feat: Keep track updatedAt as part of the SWR state

src/types.ts

@@ -156,13 +156,15 @@ export type Broadcaster<Data = any, Error = any> = (
   error?: Error,
   isValidating?: boolean,
   revalidate?: boolean,
-  populateCache?: boolean
+  populateCache?: boolean,
+  updatedAt?: number
 ) => Promise<Data>
 
 export type State<Data, Error> = {
   data?: Data
   error?: Error
   isValidating?: boolean
+  updatedAt?: number
 }
 
 export type Mutator<Data = any> = (
@@ -204,6 +206,7 @@ export interface SWRResponse<Data = any, Error = any> {
   data?: Data
   error?: Error
   mutate: KeyedMutator<Data>
+  updatedAt?: number
   isValidating: boolean
 }
 
@@ -237,7 +240,8 @@ export type RevalidateCallback = <K extends RevalidateEvent>(
 export type StateUpdateCallback<Data = any, Error = any> = (
   data?: Data,
   error?: Error,
-  isValidating?: boolean
+  isValidating?: boolean,
+  updatedAt?: number
 ) => void
 
 export interface Cache<Data = any> {

src/use-swr.ts

@@ -113,11 +113,16 @@ export const useSWRHandler = <Data = any, Error = any>(
   }
   const isValidating = resolveValidating()
 
+  const updatedAtRef = useRef<number | undefined>()
+  const getUpdatedAt = () => updatedAtRef.current
+  const updatedAt = getUpdatedAt()
+
   const [stateRef, stateDependencies, setState] = useStateWithDeps<Data, Error>(
     {
       data,
       error,
-      isValidating
+      isValidating,
+      updatedAt
     },
     unmountedRef
   )
@@ -210,6 +215,9 @@ export const useSWRHandler = <Data = any, Error = any>(
         newData = await newData
 
         if (shouldStartNewRequest) {
+          const now = Date.now()
+          updatedAtRef.current = now
+          newState.updatedAt = now
           // If the request isn't interrupted, clean it up after the
           // deduplication interval.
           setTimeout(cleanupState, config.dedupingInterval)
@@ -328,7 +336,16 @@ export const useSWRHandler = <Data = any, Error = any>(
       // Here is the source of the request, need to tell all other hooks to
       // update their states.
       if (isCurrentKeyMounted() && shouldStartNewRequest) {
-        broadcastState(cache, key, newState.data, newState.error, false)
+        broadcastState(
+          cache,
+          key,
+          newState.data,
+          newState.error,
+          false,
+          false,
+          undefined,
+          newState.updatedAt
+        )
       }
 
       return true
@@ -376,13 +393,19 @@ export const useSWRHandler = <Data = any, Error = any>(
     const onStateUpdate: StateUpdateCallback<Data, Error> = (
       updatedData,
       updatedError,
-      updatedIsValidating
+      updatedIsValidating,
+      nextUpdatedAt
     ) => {
+      updatedAtRef.current = isUndefined(nextUpdatedAt)
+        ? getUpdatedAt()
+        : nextUpdatedAt
+
       setState(
         mergeObjects(
           {
             error: updatedError,
-            isValidating: updatedIsValidating
+            isValidating: updatedIsValidating,
+            updatedAt: getUpdatedAt()
           },
           // Since `setState` only shallowly compares states, we do a deep
           // comparison here.
@@ -433,7 +456,8 @@ export const useSWRHandler = <Data = any, Error = any>(
       setState({
         data,
         error,
-        isValidating
+        isValidating,
+        updatedAt: UNDEFINED
       })
     }
 
@@ -526,6 +550,10 @@ export const useSWRHandler = <Data = any, Error = any>(
     get isValidating() {
       stateDependencies.isValidating = true
       return isValidating
+    },
+    get updatedAt() {
+      stateDependencies.updatedAt = true
+      return getUpdatedAt()
     }
   } as SWRResponse<Data, Error>
 }

src/utils/broadcast-state.ts

@@ -9,7 +9,8 @@ export const broadcastState: Broadcaster = (
   error,
   isValidating,
   revalidate,
-  broadcast = true
+  broadcast = true,
+  updatedAt
 ) => {
   const [EVENT_REVALIDATORS, STATE_UPDATERS, , FETCH] = SWRGlobalState.get(
     cache
@@ -20,7 +21,7 @@ export const broadcastState: Broadcaster = (
   // Cache was populated, update states of all hooks.
   if (broadcast && updaters) {
     for (let i = 0; i < updaters.length; ++i) {
-      updaters[i](data, error, isValidating)
+      updaters[i](data, error, isValidating, updatedAt)
     }
   }
 

src/utils/state.ts

@@ -23,7 +23,8 @@ export const useStateWithDeps = <Data, Error, S = State<Data, Error>>(
   const stateDependenciesRef = useRef<StateDeps>({
     data: false,
     error: false,
-    isValidating: false
+    isValidating: false,
+    updatedAt: false
   })
 
   /**

test/use-swr-loading.test.tsx

@@ -138,7 +138,7 @@ describe('useSWR - loading', () => {
     }
 
     renderWithConfig(<Page />)
-    screen.getByText('data,error,isValidating,mutate')
+    screen.getByText('data,error,isValidating,mutate,updatedAt')
   })
 
   it('should sync loading states', async () => {