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React downloads pending images of previous render

(Update: previously, I thought this problem was caused by React Router, but I have stripped React Router out of the code, and the problem still persists. So I have modified this question thoroughly.)
Situation:
I have paginated pages which show a list of images per page. (With "page" I mean the complete content that is visible, I don't mean a separate html file/url.) I would like to nagivate through those pages in an efficient manner.
Problem:
If I navigate quickly enough through the pages, not all images will be loaded in the current page before navigating to the next page. I would expect the browser to cancel all pending unfinished image downloads when navigating to the next page. But this doesn't happen, the browser keeps all the unfinished images pending to be downloaded, until they are all downloaded. Then the images of the page to which I navigated, will be downloaded. This causes a big delay and wasted bandwidth.
Question:
Is it possible to cancel the downloading of "pending" images of the previous page?
Demonstration code:
To test this, use the "network" tab in the developer tool of you browser. Also choose "disable cache" and set the throttle (download speed in developer tool) to something slow like "Slow 3G" otherwise the images will be loaded to fast too see the problem. Then navigate through the pages and see that the list of pending images is stacking up, then click the "disable images" button. Then, no image will be visible on the screen, but the browser still has a large number of pending image downloads open, which is wasted bandwith and causes a delay when new images need to be rendered.
You can test the code here: https://codepen.io/Devabc/pen/PowjqwZ
//This code is using imgur images to demonstrate.
class Page extends React.Component {
state = {
pageNr: 1,
imagesEnabled: true
};
onLinkClick = event => {
const number = event.target.dataset.value;
console.log("Number: " + number);
this.setState({pageNr: number});
};
onButtonClick = event => {
console.log("toggling images");
this.setState(prevState => {
return {imagesEnabled: ! prevState.imagesEnabled};
});
};
render() {
const links = _.range(0, 5).map(number => {
return (
<a href="#" data-value={number} onClick={this.onLinkClick}>
{number}
</a>
);
});
const pageNr = this.state.pageNr;
const imgStart = pageNr * 100;
const imgEnd = imgStart + 100;
const images = this.state.imagesEnabled
? <ImagesPanel imgStart={imgStart} imgEnd={imgEnd} />
: null;
return (
<div>
<h1>Page {this.state.pageNr}</h1>
<div>Page links: {links}</div>
<div><button onClick={this.onButtonClick}>{this.state.imagesEnabled ? "disable images" : "enable images"}</button></div>
<div>Images:</div>
<div>{images}</div>
</div>
);
}
}
function ImagesPanel(props) {
const images = _.range(props.imgStart, props.imgEnd).map(number => {
return (
<Image imageNr={number} />
);
});
return images;
}
function imageUrl(imageNr) {
const hash = imgurHashes[imageNr];
return "https://i.imgur.com/" + hash + ".jpg";
}
function Image(props) {
const url = imageUrl(props.imageNr);
return <img class="myImage" src={url} border='1' />
}
const imgurHashes = 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];
ReactDOM.render(
<Page />,
document.getElementById('root')
);
The screenshot below shows the list of pending images in the browser, even though no images should be rendered at this moment, because the React component that should render the images is already unmounted/removed.

