Better React 18 support

.changeset/early-terms-bow.md

@@ -0,0 +1,7 @@
+---
+"mobx-react": minor
+"mobx-react-lite": minor
+"mobx": patch
+---
+
+TODO

.vscode/settings.json

@@ -1,9 +1,11 @@
 {
     "[typescript]": {
+        "editor.defaultFormatter": "esbenp.prettier-vscode",
         "editor.formatOnSave": true,
         "editor.formatOnPaste": false
     },
     "[javascript]": {
+        "editor.defaultFormatter": "esbenp.prettier-vscode",
         "editor.formatOnSave": true,
         "editor.formatOnPaste": false
     },

packages/eslint-plugin-mobx/README.md

@@ -85,6 +85,7 @@ It's a bit opinionated and can lead to a lot of false positives depending on you
 ### mobx/no-anonymous-observer (deprecated)
 
 _Deprecated in favor of [react/display-name](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/docs/rules/display-name.md) + [componentWrapperFunctions](https://github.com/jsx-eslint/eslint-plugin-react). Example of **.eslintrc**:_
+
 ```
 {
   "rules": {
@@ -97,6 +98,7 @@ _Deprecated in favor of [react/display-name](https://github.com/jsx-eslint/eslin
   }
 }
 ```
+
 ---
 
 Forbids anonymous functions or classes as `observer` components.

packages/mobx-react-lite/package.json

@@ -36,7 +36,9 @@
         "url": "https://github.com/mobxjs/mobx/issues"
     },
     "homepage": "https://mobx.js.org",
-    "dependencies": {},
+    "dependencies": {
+        "use-sync-external-store": "^1.2.0"
+    },
     "peerDependencies": {
         "mobx": "^6.1.0",
         "react": "^16.8.0 || ^17 || ^18"

packages/mobx-react-lite/src/index.ts

@@ -16,6 +16,7 @@ export { useLocalObservable } from "./useLocalObservable"
 export { useLocalStore } from "./useLocalStore"
 export { useAsObservableSource } from "./useAsObservableSource"
 
+export { observerFinalizationRegistry as _observerFinalizationRegistry }
 export const clearTimes = observerFinalizationRegistry["finalizeAllImmediately"] ?? (() => {})
 
 export function useObserver<T>(fn: () => T, baseComponentName: string = "observed"): T {

packages/mobx-react-lite/src/useObserver.ts

@@ -1,129 +1,115 @@
-import { Reaction } from "mobx"
+import { Reaction, _getGlobalState } from "mobx"
 import React from "react"
 import { printDebugValue } from "./utils/printDebugValue"
-import { observerFinalizationRegistry } from "./utils/observerFinalizationRegistry"
 import { isUsingStaticRendering } from "./staticRendering"
+import { observerFinalizationRegistry } from "./utils/observerFinalizationRegistry"
+import { useSyncExternalStore } from "use-sync-external-store/shim"
 
-function observerComponentNameFor(baseComponentName: string) {
-    return `observer${baseComponentName}`
-}
-
+// Do not store `admRef` (even as part of a closure!) on this object,
+// otherwise it will prevent GC and therefore reaction disposal via FinalizationRegistry.
 type ObserverAdministration = {
-    /** The Reaction created during first render, which may be leaked */
-    reaction: Reaction | null
-
-    /**
-     * Whether the component has yet completed mounting (for us, whether
-     * its useEffect has run)
-     */
-    mounted: boolean
-
-    /**
-     * Whether the observables that the component is tracking changed between
-     * the first render and the first useEffect.
-     */
-    changedBeforeMount: boolean
+    reaction: Reaction | null // also serves as disposed flag
+    forceUpdate: Function | null // also serves as mounted flag
+    // BC: we will use local state version if global isn't available.
+    // It should behave as previous implementation - tearing is still present,
+    // because there is no cross component synchronization,
+    // but we can use `useSyncExternalStore` API.
+    stateVersion: any
+    name: string
+    // These don't depend on state/props, therefore we can keep them here instead of `useCallback`
+    subscribe: Parameters<typeof React.useSyncExternalStore>[0]
+    getSnapshot: Parameters<typeof React.useSyncExternalStore>[1]
 }
 
-/**
- * We use class to make it easier to detect in heap snapshots by name
- */
-class ObjectToBeRetainedByReact {}
+const mobxGlobalState = _getGlobalState()
+
+// BC
+const globalStateVersionIsAvailable = typeof mobxGlobalState.globalVersion !== "undefined"
 
