Improve useNearScreen implementation

Author: josepot

strategies/useNearScreen.js

@@ -1,27 +1,47 @@
 import {useState, useEffect} from 'react'
 
+const targetCallbacks = new WeakMap()
+const observerP =
+  typeof window !== 'undefined' &&
+  Promise.resolve(
+    typeof window.IntersectionObserver === 'undefined' &&
+      import('intersection-observer')
+  ).then(
+    () =>
+      new window.IntersectionObserver(entries => {
+        entries
+          .filter(e => e.isIntersecting)
+          .map(e => targetCallbacks.get(e.target))
+          .filter(Boolean)
+          .forEach(cb => cb())
+      })
+  )
+
+function whenIntersecting(target, cb) {
+  function unsubscribe() {
+    observerP.then(observer => {
+      observer.unobserve()
+      targetCallbacks.delete(target)
+    })
+  }
+
+  observerP.then(observer => {
+    observer.observe(target)
+    targetCallbacks.set(target, () => {
+      unsubscribe()
+      cb()
+    })
+  })
+
+  return unsubscribe
+}
+
 export const useNearScreen = ({ref}) => {
   const [show, setShow] = useState(false)
 
   useEffect(
-    function() {
-      if (!ref || !ref.current) return
-
-      Promise.resolve(
-        typeof window.IntersectionObserver !== 'undefined'
-          ? window.IntersectionObserver
-          : import('intersection-observer')
-      ).then(() => {
-        const observer = new window.IntersectionObserver(function(entries) {
-          const {isIntersecting} = entries[0]
-          if (isIntersecting) {
-            setShow(true)
-            observer.disconnect()
-          }
-        })
-        observer.observe(ref.current)
-      })
-    },
+    () =>
+      ref && ref.current && whenIntersecting(ref.current, () => setShow(true)),
     [ref]
   )