I need a preloader in JS for images. The images are fetched by a PHP script (to avoid opening the file structure to the user) and set as the attribute of the img tag. After the preload we wait for a event bound as an event listener to get fired (kind of like a slideshow).
var preloadImage = $('<img />').attr('src', 'getimage.php');
preloadImage.onLoad = function() {console.log('finished')}
The problem is, that of course onLoad does not work as it deals with the image being processed in the JS, while in this case we are waiting for the PHP to finish getting the image. The php script is just like this
<?php
header("Content-type: image/png");
readfile('image.png');
?>
Anyone has an idea how in this case i can make the JS wait till the php has finished loading?
As DaveRandom answered the question perfectly in the discussion above, here is the final code. preloadImage is a jQuery object which means that one needs to pass the .load() function.
var preloadImage = $('<img />').attr('src', 'getimage.php').load(function() {console.log('finished')})
Related
I'm using jQuery to load content dynamically when the user clicks a link. The content is just a bunch of images that are then set to display in a slideshow of sorts. My problem is, I can't seem to figure out a way to show the loaded content only AFTER the images have fully loaded. I've tried several different solutions and all of them seem to either break the script or just not work the way I want. Here's the code I'm using now:
$(document).ready(function() {
$("a#item").click( function() {
var projectName = $(this).attr('class');
$("div.slideshow").css("display", "block");
$("div.slideshow").load(projectName+".php", function() {
var slideshow = new Array();
$("div.slideshow img").each(function() {
slideshow.push($(this));
});
startSlideshow(slideshow.shift());
function startSlideshow(image) {
image.delay(400).fadeIn(150, function() {
if(slideshow.length > 0) {startSlideshow(slideshow.shift());}
else { $("div.slideshow").delay(400).fadeOut(200, function() {$("div.slideshow img").css("display", "none")}); }
});
}
});
return false;
});
});
You can also see the full demo site here: http://publicprofileproject.com/
Any input would be greatly appreciated!
You could create an array of image objects in your JavaScript, loading each image into an element of the array. Attach an event handler to the onLoad event of the images.
In the event handler, increment a count of loaded images. When your counter reaches the length of your array, the browser will have all of the images. This is the point at which you can show your slideshow.
If you do this in your page load, it will have the added advantage of pre-loading the images ready for when the user clicks your link.
I believe this question has already been answered here.
The general idea is that you specify a load event handler to display it prior to specifying the source attribute.
Alternatively, if your projects+".php" file is specifying the images in ready-made, html mark-up, then you should be able to capture the load event of the images in the file you are loading. Add the following pseudocode into your file that is being loaded.
$("img").load(function() {
// show the div on the page it is getting loaded into
alert("images should be loaded now");
});
You might be able to place it in your original code segment and potentially bind it using the live / on binding events. ex: $("img").on("load", function() {...
From the load documentation:
The load event is sent to an element when it and all sub-elements have been completely loaded. This event can be sent to any element associated with a URL: images, scripts, frames, iframes, and the window object.
Edit: Interesting discouragement for doing what it looks like you're doing:
Caveats of the load event when used with images
A common challenge developers attempt to solve using the .load() shortcut is to execute a function when an image (or collection of images) have completely loaded. There are several known caveats with this that should be noted. These are:
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
I have a block of php code that loads an image at random. I'm trying to determine when the image has loaded so I can perform some additional actions on it. Here is how I'm currently loading my image:
// Gets my image
$sql = "SELECT id FROM images WHERE user_id=$owner_user_id LIMIT 0,1";
$imgres = mysql_query($sql);
if ($imgres) {
$imgrow = mysql_fetch_array($imgres, MYSQL_ASSOC);
if (!empty($imgrow)) {
echo ('<image src="img.php?id=' . $imgrow['id'] . '" id="profile_img" style="visibility:hidden"/>');
}
}
One of the actions I need to do is get the width of the image. I'm getting the image like so:
alert("IMAGE WIDTH:"+$('#profile_img').width());
It currently returns 0 because it's being called prior to the image being loaded. I've tried adding this method call to my document.ready block but it still gets called prior to the image being loaded. Is there an easy way to determine when the image has loaded?