-function objectToBeRetainedByReactFactory() {
-    return new ObjectToBeRetainedByReact()
+function createReaction(adm: ObserverAdministration) {
+    adm.reaction = new Reaction(`observer${adm.name}`, () => {
+        if (!globalStateVersionIsAvailable) {
+            // BC
+            adm.stateVersion = Symbol()
+        }
+        // Force update won't be avaliable until the component "mounts".
+        // If state changes in between initial render and mount,
+        // `useSyncExternalStore` should handle that by checking the state version and issuing update.
+        adm.forceUpdate?.()
+    })
 }
 
-export function useObserver<T>(fn: () => T, baseComponentName: string = "observed"): T {
+export function useObserver<T>(render: () => T, baseComponentName: string = "observed"): T {
     if (isUsingStaticRendering()) {
-        return fn()
+        return render()
     }
 
-    const [objectRetainedByReact] = React.useState(objectToBeRetainedByReactFactory)
-    // Force update, see #2982
-    const [, setState] = React.useState()
-    const forceUpdate = () => setState([] as any)
-
-    // StrictMode/ConcurrentMode/Suspense may mean that our component is
-    // rendered and abandoned multiple times, so we need to track leaked
-    // Reactions.
     const admRef = React.useRef<ObserverAdministration | null>(null)
 
     if (!admRef.current) {
         // First render
-        admRef.current = {
+        const adm: ObserverAdministration = {
             reaction: null,
-            mounted: false,
-            changedBeforeMount: false
+            forceUpdate: null,
+            stateVersion: Symbol(),
+            name: baseComponentName,
+            subscribe(onStoreChange: () => void) {
+                // Do NOT access admRef here!
+                observerFinalizationRegistry.unregister(adm)
+                adm.forceUpdate = onStoreChange
+                if (!adm.reaction) {
+                    // We've lost our reaction and therefore all subscriptions.
+                    // We have to recreate reaction and schedule re-render to recreate subscriptions,
+                    // even if state did not change.
+                    createReaction(adm)
+                    adm.forceUpdate()
+                }
+
+                return () => {
+                    // Do NOT access admRef here!
+                    adm.forceUpdate = null
+                    adm.reaction?.dispose()
+                    adm.reaction = null
+                }
+            },
+            getSnapshot() {
+                // Do NOT access admRef here!
+                return globalStateVersionIsAvailable
+                    ? mobxGlobalState.stateVersion
+                    : adm.stateVersion
+            }
         }
+
+        admRef.current = adm
     }
 
     const adm = admRef.current!
 
     if (!adm.reaction) {
-        // First render or component was not committed and reaction was disposed by registry
-        adm.reaction = new Reaction(observerComponentNameFor(baseComponentName), () => {
-            // Observable has changed, meaning we want to re-render
-            // BUT if we're a component that hasn't yet got to the useEffect()
-            // stage, we might be a component that _started_ to render, but
-            // got dropped, and we don't want to make state changes then.
-            // (It triggers warnings in StrictMode, for a start.)
-            if (adm.mounted) {
-                // We have reached useEffect(), so we're mounted, and can trigger an update
-                forceUpdate()
-            } else {
-                // We haven't yet reached useEffect(), so we'll need to trigger a re-render
-                // when (and if) useEffect() arrives.
-                adm.changedBeforeMount = true
-            }
-        })
-
-        observerFinalizationRegistry.register(objectRetainedByReact, adm, adm)
+        // First render or reaction was disposed by registry before subscribe
+        createReaction(adm)
+        // StrictMode/ConcurrentMode/Suspense may mean that our component is
+        // rendered and abandoned multiple times, so we need to track leaked
+        // Reactions.
+        observerFinalizationRegistry.register(admRef, adm, adm)
     }
 
-    React.useDebugValue(adm.reaction, printDebugValue)
-
-    React.useEffect(() => {
-        observerFinalizationRegistry.unregister(adm)
-
-        adm.mounted = true
+    React.useDebugValue(adm.reaction!, printDebugValue)
 
-        if (adm.reaction) {
-            if (adm.changedBeforeMount) {
-                // Got a change before mount, force an update
-                adm.changedBeforeMount = false
-                forceUpdate()
-            }
-        } else {
-            // The reaction we set up in our render has been disposed.
-            // This can be due to bad timings of renderings, e.g. our
-            // component was paused for a _very_ long time, and our
-            // reaction got cleaned up
-
-            // Re-create the reaction
-            adm.reaction = new Reaction(observerComponentNameFor(baseComponentName), () => {
-                // We've definitely already been mounted at this point
-                forceUpdate()
-            })
-            forceUpdate()
-        }
-
-        return () => {
-            adm.reaction!.dispose()
-            adm.reaction = null
-            adm.mounted = false
-            adm.changedBeforeMount = false
-        }
-    }, [])
+    useSyncExternalStore(
+        // Both of these must be stable, otherwise it would keep resubscribing every render.
+        adm.subscribe,
+        adm.getSnapshot
+    )
 
