Add onLoadingComplete() prop to Image component (vercel#26824)

This adds a new prop, `onLoadingComplete()`, to handle the most common use case of `ref`.

I also added docs and a warning when using `ref` so we recommend the new prop instead.
 
- Fixes vercel#18398 
- Fixes vercel#22482

docs/api-reference/next/image.md

@@ -195,6 +195,10 @@ The image position when using `layout="fill"`.
 
 [Learn more](https://developer.mozilla.org/en-US/docs/Web/CSS/object-position)
 
+### onLoadingComplete
+
+A callback function that is invoked once the image is completely loaded and the placeholder has been removed.
+
 ### loading
 
 > **Attention**: This property is only meant for advanced usage. Switching an
@@ -242,6 +246,7 @@ Other properties on the `<Image />` component will be passed to the underlying
 - `srcSet`. Use
   [Device Sizes](/docs/basic-features/image-optimization.md#device-sizes)
   instead.
+- `ref`. Use [`onLoadingComplete`](#onloadingcomplete) instead.
 - `decoding`. It is always `"async"`.
 
 ## Related

packages/next/client/image.tsx

@@ -120,6 +120,7 @@ export type ImageProps = Omit<
   unoptimized?: boolean
   objectFit?: ImgElementStyle['objectFit']
   objectPosition?: ImgElementStyle['objectPosition']
+  onLoadingComplete?: () => void
 } & (StringImageProps | ObjectImageProps)
 
 const {
@@ -261,30 +262,37 @@ function defaultImageLoader(loaderProps: ImageLoaderProps) {
 
 // See https://stackoverflow.com/q/39777833/266535 for why we use this ref
 // handler instead of the img's onLoad attribute.
-function removePlaceholder(
+function handleLoading(
   img: HTMLImageElement | null,
-  placeholder: PlaceholderValue
+  placeholder: PlaceholderValue,
+  onLoadingComplete?: () => void
 ) {
-  if (placeholder === 'blur' && img) {
-    const handleLoad = () => {
-      if (!img.src.startsWith('data:')) {
-        const p = 'decode' in img ? img.decode() : Promise.resolve()
-        p.catch(() => {}).then(() => {
+  if (!img) {
+    return
+  }
+  const handleLoad = () => {
+    if (!img.src.startsWith('data:')) {
+      const p = 'decode' in img ? img.decode() : Promise.resolve()
+      p.catch(() => {}).then(() => {
+        if (placeholder === 'blur') {
           img.style.filter = 'none'
           img.style.backgroundSize = 'none'
           img.style.backgroundImage = 'none'
-        })
-      }
-    }
-    if (img.complete) {
-      // If the real image fails to load, this will still remove the placeholder.
-      // This is the desired behavior for now, and will be revisited when error
-      // handling is worked on for the image component itself.
-      handleLoad()
-    } else {
-      img.onload = handleLoad
+        }
+        if (onLoadingComplete) {
+          onLoadingComplete()
+        }
+      })
     }
   }
+  if (img.complete) {
+    // If the real image fails to load, this will still remove the placeholder.
+    // This is the desired behavior for now, and will be revisited when error
+    // handling is worked on for the image component itself.
+    handleLoad()
+  } else {
+    img.onload = handleLoad
+  }
 }
 
 export default function Image({
@@ -299,6 +307,7 @@ export default function Image({
   height,
   objectFit,
   objectPosition,
+  onLoadingComplete,
   loader = defaultImageLoader,
   placeholder = 'empty',
   blurDataURL,
@@ -401,6 +410,11 @@ export default function Image({
         )
       }
     }
+    if ('ref' in rest) {
+      console.warn(
+        `Image with src "${src}" is using unsupported "ref" property. Consider using the "onLoadingComplete" property instead.`
+      )
+    }
   }
   let isLazy =
     !priority && (loading === 'lazy' || typeof loading === 'undefined')
@@ -589,9 +603,9 @@ export default function Image({
         {...imgAttributes}
         decoding="async"
         className={className}
-        ref={(element) => {
-          setRef(element)
-          removePlaceholder(element, placeholder)
+        ref={(img) => {
+          setRef(img)
+          handleLoading(img, placeholder, onLoadingComplete)
         }}
         style={imgStyle}
       />

test/integration/image-component/default/pages/on-loading-complete.js

@@ -0,0 +1,32 @@
+import { useState } from 'react'
+import Image from 'next/image'
+
+const Page = () => (
+  <div>
+    <h1>On Loading Complete Test</h1>
+    <ImageWithMessage id="1" src="/test.jpg" />
+    <ImageWithMessage
+      id="2"
+      src={require('../public/test.png')}
+      placeholder="blur"
+    />
+  </div>
+)
+
+function ImageWithMessage({ id, src }) {
+  const [msg, setMsg] = useState('[LOADING]')
+  return (
+    <>
+      <Image
+        id={`img${id}`}
+        src={src}
+        width="400"
+        height="400"
+        onLoadingComplete={() => setMsg(`loaded img${id}`)}
+      />
+      <p id={`msg${id}`}>{msg}</p>
+    </>
+  )
+}
+
+export default Page

test/integration/image-component/default/test/index.test.js

@@ -182,6 +182,35 @@ function runTests(mode) {
     }
   })
 
+  it('should callback onLoadingComplete when image is fully loaded', async () => {
+    let browser
+    try {
+      browser = await webdriver(appPort, '/on-loading-complete')
+
+      await check(
+        () => browser.eval(`document.getElementById("img1").src`),
+        /test(.*)jpg/
+      )
+
+      await check(
+        () => browser.eval(`document.getElementById("img2").src`),
+        /test(.*).png/
+      )
+      await check(
+        () => browser.eval(`document.getElementById("msg1").textContent`),
+        'loaded img1'
+      )
+      await check(
+        () => browser.eval(`document.getElementById("msg2").textContent`),
+        'loaded img2'
+      )
+    } finally {
+      if (browser) {
+        await browser.close()
+      }
+    }
+  })
+
   it('should work when using flexbox', async () => {
     let browser
     try {