$(document).ready(function(){
$('#img_id').load(function() {
alert('image is loaded');
});
});
this will do the trick
you can bind a load event handler to your image with jquery:
http://api.jquery.com/load-event/
$("#profile_img").load(function(){
alert(("IMAGE WIDTH:"+$('#profile_img').width())
});
http://api.jquery.com/load-event/
This really has nothing to do with PHP; image loading is done client side, so you'll need to do it in the Javascript side of things. You'll be dealing with Javascript events - here's a primer:
http://www.quirksmode.org/js/introevents.html
Luckily, jQuery has a built-in function to bind to the event for you (even called load()!) which allows you to pass a callback to fire once that content is loaded.
Your best bet is to preload the image in JavaScript prior to the DOM loading at all:
<SCRIPT language="JavaScript">
preload_image = new Image(25,25);
preload_image.src="http://mydomain.com/image.gif";
</SCRIPT>
Do NOT use jQuery .load() to test when images are loaded. According to the jQuery docs:
•It doesn't work consistently nor [is it] 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
I am using the following script to display big images on mouse over the small images (example photo attached in the last). I want to show the 'loading' image (like this) while the big image is being downloaded from the server. How can this be achieved?
Note: I have asked a similar question here but I was not successful in applying the append function to the following code. Please help.
<script type="text/javascript">
function showIt(imgsrc)
{
var holder = document.getElementById('imageshow');
var newpic= new Image();
newpic.src=imgsrc;
holder.src=imgsrc;
holder.width = newpic.width;
holder.height=newpic.height;
}
</script>
<body>
/***on hover, xyz.jpg will be replaced by bigA.jpg and so on***/
<img src="smallA.jpg" onMouseOver="showIt('bigA.jpg')"/>
<img src="smallB.jpg" onMouseOver="showIt('bigB.jpg')"/>
<img src="xyz.jpg" id="imageshow" />
</body>
Images have a load event. As long as you set the load handler before the image.src is set, you should get notified when the image has successfully loaded or encounters some kind of error in loading. I do that very thing in a slideshow that I wrote so I know when the next image is ready for display and I display a wait cursor (animated gif like you're wanting) if the image has been delayed more than one second beyond it's appointed display time so the user knows what's going on.
In general, you can do something like this:
function loadImage(url, successHandler, errorHandler) {
var myImg = new Image();
myImg.onload = myLoadHandler; // universally supported
myImg.onabort = myErrorHandler; // only supported in some browsers, but no harm in listening for it
myImg.onerror = myErrorHandler;
myImg.src = url;
function myLoadHandler() {
successHandler(myImg, url);
}
function myErrorHandler() {
if (errorHandler) {
errorHandler(url);
}
}
}
Using code like this, you can display the wait cursor when you initiate the image load and hide it when the successHandler gets called.
If there were any other listeners to these events, then you should use addEventListener or attachEvent instead of onload, onabort, onerror, but if there's only one listener, you can go either way.
If the desired images are known in advance, then it's sometimes a better user experience (less waiting) to preload images that may be used later. This gets them into the browser's memory cache so they will appear instantly when needed. One can preload images either in HTML or in JS. In HTML, just insert tags into the web page for all the desired images (but hide them with CSS). In JS, just create an image array and create the image objects:
// image URLs to preload
var preloadImageURLs = [
"http://photos.smugmug.com/935492456_5tur7-M.jpg",
"http://photos.smugmug.com/835492456_968nf-M.jpg",
"http://photos.smugmug.com/735492456_3kg86-M.jpg",
];
var preloads = []; // storage for preloaded images
function preloadImages() {
var img, i;
for (i = 0; i < preloadImageURLs.length; i++) {
img = new Image();
img.src = preloadImageURLs[i];
preloads.push(img);
}
}
This will cause all the images in the preloadImageURLs array to be preloaded and available instantly later on in the life of the web page, thus preventing any user delays while waiting for images to be loaded. Obviously, there's a short amount of time for the preloaded images to actually get loaded, but for smallish images that usually happens before the user interacts with the web page so it makes for a faster feel to dynamic parts of the web page that use images.
