObservable_?: boolean
+
+    readonly weak_: boolean
 }
 
 export class CaughtException {

packages/mobx/src/core/reaction.ts

@@ -67,7 +67,8 @@ export class Reaction implements IDerivation, IReactionPublic {
         public name_: string = __DEV__ ? "Reaction@" + getNextId() : "Reaction",
         private onInvalidate_: () => void,
         private errorHandler_?: (error: any, derivation: IDerivation) => void,
-        public requiresObservable_?
+        public requiresObservable_?,
+        readonly weak_ = false
     ) {}
 
     onBecomeStale_() {

packages/mobx/src/internal.ts

@@ -8,6 +8,7 @@ but at least in this file we can magically reorder the imports with trial and er
 export * from "./utils/global"
 export * from "./errors"
 export * from "./utils/utils"
+export * from "./utils/weakset"
 export * from "./api/decorators"
 export * from "./core/atom"
 export * from "./utils/comparer"