Use WeakRef for observers

[bjb568]

This PR changes the observers of an observable to use a weak reference, which allows computed values with the keepAlive option to be garbage collected.

In the following code, without this PR, memory usage grows without bound. With this PR, buffer is able to be disposed.

import { observable, computed } from "mobx";

const box = observable.box(1);

setInterval(() => {
  const megabytes = performance.memory.usedJSHeapSize / 1e6;
  console.log(`${megabytes.toFixed(2)} MB used.`);

  const buffer = observable.box(new ArrayBuffer(100 * 1e6));
  computed(() => buffer.get().byteLength * box.get(), {
    keepAlive: true,
  }).get();
}, 1000);

With the following code, since it is creating a reaction, it shouldn't be disposed. This is accomplished by having a globalState.activeReactions set to prevent garbage collection. Both with and without this PR, memory will grow without bound (autoruns still need to be disposed of).

import { observable, computed } from "mobx";

const box = observable.box(1);

setInterval(() => {
  const megabytes = performance.memory.usedJSHeapSize / 1e6;
  console.log(`${megabytes.toFixed(2)} MB used.`);

  const buffer = observable.box(new ArrayBuffer(100 * 1e6));
  autorun(() => buffer.get().byteLength * box.get());
}, 1000);

Since this PR deals with garbage collection, it's difficult to write unit tests verifying this behavior, so the verification was done in a browser.

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

[changeset-bot]

🦋 Changeset detected

Latest commit: b61de45

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mobx	Minor

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