mobx-react-lite/src/useObserver.ts

import { Reaction } from "mobx"
import React from "react"
import { printDebugValue } from "./utils/printDebugValue"
import {
    addReactionToTrack,
    IReactionTracking,
    recordReactionAsCommitted
} from "./utils/reactionCleanupTracking"
import { isUsingStaticRendering } from "./staticRendering"

function observerComponentNameFor(baseComponentName: string) {
    return `observer${baseComponentName}`
}

/**
 * We use class to make it easier to detect in heap snapshots by name
 */
class ObjectToBeRetainedByReact {}

function objectToBeRetainedByReactFactory() {
    return new ObjectToBeRetainedByReact()
}

export function useObserver<T>(fn: () => T, baseComponentName: string = "observed"): T {
    if (isUsingStaticRendering()) {
        return fn()
    }

    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 reactionTrackingRef = React.useRef<IReactionTracking | null>(null)

    if (!reactionTrackingRef.current) {
        // First render for this component (or first time since a previous
        // reaction from an abandoned render was disposed).

        const newReaction = 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 (trackingData.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.
                trackingData.changedBeforeMount = true
            }
        })

        const trackingData = addReactionToTrack(
            reactionTrackingRef,
            newReaction,
            objectRetainedByReact
        )
    }

    const { reaction } = reactionTrackingRef.current!
    React.useDebugValue(reaction, printDebugValue)

    React.useEffect(() => {
        // Called on first mount only
        recordReactionAsCommitted(reactionTrackingRef)

        if (reactionTrackingRef.current) {
            // Great. We've already got our reaction from our render;
            // all we need to do is to record that it's now mounted,
            // to allow future observable changes to trigger re-renders
            reactionTrackingRef.current.mounted = true
            // Got a change before first mount, force an update
            if (reactionTrackingRef.current.changedBeforeMount) {
                reactionTrackingRef.current.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
            reactionTrackingRef.current = {
                reaction: new Reaction(observerComponentNameFor(baseComponentName), () => {
                    // We've definitely already been mounted at this point
                    forceUpdate()
                }),
                mounted: true,
                changedBeforeMount: false,
                cleanAt: Infinity
            }
            forceUpdate()
        }

        return () => {
            reactionTrackingRef.current!.reaction.dispose()
            reactionTrackingRef.current = null
        }
    }, [])

    // 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 exception
    reaction.track(() => {
        try {
            rendering = fn()
        } catch (e) {
            exception = e
        }
    })

    if (exception) {
        throw exception // re-throw any exceptions caught during rendering
    }

    return rendering
}