feat(useIntersectionObserver): allow lazy roots

package.json

@@ -57,6 +57,7 @@
     "@babel/cli": "^7.12.10",
     "@babel/core": "^7.12.10",
     "@babel/preset-typescript": "^7.12.7",
+    "@testing-library/react-hooks": "^7.0.0",
     "@types/enzyme": "^3.10.8",
     "@types/jest": "^26.0.19",
     "@types/lodash": "^4.14.167",

src/useIntersectionObserver.ts

@@ -9,11 +9,15 @@ import useEventCallback from './useEventCallback'
  * a DOM Element that returns it's entries as they arrive.
  *
  * @param element The DOM element to observe
- * @param init IntersectionObserver options
+ * @param init IntersectionObserver options with a notable change,
+ * unlike a plain IntersectionObserver `root: null` means "not provided YET",
+ * and the hook will wait until it receives a non-null value to set up the observer.
+ * This change allows for easier syncing of element and root values in a React
+ * context.
  */
 function useIntersectionObserver<TElement extends Element>(
   element: TElement | null | undefined,
-  options: IntersectionObserverInit,
+  options?: IntersectionObserverInit,
 ): IntersectionObserverEntry[]
 /**
  * Setup an [`IntersectionObserver`](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver) on
@@ -22,16 +26,21 @@ function useIntersectionObserver<TElement extends Element>(
  *
  * @param element The DOM element to observe
  * @param callback A listener for intersection updates.
- * @param init IntersectionObserver options
+ * @param init IntersectionObserver options with a notable change,
+ * unlike a plain IntersectionObserver `root: null` means "not provided YET",
+ * and the hook will wait until it receives a non-null value to set up the observer.
+ * This change allows for easier syncing of element and root values in a React
+ * context.
+ *
  */
 function useIntersectionObserver<TElement extends Element>(
   element: TElement | null | undefined,
   callback: IntersectionObserverCallback,
-  options: IntersectionObserverInit,
+  options?: IntersectionObserverInit,
 ): void
 function useIntersectionObserver<TElement extends Element>(
   element: TElement | null | undefined,
-  callbackOrOptions: IntersectionObserverCallback | IntersectionObserverInit,
+  callbackOrOptions?: IntersectionObserverCallback | IntersectionObserverInit,
   maybeOptions?: IntersectionObserverInit,
 ): void | IntersectionObserverEntry[] {
   let callback: IntersectionObserverCallback | undefined
@@ -47,8 +56,10 @@ function useIntersectionObserver<TElement extends Element>(
 
   const handler = useEventCallback(callback || setEntry)
 
+  // We wait for element to exist before constructing
   const observer = useStableMemo(
     () =>
+      root !== null &&
       typeof IntersectionObserver !== 'undefined' &&
       new IntersectionObserver(handler, {
         threshold,