The problem is not caused by React. I'm not sure who to blame for this problem, it could be the Document Object Model standard, HTML or the browsers implementing it. The problem is that once an image is attached to the DOM (when the browser starts to render a HTML page), the browsers will start to download it, even when the image is later removed from the DOM.
There are however a number of solutions to solve this problem.
Solution 1: src attribute modification to cancel downloads
The src attribute is mandatory in the img element of HTML, but setting it to an empty string (clearing), results in cancellation of the download of the image. (This may not work on all browsers.)
This solution is described here: How to cancel an image from loading and demonstrated here https://jsbin.com/zakeqaseru/1/edit?output and here https://jsfiddle.net/nw34gLgt/
(Use the browser developer tools to monitor whether the image download is cancelled.)
The src can be changed to the empty string using setAttribute(). Setting it to null has the same effect as setting it to the string "null", which would also work, but is less clean.
Removing the src attribute doesn't work on most browsers.
Whether this solution works may differ per browser. Older browsers may cause the web page itself to be requested when src is removed, as mentioned here: https://gtmetrix.com/avoid-empty-src-or-href.html
This could be a very big performance hit on both the brower and the web server.
In case the browser cannot deal with this, a similar solution would be to use the background-image property on a HTML element like a div instead of an img.
This src-clearing solution is implemented in React-Image:
https://github.com/mbrevda/react-image/pull/223
this.i.src = ''
try {
delete this.i.src
} catch (e) {
// On Safari in Strict mode this will throw an exception,
// - https://github.com/mbrevda/react-image/issues/187
// We don't need to do anything about it.
}
delete this.i
A solution in React would be something like this:
class Img extends React.Component {
constructor(props) {
super(props);
this.imgRef = React.createRef();
}
componentWillUnmount() {
try {
this.imgRef.current.src = '';
delete this.imgRef.current.src;
} catch (e) {}
}
render() {
return (
<img {...this.props} ref={this.imgRef} />
);
}
}
Then you can use it like this:
<Img src="...some image url...." />
Solution 2: lazy loading src
By setting the src attribute under a programmatic condition, you can make sure that images are only loaded when needed. This solution can also be combined with other solutions to make sure that images that are downloaded, are cancelled when the associated React elements are unmounted.
The data-src (custom data) attribute is often used in this approach, to copy its value to src when an image needs to be rendered.
This solution may still download unmounted images.
Solution 3: Chrome's native lazy-loading
This solution exists since Chrome 76 (July 2019?). Chrome has a loading attribute for images and iframes, which can be set to "lazy" to defer downloading of the resource until it reaches a calculated distance from the viewport.
The loading attribute isn't supported by many other browser.
For more info, see:
https://web.dev/native-lazy-loading/
https://github.com/scott-little/lazyload/
Lazy-loading request for Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1542784
This solution may still download unmounted images.
Additional information sources
https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video
https://imagekit.io/blog/lazy-loading-images-complete-guide/

Angular 4 refresh <img src={{ ... }} > when the value changes

I am using Angular 4 and I have a component where I want to change my user current photo.. so the html code that displays the current user photo is this..
<div class="profilphoto">
<img src= {{currentphoto}}>
</div>
currentphoto contains the current user photo url and I get it from firebase..
After that I display a Gallery of photos so the user can select one and change his profil photo using a form.. the submit code is the following and works fine
onSubmit = function(form) {
this.photoUrl = form.photoUrl;
firebase.database().ref("users/"+this.authService.cusername+"/photo").update({
url: form.photoUrl
}).then(()=>{
this.currentphoto = this.authService.photourl;
console.log("currentphoto:"+this.currentphoto);
}
);
}
Everything works fine except that despite the currentphoto value has changed and database url upadated the html still displays the previous user's image and its annoying(if I refresh the page it shows the new profil image).. Is there any way I can make
<div class="profilphoto">
<img src= {{currentphoto}}>
</div>
detect when currentphoto changes value and Display the image with the new src value??

Try calling the changedetection manually,
constructor(private cdRef: ChangeDetectorRef){
}
Once you set the image
this.currentphoto = this.authService.photourl;
this.cdRef.detectChanges();

This is probably not relevant anymore but in case it could help people get this solved faster, here it is.
If you want to update the image after it has changed write your src like this:
src='{{currentphoto}}?d={{clock_tick}}'
Initialize your clock tick when you first load your component (I use Date.now()) and again after you update the image. This will tell your browser that you updated the image and it will reload it for you.
It worked for me.

Without the entire code of your component/service is hard to know what is the problem, but the best shot is in this line:
onSubmit = function(form) {
probably it's a problem with the 'this' value.
Insert a console.log(this) below this line (inside the function) and check if 'this' is a reference to the component instance or to the window.
Try to use arrow function:
onSubmit = form => {
//do what you need here, call the service, etc...
//and then set the this.currentphoto
}

Have you tried this:
<div class="profilphoto">
<img [src]="currentphoto" >
</div>

check if multiple images loaded within certain timeinterval; if not then replace with other url

Problem- I am displaying some images on a page which are being served by some proxy server. In each page I am displaying 30 images ( 6 rows - 5 in each row). Here if due to overload or due to any other issue if proxy server could not able to server images( either all images or some of them) in 6 seconds then I want to replace unloaded image url with some other url using javascript so that I could display 30 images at the end.
What I tried is below.
objImg = new Image();
objImg.src = 'http://www.menucool.com/slider/prod/image-slider-4.jpg';
if(!objImg.complete)
{
alert('image not loaded');
}else{
img.src = 'http://www.menucool.com/slider/prod/image-slider-4.jpg';
}
I also tried with below code.
$('img[id^="picThumbImg_"]').each(function(){
if($(this).load()) {
//it will display loaded image id's to console
window.console.log($(this).attr('id'));
}
});
I could not use set time-out for each image because it will delay all page load.
I checked other similar question on stack-overflow but no solution worked me perfectly as I need to display multiple images.Please guide how to proceed.

