jQuery .load(function) synchronous - javascript

What is the best way to use the jQuery load function synchronously.
I need to load an image but can't execute the next line of code until that image has loaded.
I could loop a variable until the load has completed but was wondering if there was a better way of doing that.
var img = jQuery('<img src="' + url + '"/>').load(function () {
});
//Run code here once img load has comlpeted.

You can also use CallBack function to get Synchronous Behaviour -
var result = $('#main-container').load( 'html/Welcomeform.html',
function () {
if($("textarea").find("#mail-id")){
$("textarea#mail-id").val(email_id);
}
} );

From what I know, the load event will always fire asynchronously, except if the image is already cached (in some browsers). The only reliable solution is to put the code in a callback like you did. However, to make sure the load handler will always be fired in all browsers, even if the image is cached, make sure to add the handler before setting the src property of the image.

var img = jQuery('<img src="' + url + '"/>').load(runner);
function runner() {
//run code here once image is loaded
}

I arrived here looking for a similar solution. From the reads, it is not possible with .load, you need to use an AJAX request, as the question comment points out.
In my case I need to load a html file and I have added a transition to change the content. The old content were showed before the new one after the transition even if I was showing the content inside the load callback.
var main = $("#main");
main.load("about.html", displaySection);
function displaySection () {
main.show('blind');
}
My workaround has been to run the transition that shows the loaded content inside a timeout function with a of 200 for the delay parameter.
var main = $("#main");
main.load("about.html", displaySection);
function displaySection () {
setTimeout(function() {
main.show('blind');
}, 200);
}
The problem could be if the connection is so slow that the new page takes more than 200 ms to load, but in this case I wonder the callback will be launched later on. I don't understand why is not working without the timeout that I feel quite ugly, but it solved my problem ... just in case any other has not given a thought on this possibility.

The callback function in load() will fire once the basic elements of the screen have been retrieved, (the actual html) but doesn't wait for images to finish, you can use this to make it wait.
$('#holder').load(function() {
var imgcount = $('#holder img').length;
$('#holder img').load(function(){
imgcount--;
if (imgcount == 0) {
/* now they're all loaded, let's display them! */
}
});
});

Related

How to automatically read a web page when refreshed [duplicate]

