Recheck a variable's value in commit phase

Author: Jenn Creighton

src/react/hooks/__tests__/useReactiveVar.test.tsx

@@ -169,4 +169,72 @@ describe("useReactiveVar Hook", () => {
       console.error = error;
     }).then(resolve, reject);
   });
+
+  describe("useEffect", () => {
+    itAsync("works if updated higher in the component tree", async (resolve, reject) => {
+      const counterVar = makeVar(0);
+
+      function ComponentOne() {
+        const count = useReactiveVar(counterVar);
+
+        useEffect(() => {
+          counterVar(1);
+        }, []);
+
+        return (<div>{count}</div>);
+      }
+
+      function ComponentTwo() {
+        const count = useReactiveVar(counterVar);
+
+        return (<div>{count}</div>);
+      }
+
+      const { getAllByText } = render(
+        <>
+          <ComponentOne />
+          <ComponentTwo />
+        </>
+      );
+
+      await wait(() => {
+        expect(getAllByText("1")).toHaveLength(2);
+      });
+
+      resolve();
+    });
+
+    itAsync("works if updated lower in the component tree", async (resolve, reject) => {
+      const counterVar = makeVar(0);
+
+      function ComponentOne() {
+        const count = useReactiveVar(counterVar);
+
+        return (<div>{count}</div>);
+      }
+
+      function ComponentTwo() {
+        const count = useReactiveVar(counterVar);
+
+        useEffect(() => {
+          counterVar(1);
+        }, []);
+
+        return (<div>{count}</div>);
+      }
+
+      const { getAllByText } = render(
+        <>
+          <ComponentOne />
+          <ComponentTwo />
+        </>
+      );
+
+      await wait(() => {
+        expect(getAllByText("1")).toHaveLength(2);
+      });
+
+      resolve();
+    });
+  });
 });

src/react/hooks/useReactiveVar.ts

@@ -18,5 +18,15 @@ export function useReactiveVar<T>(rv: ReactiveVar<T>): T {
   //   const mute = rv.onNextChange(setValue);
   //   return () => mute();
   // }, [value])
+
+  // We check the variable's value in this useEffect and schedule an update if
+  // the value has changed. This check occurs once, on the initial render, to avoid
+  // a useEffect higher in the component tree changing a variable's value
+  // before the above useEffect can set the onNextChange handler. Note that React
+  // will not schedule an update if setState is called with the same value as before.
+  useEffect(() => {
+    setValue(rv())
+  }, []);
+
   return value;
 }