You don't have to wait 6 seconds, or using TimeOut. You can check if the images are loaded or not using the onload Javascript/Jquery event. I know, it will take a little bit to dispatch the onerror event, let see:
Why don't use the load Jquery event on the window or the image itself?
$(window).load(function(){
//check each image
})
Disadvantage:
It will wait for other resources like scripts, stylesheets & flash, and not just images, which may or may not be OK to you.
If the image loads from the cache, some browsers may not fire off the event (included yours and that's why your code is not working)
Why don't use the error Jquery event on the image itself?
$('img[id^="picThumbImg_"]').error(function(){
//image loading error
})
Disadvantages:
It doesn't work consistently nor reliably cross-browser
It doesn't fire correctly in WebKit if the image src is set to the same src as before
It doesn't correctly bubble up the DOM tree
Can cease to fire for images that already live in the browser's cache
Note:: Error is almost the same that the load event
Improving the code!:
$('img[id^="picThumbImg_"]').one('error', function() {
// image load error
}).each(function() {
if(!this.complete) $(this).error();
});
This will avoid few things of the previous code, but you still will have to wait if it's a 404 and you're replacing it in the onerror event, that will take a little bit right?
So, what now!
You can use this awesome plugin!. Once you add the reference, you just have to use something like:
var imgLoad = imagesLoaded('#img-container');
imgLoad.on( 'always', function() {
// detect which image is broken
for ( var i = 0, len = imgLoad.images.length; i < len; i++ ) {
if(!imgLoad.images[i].isLoaded){
//changing the src
imgLoad.images[i].img.src = imgLoad.images[i].img.getAttribute("data-src2");
}
}
});
Your HTML markup should look like:
<div id="img-container">
<div class="row">
...
</div>
<div class="row">
<img src="original-path.jpg" data-src2="alternative-path.jpg">
...
</div>
<div class="row">
...
</div>
</div>
Note: You don't need jQuery in this case and this plugin is suggested by Paul Irish ;)

Give all your images a specific class. Loop through your images and use .load() to check if loaded, example below...
Detect image load

Dynamically created image doesn't show when <img>'s src is refreshed to point to it

I have a javascript function
generateGraph() {
...
jQuery.get('chart.php?month='+month+'&year='+year);
document.getElementById('graph').src = 'charts/'+month+'_'+year+'.png';
...
}
which is called onChange. It basically creates, if not exists, or refreshes if already exists, a PNG image (using the first line above) and supposedly displays the generated image (using the second line above).
The first line works, but the second one doesn't. Meaning, the image is generated, but the second line can't display the image, unless the page is reloaded, or if the image being referred to already existed before the page was loaded (meaning the 1st line just "refreshed" the image)
Any ideas as to why this is and how to fix it?
Thanks much for your help in advance!

Try this:
generateGraph() {
...
jQuery.get('chart.php?month='+month+'&year='+year, function() {
document.getElementById('graph').src = 'charts/'+month+'_'+year+'.png';
// as mentioned by others this also works and is in the jquery style:
// $("#graph").attr("src", "charts/" + month + "_" + year + ".png");
});
...
}
If I understand everything correctly, a call to chart.php generates an image at charts/month_year.png. If this is correct, your original code sent a request for the image immediately after sending a request to generate that image. This doesn't give enough time to the server to create and post the image to charts/month_year.png, which is why nothing appeared.

Update the src within $.get's success callback, to ensure that it happens upon completion of the request:
generateGraph() {
...
jQuery.get('chart.php?month='+month+'&year='+year, function() {
jQuery('#graph').attr("src",'charts/'+month+'_'+year+'.png');
});
...
}

It looks like your image generation process takes nonzero time, and assigning src right after jQuery call just does not fit in time to get the image to be ready, and after you reload the page, the image is ready for sure. So add some AJAX verification for the image creation readiness.

You can solve this problem by creating an image that is never displayed. That will download the image file. Then, when this image is loaded, swap out the one on your page with the downloaded image:
var newImage = new Image();
newImage.onload = function(){
// Update existing image src on your page
}
}
newImage.src = "URL to your image";