This question already has answers here:
How to make JavaScript execute after page load?
(25 answers)
Closed 1 year ago.
I am using following code to execute some statements after page load.
<script type="text/javascript">
window.onload = function () {
newInvite();
document.ag.src="b.jpg";
}
</script>
But this code does not work properly. The function is called even if some images or elements are loading. What I want is to call the function the the page is loaded completely.
this may work for you :
document.addEventListener('DOMContentLoaded', function() {
// your code here
}, false);
or
if your comfort with jquery,
$(document).ready(function(){
// your code
});
$(document).ready() fires on DOMContentLoaded, but this event is not being fired consistently among browsers. This is why jQuery will most probably implement some heavy workarounds to support all the browsers. And this will make it very difficult to "exactly" simulate the behavior using plain Javascript (but not impossible of course).
as Jeffrey Sweeney and J Torres suggested, i think its better to have a setTimeout function, before firing the function like below :
setTimeout(function(){
//your code here
}, 3000);
JavaScript
document.addEventListener('readystatechange', event => {
// When HTML/DOM elements are ready:
if (event.target.readyState === "interactive") { //does same as: ..addEventListener("DOMContentLoaded"..
alert("hi 1");
}
// When window loaded ( external resources are loaded too- `css`,`src`, etc...)
if (event.target.readyState === "complete") {
alert("hi 2");
}
});
same for jQuery:
$(document).ready(function() { //same as: $(function() {
alert("hi 1");
});
$(window).load(function() {
alert("hi 2");
});
NOTE: - Don't use the below markup ( because it overwrites other same-kind declarations ) :
document.onreadystatechange = ...
I'm little bit confuse that what you means by page load completed, "DOM Load" or "Content Load" as well? In a html page load can fire event after two type event.
DOM load: Which ensure the entire DOM tree loaded start to end. But not ensure load the reference content. Suppose you added images by the img tags, so this event ensure that all the img loaded but no the images properly loaded or not. To get this event you should write following way:
document.addEventListener('DOMContentLoaded', function() {
// your code here
}, false);
Or using jQuery:
$(document).ready(function(){
// your code
});
After DOM and Content Load: Which indicate the the DOM and Content load as well. It will ensure not only img tag it will ensure also all images or other relative content loaded. To get this event you should write following way:
window.addEventListener('load', function() {...})
Or using jQuery:
$(window).on('load', function() {
console.log('All assets are loaded')
})
If you can use jQuery, look at load. You could then set your function to run after your element finishes loading.
For example, consider a page with a simple image:
<img src="book.png" alt="Book" id="book" />
The event handler can be bound to the image:
$('#book').load(function() {
// Handler for .load() called.
});
If you need all elements on the current window to load, you can use
$(window).load(function () {
// run code
});
If you cannot use jQuery, the plain Javascript code is essentially the same amount of (if not less) code:
window.onload = function() {
// run code
};
If you wanna call a js function in your html page use onload event. The onload event occurs when the user agent finishes loading a window or all frames within a FRAMESET. This attribute may be used with BODY and FRAMESET elements.
<body onload="callFunction();">
....
</body>
You're best bet as far as I know is to use
window.addEventListener('load', function() {
console.log('All assets loaded')
});
The #1 answer of using the DOMContentLoaded event is a step backwards since the DOM will load before all assets load.
Other answers recommend setTimeout which I would strongly oppose since it is completely subjective to the client's device performance and network connection speed. If someone is on a slow network and/or has a slow cpu, a page could take several to dozens of seconds to load, thus you could not predict how much time setTimeout will need.
As for readystatechange, it fires whenever readyState changes which according to MDN will still be before the load event.
Complete
The state indicates that the load event is about to fire.
This way you can handle the both cases - if the page is already loaded or not:
document.onreadystatechange = function(){
if (document.readyState === "complete") {
myFunction();
}
else {
window.onload = function () {
myFunction();
};
};
}
you can try like this without using jquery
window.addEventListener("load", afterLoaded,false);
function afterLoaded(){
alert("after load")
}
Alternatively you can try below.
$(window).bind("load", function() {
// code here });
This works in all the case. This will trigger only when the entire page is loaded.
window.onload = () => {
// run in onload
setTimeout(() => {
// onload finished.
// and execute some code here like stat performance.
}, 10)
}
If you're already using jQuery, you could try this:
$(window).bind("load", function() {
// code here
});
I can tell you that the best answer I found is to put a "driver" script just after the </body> command. It is the easiest and, probably, more universal than some of the solutions, above.
The plan: On my page is a table. I write the page with the table out to the browser, then sort it with JS. The user can resort it by clicking column headers.
After the table is ended a </tbody> command, and the body is ended, I use the following line to invoke the sorting JS to sort the table by column 3. I got the sorting script off of the web so it is not reproduced here. For at least the next year, you can see this in operation, including the JS, at static29.ILikeTheInternet.com. Click "here" at the bottom of the page. That will bring up another page with the table and scripts. You can see it put up the data then quickly sort it. I need to speed it up a little but the basics are there now.
</tbody></body><script type='text/javascript'>sortNum(3);</script></html>
MakerMikey
I tend to use the following pattern to check for the document to complete loading. The function returns a Promise (if you need to support IE, include the polyfill) that resolves once the document completes loading. It uses setInterval underneath because a similar implementation with setTimeout could result in a very deep stack.
function getDocReadyPromise()
{
function promiseDocReady(resolve)
{
function checkDocReady()
{
if (document.readyState === "complete")
{
clearInterval(intervalDocReady);
resolve();
}
}
var intervalDocReady = setInterval(checkDocReady, 10);
}
return new Promise(promiseDocReady);
}
Of course, if you don't have to support IE:
const getDocReadyPromise = () =>
{
const promiseDocReady = (resolve) =>
{
const checkDocReady = () =>
((document.readyState === "complete") && (clearInterval(intervalDocReady) || resolve()));
let intervalDocReady = setInterval(checkDocReady, 10);
}
return new Promise(promiseDocReady);
}
With that function, you can do the following:
getDocReadyPromise().then(whatIveBeenWaitingToDo);
call a function after complete page load set time out
setTimeout(function() {
var val = $('.GridStyle tr:nth-child(2) td:nth-child(4)').text();
for(var i, j = 0; i = ddl2.options[j]; j++) {
if(i.text == val) {
ddl2.selectedIndex = i.index;
break;
}
}
}, 1000);
Try this jQuery:
$(function() {
// Handler for .ready() called.
});
Put your script after the completion of body tag...it works...

jQuery: Detecting if browser is done rendering after append()

