How to create a JavaScript callback for knowing when an image is loaded?

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太阳男子

I want to know when an image has finished loading. Is there a way to do it with a callback?

If not, is there a way to do it at all?

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甜味超标

2020-11-22 05:45

If you are using React.js, you could do this:

render() {

// ...

<img 
onLoad={() => this.onImgLoad({ item })}
onError={() => this.onImgLoad({ item })}

src={item.src} key={item.key}
ref={item.key} />

// ... }

Where:

- onLoad (...) now will called with something like this: { src: "https://......png", key:"1" } you can use this as "key" to know which images is loaded correctly and which not.
- onError(...) it is the same but for errors.
- the object "item" is something like this { key:"..", src:".."} you can use to store the images' URL and key in order to use in a list of images.

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太阳男子

2020-11-22 05:51

Image.onload() will often work.

To use it, you'll need to be sure to bind the event handler before you set the src attribute.

Related Links:

Mozilla on Image.onload()

Example Usage:

    window.onload = function () {

        var logo = document.getElementById('sologo');

        logo.onload = function () {
            alert ("The image has loaded!");		
        };

        setTimeout(function(){
            logo.src = 'https://edmullen.net/test/rc.jpg';         
        }, 5000);
    };

 <html>
    <head>
    <title>Image onload()</title>
    </head>
    <body>

    <img src="#" alt="This image is going to load" id="sologo"/>

    <script type="text/javascript">

    </script>
    </body>
    </html>

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一整个雨季

2020-11-22 05:52

Here is jQuery equivalent:

var $img = $('img');

if ($img.length > 0 && !$img.get(0).complete) {
   $img.on('load', triggerAction);
}

function triggerAction() {
   alert('img has been loaded');
}

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攒了一身酷

2020-11-22 05:57

Not suitable for 2008 when the question was asked, but these days this works well for me:

async function newImageSrc(src) {
  // Get a reference to the image in whatever way suits.
  let image = document.getElementById('image-id');

  // Update the source.
  img.src = src;

  // Wait for it to load.
  await new Promise((resolve) => { image.onload = resolve; });

  // Done!
  console.log('image loaded! do something...');
}

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