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()
     }