<img id=access src=loading.gif>
<script>
window.onload=function(){
document.getElementById('access').src='access.jpg';
}
</script>
Hope this helps.
Here is what I am trying to accomplish. I have a form that uses jQuery to make an AJAX call to a PHP file. The PHP file interacts with a database, and then creates the page content to return as the AJAX response; i.e. this page content is written to a new window in the success function for the $.ajax call. As part of the page content returned by the PHP file, I have a straightforward HTML script tag that has a JavaScript file. Specifically:
<script type="text/javascript" src="pageControl.js"></script>
This is not echoed in the php (although I have tried that), it is just html. The pageControl.js is in the same directory as my php file that generates the content.
No matter what I try, I can't seem to get the pageControl.js file included or working in the resulting new window created in response to success in the AJAX call. I end up with errors like "Object expected" or variable not defined, leading me to believe the file is not getting included. If I copy the JavaScript directly into the PHPfile, rather than using the script tag with src, I can get it working.
Is there something I am missing here about scope resolution between calling file, php, and the jQuery AJAX? I am going to want to include javascript files this way in the future and would like to understand what I am doing wrong.
Hello again:
I have worked away at this issue, and still no luck. I am going to try and clarify what I am doing, and maybe that will bring something to mind. I am including some code as requested to help clarify things a bit.
Here is the sequence:
User selects some options, and clicks submit button on form.
The form button click is handled by jQuery code that looks like this:
$(document).ready(function() {
$("#runReport").click(function() {
var report = $("#report").val();
var program = $("#program").val();
var session = $("#session").val();
var students = $("#students").val();
var dataString = 'report=' +report+
'&program=' +program+
'&session=' +session+
'&students=' +students;
$.ajax({
type: "POST",
url: "process_report_request.php",
cache: false,
data: dataString,
success: function(pageContent) {
if (pageContent) {
$("#result_msg").addClass("successMsg")
.text("Report created.");
var windowFeatures = "width=800,menubar=yes,scrollbars=1,resizable=1,status=yes";
// open a new report window
var reportWindow = window.open("", "newReportWindow", windowFeatures);
// add the report data itself returned from the AJAX call
reportWindow.document.write(pageContent);
reportWindow.document.close();
}
else {
$("#result_msg").addClass("failedMsg")
.text("Report creation failed.");
}
}
}); // end ajax call
// return false from click function to prevent normal submit handling
return false;
}); // end click call
}); // end ready call
This code performs an AJAX call to a PHP file (process_report_request.php) that creates the page content for the new window. This content is taken from a database and HTML. In the PHP file I want to include another javascript file in the head with javascript used in the new window. I am trying to include it as follows
<script src="/folder1/folder2/folder3/pageControl.js" type="text/javascript"></script>
Changed path folder names to protect the innocent :)
The pageControl.js file is actually in the same folder as the jQuery code file and the php file, but I am trying the full path just to be safe. I am also able to access the js file using the URL in the browser, and I can successfully include it in a static html test page using the script src tag.
After the javascript file is included in the php file, I have a call to one of its functions as follows (echo from php):
echo '<script type="text/javascript" language="javascript">writePageControls();</script>';
So, once the php file sends all the page content back to the AJAX call, then the new window is opened, and the returned content is written to it by the jQuery code above.
The writePageControls line is where I get the error "Error: Object expected" when I run the page. However, since the JavaScript works fine in both the static HTML page and when included "inline" in the PHP file, it is leading me to think this is a path issue of some kind.
Again, no matter what I try, my calls to the functions in the pageControls.js file do not work. If I put the contents of the pageControl.js file in the php file between script tags and change nothing else, it works as expected.