In a jQuery project, I am adding several images to a page after it has finished loading, using the append() function. After all the appends are done, I need to call a function that detects the width and height of each of these new elements, so I can properly align them in the page.
This is working well on my rather slow development PC. But when testing it on a fast machine in a quick browser (e.g. Chrome), it turns out that calling the layout function right after the appends are done leads to random, weird behavior. Executing the layout function again with a slight delay solves the problem. I guess "javascript is done appending the new elements" is not the same as "the browser is done displaying them", so measuring their size is not yet possible.
So what I need is some way to tell when the new elements are actually there, so that I can reliably detect their widths and heights. I don't think document.ready() would help in this case, since the elements are added by a function that is called in document.load().
This is the code I copied from another stackoverflow thread (by a dude named Pointy, thanks by the way!)
function called_by_document_load() {
var promises = [];
for (var i=0; i<my_pics.length; i++) {
(function(url, promise) {
var img = new Image();
img.onload = function () {
promise.resolve();
};
img.src = url;
}) (my_pics[i]['url'], promises[i] = $.Deferred());
}
$.when.apply($, promises).done(function() {
// add the new images using append()
// when done, call my own layout function
}
... with the array my_pics containing the URLs of the images to load.
Any ideas? :-)
Thanks in advance,
snorri
After adding the new images using append, yield the control to the browser so that the browser can update the ui and then invoke your layout function. You can do so my using a setTimeout with delay set to 0.
$.when.apply($, promises).done(function() {
// add the new images using append()
setTimeout(function () {
// when done, call my own layout function
}, 0);
}
There are many SO threads that explain why this works and is useful.
Here is one good explanation
Also, worth watching is the video by Philip Roberts on browser's event loop.

Unable to get setTimeout to work in javascript

I am using jquery to slide up an down a div.
For some reason setTimeout is not working (looks like a function scope issue).
Not able to figure out what is wrong with the below code.
(both functions are inside $(document).ready(function(){ } )
$('.slider-thumb').click(function(){
var source = $(this).attr("src");
$('#image_view').css('background-image',"url("+source+")");
$('#image_view').slideDown(1000, calbck);
initiate_timeout();
function calbck(){}
});
function initiate_timeout(){
var time_out = setTimeout(function() {
$('#image_view').slideUp(1000, calbck);
},2000);
}
Indenting the source code shows that you indeed have some scoping issue. The calbck function is private to the click handler function, and is thus not visible to initiate_timeout function. Either make calbck a top-level function, or make initiate_timeout a function local to the click handler function.
$('.slider-thumb').click(function() {
var source = $(this).attr("src");
$('#image_view').css('background-image',"url("+source+")");
$('#image_view').slideDown(1000, calbck);
initiate_timeout();
function calbck(){}
});
function initiate_timeout(){
var time_out = setTimeout(function() {
$('#image_view').slideUp(1000, calbck);
}, 2000);
}
You have some JS errors and scoping issues. Why would you ever have debugging turned off when trying to troubleshoot an error? Change your code to this:
$('.slider-thumb').click(function(){
var source = $(this).attr("src");
$('#image_view').css('background-image',"url("+source+")");
$('#image_view').slideDown(1000);
setTimeout(function() {
$('#image_view').slideUp(1000);
},2000); // will start 2 seconds after slideDown starts (which is 1 second after it completes)
});
or even better, use the completion function of the first animation to set the timer:
$('.slider-thumb').click(function(){
var source = $(this).attr("src");
$('#image_view').css('background-image',"url("+source+")");
$('#image_view').slideDown(1000, setTimeout(function() {
$('#image_view').slideUp(1000);
},1000)); // will stay open for 1 second before sliding up again
});
Or, even better, using jQuery's delay/queuing, you can do this:
$('.slider-thumb').click(function(){
var source = $(this).attr("src");
$('#image_view').css('background-image',"url("+source+")");
$('#image_view').slideDown(1000).delay(1000).slideUp(1000); // delay 1 sec between effects
});
The calbck you were trying to pass to slideUp was not defined in the scope you were using it (inside of initiate_timout()). It was private to your click handler.
jQuery probably has better ways to chain effects than using your own timer, but I see no reason why this code shouldn't work if it matches your HTML.
Note: if your background image wasn't already pre-cached, it may not be loaded right away when your slideDown starts.
Are you getting any errors? You're passing an undefined value as calbck to slideUp in the setTimeout function.
It works for me (with my modified organization): http://jsfiddle.net/v3cjG/1/
And when I run your exact code ( http://jsfiddle.net/v3cjG/2/ ) I get an error for "Can't find variable calbck"--the var I said you were passing without defining.

jQuery .load() not firing on images (probably caching?)

