Fix #3747: computed values can get stale if backing observable was cr…

…eated and updated outside tracking context (#3748)

.changeset/witty-falcons-explode.md

@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Fixed: #3747, computed values becoming stale if the underlying observable was created and updated outside a reactive context

packages/mobx/__tests__/v5/base/observables.js

@@ -2382,6 +2382,7 @@ test("state version updates correctly", () => {
  o.x++
  })
 
+ // expect(o.x).toBe(4) is 1?
  prevStateVersion = getGlobalState().stateVersion
  o.x++
  expect(o.x).toBe(5)
@@ -2505,3 +2506,13 @@ test("state version does not update on observable creation", () => {
  check(() => mobx.observable([0], { proxy: true }))
  check(() => mobx.computed(() => 0))
 })
+
+test("#3747", () => {
+ mobx.runInAction(() => {
+ const o = observable.box(0)
+ const c = computed(() => o.get())
+ expect(c.get()).toBe(0)
+ o.set(1)
+ expect(c.get()).toBe(1) // would fail
+ })
+})

packages/mobx/src/core/atom.ts

@@ -68,21 +68,19 @@ export class Atom implements IAtom {
  * Invoke this method _after_ this method has changed to signal mobx that all its observers should invalidate.
  */
  public reportChanged() {
- if (globalState.inBatch && this.batchId_ === globalState.batchId) {
- // Called from the same batch this atom was created in.
- return
+ if (!globalState.inBatch || this.batchId_ !== globalState.batchId) {
+ // We could update state version only at the end of batch,
+ // but we would still have to switch some global flag here to signal a change.
+ globalState.stateVersion =
+ globalState.stateVersion < Number.MAX_SAFE_INTEGER
+ ? globalState.stateVersion + 1
+ : Number.MIN_SAFE_INTEGER
+ // Avoids the possibility of hitting the same globalState.batchId when it cycled through all integers (necessary?)
+ this.batchId_ = NaN
  }
- // Avoids the possibility of hitting the same globalState.batchId when it cycled through all integers (necessary?)
- this.batchId_ = NaN
 
  startBatch()
  propagateChanged(this)
- // We could update state version only at the end of batch,
- // but we would still have to switch some global flag here to signal a change.
- globalState.stateVersion =
- globalState.stateVersion < Number.MAX_SAFE_INTEGER
- ? globalState.stateVersion + 1
- : Number.MIN_SAFE_INTEGER
  endBatch()
  }