fix timing issue in component updates due to consecutive dispatches

Author: MrWolfZ

src/components/connectAdvanced.js

@@ -253,6 +253,7 @@ export default function connectAdvanced(
       const lastChildProps = useRef()
       const lastWrapperProps = useRef(wrapperProps)
       const childPropsFromStoreUpdate = useRef()
+      const renderIsScheduled = useRef(false)
 
       const actualChildProps = usePureOnlyMemo(() => {
         // Tricky logic here:
@@ -282,6 +283,7 @@ export default function connectAdvanced(
         // We want to capture the wrapper props and child props we used for later comparisons
         lastWrapperProps.current = wrapperProps
         lastChildProps.current = actualChildProps
+        renderIsScheduled.current = false
 
         // If the render was from a store update, clear out that reference and cascade the subscriber update
         if (childPropsFromStoreUpdate.current) {
@@ -328,14 +330,17 @@ export default function connectAdvanced(
 
           // If the child props haven't changed, nothing to do here - cascade the subscription update
           if (newChildProps === lastChildProps.current) {
-            notifyNestedSubs()
+            if (!renderIsScheduled.current) {
+              notifyNestedSubs()
+            }
           } else {
             // Save references to the new child props.  Note that we track the "child props from store update"
             // as a ref instead of a useState/useReducer because we need a way to determine if that value has
             // been processed.  If this went into useState/useReducer, we couldn't clear out the value without
             // forcing another re-render, which we don't want.
             lastChildProps.current = newChildProps
             childPropsFromStoreUpdate.current = newChildProps
+            renderIsScheduled.current = true
 
             // If the child props _did_ change (or we caught an error), this wrapper component needs to re-render
             forceComponentUpdateDispatch({

test/components/connect.spec.js

@@ -3182,6 +3182,54 @@ describe('React', () => {
 
         expect(reduxCountPassedToMapState).toEqual(3)
       })
+
+      it('should ensure top-down updates for consecutive batched updates', () => {
+        const INC = 'INC'
+        const reducer = (c = 0, { type }) => (type === INC ? c + 1 : c)
+        const store = createStore(reducer)
+
+        let executionOrder = []
+        let expectedExecutionOrder = [
+          'parent map',
+          'parent render',
+          'child map',
+          'child render'
+        ]
+
+        const ChildImpl = () => {
+          executionOrder.push('child render')
+          return <div>child</div>
+        }
+
+        const Child = connect(state => {
+          executionOrder.push('child map')
+          return { state }
+        })(ChildImpl)
+
+        const ParentImpl = () => {
+          executionOrder.push('parent render')
+          return <Child />
+        }
+
+        const Parent = connect(state => {
+          executionOrder.push('parent map')
+          return { state }
+        })(ParentImpl)
+
+        rtl.render(
+          <ProviderMock store={store}>
+            <Parent />
+          </ProviderMock>
+        )
+
+        executionOrder = []
+        rtl.act(() => {
+          store.dispatch({ type: INC })
+          store.dispatch({ type: '' })
+        })
+
+        expect(executionOrder).toEqual(expectedExecutionOrder)
+      })
     })
 
     it("should enforce top-down updates to ensure a deleted child's mapState doesn't throw errors", () => {