     // render the original component, but have the
     // reaction track the observables, so that rendering
     // can be invalidated (see above) once a dependency changes
-    let rendering!: T
+    let renderResult!: T
     let exception
-    adm.reaction.track(() => {
+    adm.reaction!.track(() => {
         try {
-            rendering = fn()
+            renderResult = render()
         } catch (e) {
             exception = e
         }
@@ -133,5 +119,5 @@ export function useObserver<T>(fn: () => T, baseComponentName: string = "observe
         throw exception // re-throw any exceptions caught during rendering
     }
 
-    return rendering
+    return renderResult
 }

packages/mobx-react/__tests__/__snapshots__/observer.test.tsx.snap

@@ -18,6 +18,8 @@ exports[`#797 - replacing this.render should trigger a warning 1`] = `
 }
 `;
 
+exports[`#3492 should not cause warning by calling forceUpdate on uncommited components 1`] = `[MockFunction]`;
+
 exports[`Redeclaring an existing observer component as an observer should log a warning 1`] = `
 [MockFunction] {
   "calls": Array [
@@ -35,23 +37,7 @@ exports[`Redeclaring an existing observer component as an observer should log a
 }
 `;
 
-exports[`SSR works #3448 1`] = `
-[MockFunction] {
-  "calls": Array [
-    Array [
-      "The reactive render of an observer class component (TestCmp)
-                was overridden after MobX attached. This may result in a memory leak if the
-                overridden reactive render was not properly disposed.",
-    ],
-  ],
-  "results": Array [
-    Object {
-      "type": "return",
-      "value": undefined,
-    },
-  ],
-}
-`;
+exports[`SSR works #3448 1`] = `[MockFunction]`;
 
 exports[`issue 12 1`] = `
 <div>

packages/mobx-react/__tests__/finalizationRegistry.tsx

@@ -0,0 +1,88 @@
+import { cleanup, render, waitFor } from "@testing-library/react"
+import * as mobx from "mobx"
+import * as React from "react"
+
+// @ts-ignore
+import gc from "expose-gc/function"
+import { observer } from "../src"
+
+afterEach(cleanup)
+
+function sleep(time: number) {
+    return new Promise<void>(res => {
+        setTimeout(res, time)
+    })
+}
+
+// TODO dunno why it's not available
+declare class WeakRef<T> {
+    constructor(object: T)
+    deref(): T | undefined
+}
+
+test("should not prevent GC of uncomitted components", async () => {
+    expect(typeof globalThis.FinalizationRegistry).toBe("function")
+
+    // This specific setup causes first instance of A not being commited.
+    // This is checked by comparing constructor and componentDidMount invocation counts.
+    // There is no profound reason why that's the case, if you know a simpler or more robust setup
+    // feel free to change this.
+
+    const o = mobx.observable({ x: 0 })
+    let aConstructorCount = 0
+    let aMountCount = 0
+
+    let firstARef: WeakRef<React.Component>
+
+    @observer
+    class A extends React.Component<any> {
+        constructor(props) {
+            super(props)
+            if (aConstructorCount === 0) {
+                firstARef = new WeakRef(this)
+            }
+            aConstructorCount++
+        }
+        componentDidMount(): void {
+            aMountCount++
+        }
+        render() {
+            return (
+                <React.Suspense fallback="fallback">
+                    <LazyB />
+                    {o.x}
+                </React.Suspense>
+            )
+        }
+    }
+
+    class B extends React.Component {
+        render() {
+            return "B"
+        }
+    }
+
+    const LazyA = React.lazy(() => Promise.resolve({ default: A }))
+    const LazyB = React.lazy(() => Promise.resolve({ default: B }))
+
+    function App() {
+        return (
+            <React.Suspense fallback="fallback">
+                <LazyA />
+            </React.Suspense>
+        )
+    }
+
+    const { unmount, container } = render(<App />)
+
+    expect(container).toHaveTextContent("fallback")
+    await waitFor(() => expect(container).toHaveTextContent("B0"))
+    expect(aConstructorCount).toBe(2)
+    expect(aMountCount).toBe(1)
+
+    gc()
+    await sleep(50)
+    expect(firstARef!.deref()).toBeUndefined()
+
+    unmount()
+})