I have some pretty basic jQuery code:
...
$(this).find('img').load(function(){
loadedImages++;
if(loadedImages == $this.find('img').length){
...
However, thats not firing consistently. It fires if i do a hard refresh or close my browser, but a normal refresh, or just hitting the same URL twice at any time without erasing the cache makes the .load() never fire.
Any ideas on how to fix this?
I think this has been discussed before. It’s not the caching per se that is the problem but the timing: the images may already have been loaded by the time the event handler is attached (so it never gets fired). This may also occur if no caching happens, for example in a multithreaded browser on a very fast connection. Fortunately, there is a property .complete that you can use:
var load_handler = function() {
loadedImages++;
…
}
$(this).find('img').filter(function() {
return this.complete;
}).each(load_handler).end().load(load_handler);
You can also create your own event attach function:
jQuery.fn.extend({
ensureLoad: function(handler) {
return this.each(function() {
if(this.complete) {
handler.call(this);
} else {
$(this).load(handler);
}
});
}
});
And then call it as follows:
$(this).find('img').ensureLoad(function(){
loadedImages++;
if(loadedImages == $this.find('img').length){
…
});
A way would be to add a "dummy variable" at the end of the URL that you use to grab the image... such as the time in milliseconds appended as a query string param.
Edit: Preventing the Browser to Cache images is a very bad idea in 99% of the cases as it will slow down your application. Instead you should check if an image is already loaded, and if not wait for the load event to complete.
As Ron and El Guapo said, the answer is to add a query at the end of the URL. I did this like this:
$(this).find('img').each(function(){
$(this).attr('src',$(this).attr('src')+'?'+new Date().getTime())
}).load(function(){
//This will always run now!

Javascript block script execution

I need to do something like this:
Execute a piece of code
Start to load an image and block the script execution
When the image is loaded resume the execution
Execute the rest of the code
I know that the simplest way is to assign a function on the onload event of the image and then execute the rest of the code in the function, but if it's possible i want to have a "linear" behaviour blocking the script execution and then resume it.
So, is there a cross-browser way to do this?
The only way to block script execution is to use a loop, which will also lock up most browsers and prevent any interaction with your web page.
Firefox, Chrome, Safari, Opera and IE all support the complete property, which was finally standardised in HTML5. This means you can use a while loop to halt script execution until the image has finished downloading.
var img = new Image();
img.src = "/myImage.jpg";
document.body.appendChild(img);
while (!img.complete)
{
// do nothing...
}
// script continues after image load
That being said, I think you should look at ways of achieving your goal without locking up the browser.
If you don't mind having a preprocessing step, try Narrative Javascript, which you can
image.onload = new EventNotifier();
image.onload.wait->();
This suggestion is not exactly what you asked for, but I offer it as a possible alternative.
Create a CSS class with the background-image you want to use. When your app starts, assign this CSS class to a DIV that is either hidden out of site or sized to zero by zero pixels. This will ensure the image is loaded from the server. When you want to load the image (step two above), use the CSS class you create; this will happen quickly. Maybe quickly enough that you need not block the subsequent code execution?
I wouldn't try to block script execution completely, as that could make the browser slow down, or even alert the user that a script is taking too long to execute.
What you can do is 'linearize' your code by using events to finish work. You will need to add a time out to the function, as the image may never load.
Example:
var _img = null;
var _imgDelay = 0;
var _finished = false;
function startWork(){
_img = document.createElement('img');
_img.onload = onImgLoaded;
_img.src = 'yourimg.png';
// append img tag to parent element here
// this is a time out function in case the img never loads,
// or the onload event never fires (which can happen in some browsers)
imgTimeout();
}
function imgTimeout(){
if (_img.complete){
// img is really done loading
finishWork();
}
else{
// calls recursively waiting for the img to load
// increasing the wait time with each call, up to 12s
_imgDelay += 3000;
if (_imgDelay <= 12000){ // waits up to 30 seconds
setTimeout(imgTimeout, _imgDelay);
}
else{
// img never loaded, recover here.
}
}
}
function onImgLoaded(){
finishWork();
}
function finishWork(){
if (!_finished){
// continue here
_finished = true;
}
}
You can use xmlhttprequest and use synchronous mode.
var url = "image.php?sleep=3";
var img = new Image;
var sjax = new XMLHttpRequest();
img.src = url;
sjax.open("GET", url, false);
sjax.send(null);
alert(img.complete);
The trick here is we load the same image twice, first by using the Image object, and also by using ajax in synchronous mode. The Image object isn't needed, I just assumed that's how you want to load it. The key though is that if ajax completes, then the image will be fully downloaded an in the browser's cache. As such, the image will also be available for use by the Image object.
This does assume that the image is served with cache friendly http headers. Otherwise, it's behavior might vary in different browsers.

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