How to show a spinner while loading an image via JavaScript

I'm currently working on a web application which has a page which displays a single chart (a .png image). On another part of this page there are a set of links which, when clicked, the entire page reloads and looks exactly the same as before except for the chart in the middle of the page.
What I want to do is when a link is clicked on a page just the chart on the page is changed. This will speed things up tremendously as the page is roughly 100kb large, and don't really want to reload the entire page just to display this.
I've been doing this via JavaScript, which works so far, using the following code
document.getElementById('chart').src = '/charts/10.png';
The problem is that when the user clicks on the link, it may take a couple of seconds before the chart changes. This makes the user think that their click hasn't done anything, or that the system is slow to respond.
What I want to happen is display a spinner / throbber / status indicator, in place of where the image is while it is loading, so when the user clicks the link they know at least the system has taken their input and is doing something about it.
I've tried a few suggestions, even using a psudo time out to show a spinner, and then flick back to the image.
A good suggestion I've had is to use the following
<img src="/charts/10.png" lowsrc="/spinner.gif"/>
Which would be ideal, except the spinner is significantly smaller than the chart which is being displayed.
Any other ideas?

I've used something like this to preload an image and then automatically call back to my javascript when the image is finished loading. You want to check complete before you setup the callback because the image may already be cached and it may not call your callback.
function PreloadImage(imgSrc, callback){
var objImagePreloader = new Image();
objImagePreloader.src = imgSrc;
if(objImagePreloader.complete){
callback();
objImagePreloader.onload=function(){};
}
else{
objImagePreloader.onload = function() {
callback();
// clear onLoad, IE behaves irratically with animated gifs otherwise
objImagePreloader.onload=function(){};
}
}
}

You could show a static image that gives the optical illusion of a spinny-wheel, like these.

Using the load() method of jQuery, it is easily possible to do something as soon as an image is loaded:
$('img.example').load(function() {
$('#spinner').fadeOut();
});
See: http://api.jquery.com/load-event/

Use the power of the setTimeout() function (More info) - this allows you set a timer to trigger a function call in the future, and calling it won't block execution of the current / other functions (async.).
Position a div containing the spinner above the chart image, with it's css display attribute set to none:
<div> <img src="spinner.gif" id="spinnerImg" style="display: none;" /></div>
The nbsp stop the div collapsing when the spinner is hidden. Without it, when you toggle display of the spinner, your layout will "twitch"
function chartOnClick() {
//How long to show the spinner for in ms (eg 3 seconds)
var spinnerShowTime = 3000
//Show the spinner
document.getElementById('spinnerImg').style.display = "";
//Change the chart src
document.getElementById('chart').src = '/charts/10.png';
//Set the timeout on the spinner
setTimeout("hideSpinner()", spinnerShowTime);
}
function hideSpinner() {
document.getElementById('spinnerImg').style.display = "none";
}

Use CSS to set the loading animation as a centered background-image for the image's container.
Then when loading the new large image, first set the src to a preloaded transparent 1 pixel gif.
e.g.
document.getElementById('mainimg').src = '/images/1pix.gif';
document.getElementById('mainimg').src = '/images/large_image.jpg';
While the large_image.jpg is loading, the background will show through the 1pix transparent gif.

Building on Ed's answer, I would prefer to see something like:
function PreLoadImage( srcURL, callback, errorCallback ) {
var thePic = new Image();
thePic.onload = function() {
callback();
thePic.onload = function(){};
}
thePic.onerror = function() {
errorCallback();
}
thePic.src = srcURL;
}
Your callback can display the image in its proper place and dispose/hide of a spinner, and the errorCallback prevents your page from "beachballing". All event driven, no timers or polling, plus you don't have to add the additional if statements to check if the image completed loading while you where setting up your events - since they're set up beforehand they'll trigger regardless of how quickly the images loads.

Some time ago I have written a jQuery plugin which handles displaying a spinner automatically http://denysonique.github.com/imgPreload/
Looking in to its source code should help you with detecting when to display the spinner and with displaying it in the centre of the loaded image.

I like #duddle's jquery method but find that load() isn't always called (such as when the image is retrieved from cache in IE). I use this version instead:
$('img.example').one('load', function() {
$('#spinner').remove();
}).each(function() {
if(this.complete) {
$(this).trigger('load');
}
});
This calls load at most one time and immediately if it's already completed loading.

put the spinner in a div the same size as the chart, you know the height and width so you can use relative positioning to center it correctly.

Aside from the lowsrc option, I've also used a background-image on the img's container.

Be aware that the callback function is also called if the image src doesn't exist (http 404 error). To avoid this you can check the width of the image, like:
if(this.width == 0) return false;

#iAn's solution looks good to me. The only thing I'd change is instead of using setTimeout, I'd try and hook into the images 'Load' event. This way, if the image takes longer than 3 seconds to download, you'll still get the spinner.
On the other hand, if it takes less time to download, you'll get the spinner for less than 3 seconds.

I would add some random digits to avoid the browser cache.