test/useIntersectionObserver.test.tsx

@@ -0,0 +1,94 @@
+import useIntersectionObserver from '../src/useIntersectionObserver'
+import useCallbackRef from '../src/useCallbackRef'
+import React from 'react'
+import { renderHook, act } from '@testing-library/react-hooks'
+
+describe('useIntersectionObserver', () => {
+  let observers: any[] = []
+  beforeEach(() => {
+    ;(window as any).IntersectionObserver = class IntersectionObserverMock {
+      observe: jest.Mock<any, any>
+      unobserve: jest.Mock<any, any>
+      args: [IntersectionObserverCallback, IntersectionObserverEntryInit]
+      constructor(handler: any, init: any) {
+        this.args = [handler, init]
+        this.observe = jest.fn()
+        this.unobserve = jest.fn()
+        observers.push(this)
+      }
+    }
+  })
+
+  afterEach(() => {
+    observers = []
+  })
+
+  it('should observe element', async () => {
+    const element = document.createElement('span')
+
+    const { result } = renderHook(() => useIntersectionObserver(element))
+    const entry = {}
+    expect(result.current).toEqual([])
+
+    act(() => {
+      observers[0].args[0]([entry])
+    })
+
+    expect(result.current[0]).toStrictEqual(entry)
+  })
+
+  it('should wait for element', async () => {
+    const element = document.createElement('span')
+
+    const { result, rerender, unmount } = renderHook(
+      ({ element }) => useIntersectionObserver(element),
+      { initialProps: { element: null as any } },
+    )
+
+    expect(result.current).toEqual([])
+
+    expect(observers[0].observe).not.toBeCalled()
+
+    rerender({ element })
+
+    expect(observers[0].observe).toBeCalledTimes(1)
+
+    unmount()
+
+    expect(observers[0].unobserve).toBeCalledTimes(1)
+  })
+
+  it('should wait for root to set up observer', async () => {
+    const root = document.createElement('div')
+    const element = document.createElement('span')
+
+    const { result, rerender } = renderHook(
+      (root: any) => useIntersectionObserver(element, { root }),
+      { initialProps: null },
+    )
+
+    expect(observers).toHaveLength(0)
+
+    rerender(root)
+
+    expect(observers).toHaveLength(1)
+    expect(observers[0].observe).toBeCalledTimes(1)
+  })
+
+  it('should accept a callback', async () => {
+    const spy = jest.fn()
+    const element = document.createElement('span')
+
+    const { result } = renderHook(() => useIntersectionObserver(element, spy))
+
+    expect(result.current).toEqual(undefined)
+
+    const entry = {}
+    act(() => {
+      observers[0].args[0]([entry, observers[0]])
+    })
+
+    expect(spy).toBeCalledTimes(1)
+    expect(spy).toHaveBeenLastCalledWith([entry, observers[0]])
+  })
+})

yarn.lock

@@ -1093,6 +1093,13 @@
     core-js-pure "^3.0.0"
     regenerator-runtime "^0.13.4"
 
+"@babel/runtime@^7.12.5":
+  version "7.14.6"
+  resolved "https://registry.yarnpkg.com/@babel/runtime/-/runtime-7.14.6.tgz#535203bc0892efc7dec60bdc27b2ecf6e409062d"
+  integrity sha512-/PCB2uJ7oM44tz8YhC4Z/6PeOKXp4K588f+5M3clr1M4zbqztlo0XEfJ2LEzj/FgwfgGcIdl8n7YYjTCI0BYwg==
+  dependencies:
+    regenerator-runtime "^0.13.4"
+
 "@babel/runtime@^7.5.5", "@babel/runtime@^7.8.4":
   version "7.13.9"
   resolved "https://registry.yarnpkg.com/@babel/runtime/-/runtime-7.13.9.tgz#97dbe2116e2630c489f22e0656decd60aaa1fcee"
@@ -1472,6 +1479,17 @@
   dependencies:
     "@sinonjs/commons" "^1.7.0"
 
+"@testing-library/react-hooks@^7.0.0":
+  version "7.0.0"
+  resolved "https://registry.yarnpkg.com/@testing-library/react-hooks/-/react-hooks-7.0.0.tgz#dd6d37a7e018f147a3b9153137f10e013be8472b"
+  integrity sha512-WFBGH8DWdIGGBHt6PBtQPe2v4Kbj9vQ1sQ9qLBTmwn1PNggngint4MTE/IiWCYhPbyTW3oc/7X62DObMn/AjQQ==
+  dependencies:
+    "@babel/runtime" "^7.12.5"
+    "@types/react" ">=16.9.0"
+    "@types/react-dom" ">=16.9.0"
+    "@types/react-test-renderer" ">=16.9.0"
+    react-error-boundary "^3.1.0"
+
 "@tootallnate/once@1":
   version "1.1.2"
   resolved "https://registry.yarnpkg.com/@tootallnate/once/-/once-1.1.2.tgz#ccb91445360179a04e7fe6aff78c00ffc1eeaf82"
@@ -1637,6 +1655,20 @@
   resolved "https://registry.yarnpkg.com/@types/q/-/q-1.5.4.tgz#15925414e0ad2cd765bfef58842f7e26a7accb24"
   integrity sha512-1HcDas8SEj4z1Wc696tH56G8OlRaH/sqZOynNNB+HF0WOeXPaxTtbYzJY2oEfiUxjSKjhCKr+MvR7dCHcEelug==
 
