feat: refreshInterval as a function

src/types.ts

@@ -35,7 +35,7 @@ export interface PublicConfiguration<
   loadingTimeout: number
   focusThrottleInterval: number
   dedupingInterval: number
-  refreshInterval?: number
+  refreshInterval?: number | ((latestData: Data | undefined) => number)
   refreshWhenHidden?: boolean
   refreshWhenOffline?: boolean
   revalidateOnFocus: boolean

src/use-swr.ts

@@ -3,7 +3,13 @@ import { defaultConfig } from './utils/config'
 import { SWRGlobalState, GlobalState } from './utils/global-state'
 import { IS_SERVER, rAF, useIsomorphicLayoutEffect } from './utils/env'
 import { serialize } from './utils/serialize'
-import { isUndefined, UNDEFINED, OBJECT, mergeObjects } from './utils/helper'
+import {
+  isUndefined,
+  UNDEFINED,
+  OBJECT,
+  mergeObjects,
+  isFunction
+} from './utils/helper'
 import ConfigProvider from './utils/config-context'
 import { useStateWithDeps } from './utils/state'
 import { withArgs } from './utils/resolve-args'
@@ -440,11 +446,17 @@ export const useSWRHandler = <Data = any, Error = any>(
     let timer: any
 
     function next() {
+      // Use the passed interval
+      // ...or invoke the function with the updated data to get the interval
+      const interval = isFunction(refreshInterval)
+        ? refreshInterval(data)
+        : refreshInterval
+
       // We only start next interval if `refreshInterval` is not 0, and:
       // - `force` is true, which is the start of polling
       // - or `timer` is not 0, which means the effect wasn't canceled
-      if (refreshInterval && timer !== -1) {
-        timer = setTimeout(execute, refreshInterval)
+      if (interval && timer !== -1) {
+        timer = setTimeout(execute, interval)
       }
     }
 

test/use-swr-refresh.test.tsx

@@ -374,4 +374,64 @@ describe('useSWR - refresh', () => {
     expect(fetcherWithToken).toBeCalledTimes(5)
     expect(fetcherWithToken).toHaveBeenLastCalledWith(`1-${key}`)
   })
+
+  it('should allow using function as an interval', async () => {
+    let count = 0
+
+    const key = createKey()
+    function Page() {
+      const { data } = useSWR(key, () => count++, {
+        refreshInterval: () => 200,
+        dedupingInterval: 100
+      })
+      return <div>count: {data}</div>
+    }
+
+    renderWithConfig(<Page />)
+
+    // hydration
+    screen.getByText('count:')
+
+    // mount
+    await screen.findByText('count: 0')
+
+    await act(() => advanceTimers(200)) // update
+    screen.getByText('count: 1')
+    await act(() => advanceTimers(50)) // no update
+    screen.getByText('count: 1')
+    await act(() => advanceTimers(150)) // update
+    screen.getByText('count: 2')
+  })
+
+  it('should pass updated data to refreshInterval', async () => {
+    let count = 1
+
+    const key = createKey()
+    function Page() {
+      const { data } = useSWR(key, () => count++, {
+        refreshInterval: updatedCount => updatedCount * 1000,
+        dedupingInterval: 100
+      })
+      return <div>count: {data}</div>
+    }
+
+    renderWithConfig(<Page />)
+
+    // hydration
+    screen.getByText('count:')
+
+    // mount
+    await screen.findByText('count: 1')
+
+    await act(() => advanceTimers(1000)) // updated after 1s
+    screen.getByText('count: 2')
+    await act(() => advanceTimers(1000)) // no update
+    screen.getByText('count: 2')
+    await act(() => advanceTimers(1000)) // updated after 2s
+    screen.getByText('count: 3')
+    await act(() => advanceTimers(2000)) // no update
+    screen.getByText('count: 3')
+    await act(() => advanceTimers(1000)) // updated after 3s
+    screen.getByText('count: 4')
+  })
 })