The load event triggered by an HTML element does not bubble up to the window object by default. Therefore, when an image is loaded using an img HTML element, if you need to notify the window object, you must manually dispatch the event to it.
An example:
HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<img id="image" src="https://placehold.co/100x100" alt="Placeholder Image" />
<script src="src/script.js"></script>
</body>
</html>
JS
document.getElementById('image').addEventListener('load', function() {
console.log('Image load event');
window.dispatchEvent(new Event('load')); // Manually dispatch load event to window
});
window.addEventListener('load', function() {
console.log('Window load event');
});
Console output: the second log 'Window Load Event' is triggered by calling dispatchEvent
Image load event
Window load event
Window load event
Examples of Updating Text Based on Image Loading Status in React
Example 1: Using HTML 'complete' Property
Simulating an API call to fetch an image URL and passing it to the Banner component triggers a re-render when imgUrl changes, allowing the correct value of completed to be captured at the second render stage.
import { useRef , useState , useEffect } from "react";
export default function App() {
const [imgUrl, setImgUrl] = useState("");
useEffect(() => {
// simulate calling API to get image url
const loadImage = () => {
setTimeout(() => {
setImgUrl("https://placehold.co/200x100")
}, 1500);
}
loadImage();
}, []);
return <Banner imgUrl={imgUrl} />
}
function Banner({ imgUrl }) {
const ref = useRef(null);
// this works when the imgUrl changes, triggering a re-render. Otherwise, the value of completed will always be false
const completed = Boolean(ref.current?.complete);
return (
<div>
<img ref={ref} src={imgUrl}
onLoad={() => console.log("loaded")}
onError={() => console.log("error")}/>
<p>{completed ? "Image loaded" : "Image is not loaded"}</p>
</div>
);
}
Example 2: Using onLoad Event
Update the component state in the onLoad event callback
function StaticBanner() {
const [loaded, setLloaded] = useState(false);
return (
<div>
<img src="https://placehold.co/100x100"
onLoad={() => {
console.log("StaticBanner img loaded");
setLloaded(true);
}}
onError={() => console.log("StaticBanner img error")}/>
<p>{loaded ? "Image loaded" : "Image is not loaded"}</p>
</div>
);
}
What if a Banner lives in the DOM and does not unmount, but the img url that is passed into the component as a prop that changes constantly?
To be continued...
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