+"@types/react-dom@>=16.9.0":
+  version "17.0.7"
+  resolved "https://registry.yarnpkg.com/@types/react-dom/-/react-dom-17.0.7.tgz#b8ee15ead9e5d6c2c858b44949fdf2ebe5212232"
+  integrity sha512-Wd5xvZRlccOrCTej8jZkoFZuZRKHzanDDv1xglI33oBNFMWrqOSzrvWFw7ngSiZjrpJAzPKFtX7JvuXpkNmQHA==
+  dependencies:
+    "@types/react" "*"
+
+"@types/react-test-renderer@>=16.9.0":
+  version "17.0.1"
+  resolved "https://registry.yarnpkg.com/@types/react-test-renderer/-/react-test-renderer-17.0.1.tgz#3120f7d1c157fba9df0118dae20cb0297ee0e06b"
+  integrity sha512-3Fi2O6Zzq/f3QR9dRnlnHso9bMl7weKCviFmfF6B4LS1Uat6Hkm15k0ZAQuDz+UBq6B3+g+NM6IT2nr5QgPzCw==
+  dependencies:
+    "@types/react" "*"
+
 "@types/react@*", "@types/react@^17.0.0":
   version "17.0.2"
   resolved "https://registry.yarnpkg.com/@types/react/-/react-17.0.2.tgz#3de24c4efef902dd9795a49c75f760cbe4f7a5a8"
@@ -1645,6 +1677,20 @@
     "@types/prop-types" "*"
     csstype "^3.0.2"
 
+"@types/react@>=16.9.0":
+  version "17.0.11"
+  resolved "https://registry.yarnpkg.com/@types/react/-/react-17.0.11.tgz#67fcd0ddbf5a0b083a0f94e926c7d63f3b836451"
+  integrity sha512-yFRQbD+whVonItSk7ZzP/L+gPTJVBkL/7shLEF+i9GC/1cV3JmUxEQz6+9ylhUpWSDuqo1N9qEvqS6vTj4USUA==
+  dependencies:
+    "@types/prop-types" "*"
+    "@types/scheduler" "*"
+    csstype "^3.0.2"
+
+"@types/scheduler@*":
+  version "0.16.1"
+  resolved "https://registry.yarnpkg.com/@types/scheduler/-/scheduler-0.16.1.tgz#18845205e86ff0038517aab7a18a62a6b9f71275"
+  integrity sha512-EaCxbanVeyxDRTQBkdLb3Bvl/HK7PBK6UJjsSixB0iHKoWxE5uu2Q/DgtpOhPIojN0Zl1whvOd7PoHs2P0s5eA==
+
 "@types/schema-utils@^2.4.0":
   version "2.4.0"
   resolved "https://registry.yarnpkg.com/@types/schema-utils/-/schema-utils-2.4.0.tgz#9983012045d541dcee053e685a27c9c87c840fcd"
@@ -7915,6 +7961,13 @@ react-dom@^16.13.0:
     prop-types "^15.6.2"
     scheduler "^0.19.1"
 
+react-error-boundary@^3.1.0:
+  version "3.1.3"
+  resolved "https://registry.yarnpkg.com/react-error-boundary/-/react-error-boundary-3.1.3.tgz#276bfa05de8ac17b863587c9e0647522c25e2a0b"
+  integrity sha512-A+F9HHy9fvt9t8SNDlonq01prnU8AmkjvGKV4kk8seB9kU3xMEO8J/PQlLVmoOIDODl5U2kufSBs4vrWIqhsAA==
+  dependencies:
+    "@babel/runtime" "^7.12.5"
+
 react-error-overlay@^6.0.9:
   version "6.0.9"
   resolved "https://registry.yarnpkg.com/react-error-overlay/-/react-error-overlay-6.0.9.tgz#3c743010c9359608c375ecd6bc76f35d93995b0a"