Based on what some of you have already said, I am wondering if the path resolution to the newly opened window is not correct. But I don't understand why because I am using the full path. Also to confuse matters even more, my linked stylesheet works just fine from the PHP file.
Apologies for how long this is, but if anyone has the time to look at this further, I would greatly appreciate it. I am stumped. I am a novice when it comes to a lot of this, so if there is just a better way to do this and avoid this problem, I am all ears (or eyes I suppose...)
I have also had problems with a similar issue to this, and this was a real headache. The following approach may not be elegant, but it worked for me.
Make sure that your php file, just outputs what you want in your
body
Add jquery to the window head dynamically
Add any external script files to the window head dynamically
use jQuery html on the window's document to call html() with your loaded content on the body, so that scripts are evaluated.
For example, in your ajax success:
success: function(pageContent) {
var windowFeatures = "width=800,menubar=yes,scrollbars=1,resizable=1,status=yes";
var reportWindow = window.open("", "newReportWindow", windowFeatures);
// boilerplate
var boilerplate = "<html><head></head><body></body></html>";
reportWindow.document.write(boilerplate);
var head = reportWindow.document.getElementsByTagName("head")[0];
var jquery = reportWindow.document.createElement("script");
jquery.type = "text/javascript";
jquery.src = "http://code.jquery.com/jquery-1.7.min.js";
head.appendChild(jquery);
var js = reportWindow.document.createElement("script");
js.type = "text/javascript";
js.src = "/folder1/folder2/folder3/pageControl.js";
js.onload= function() {
reportWindow.$("body").html(pageContent);
};
head.appendChild(js);
reportWindow.document.close();
}
Good luck!
It probably isn't looking where you think it is looking to grab your javascript file.
Try a server-relative format like this:
<script src="/some/path/to/pageControl.js"></script>
If that still isn't working, verify that you can type the url to your script file into your browser and get it to download.
Make sure that you have that within either <head> or <body> of the HTML page. Also, I'd double check the path to the .js file. You could do that by pasting "pageControl.js" at the root of your web address.
Things to look for:
Use Firebug (NET tab) to check if the js file is loaded with status 200. Also check in the Console tab for any javascript errors.
Are you using HTML5 offline. If you do, maybe it serves a cached version that doesn't include your <script> tag.
View the page source and make sure it includes the script tag.
Change the source attribute to absolute path: <script src="http://www.example.com/js/pageControl.js" type="text/javascript"></script>
Visit http://www.example.com/js/pageControl.js and make sure it shows correctly.
Try to place the <script> right after the <head> so that it loads first.
This is all I could think of.
You can dynamically load script by creating the element and then append it to head or other element:
reportWindow.document.write(pageContent);
var script = document.createElement('script');
script.src = 'pageControl.js';
script.type = 'text/javascript';
reportWindow.document.getElementsByTagName('head')[0].appendChild(script);
reportWindow.document.close();
Have you tried using the jquery $("#target_div").load(...)
This also executes JS inside the output...
Read this doc to find out how to use it :
http://api.jquery.com/load/
To me it sounds like you're expecting an unloaded script to work.
Try taking a look here: http://ensure.codeplex.com/SourceControl/changeset/view/9070#201379
This is a bit of javascript that ensures that the script is loaded properly before access is attempted. You can use this either as lazy loading (loading javascript files only when required), or, as I interpret your problem, loading a script based on the result of ajax calls.
What's probably happening is, you're echoing a string via an ajax callback, not inserting an element. External scripts require a second GET call to load their contents, which isn't happening - only the first call happened. So, when the first call includes the inline code, the DOM doesn't have to make an additional GET request to fetch the contents. If the DOM doesn't see the script, the DOM won't execute it, which means it's just some random tag.
There's a very fast way to find out. In Chrome (or Firefox with the Firebug plugin installed), check the console > scripts dropdown to see all the loaded scripts. If it's not listed, it's not loaded and the script tag you see in the markup is otherwise inert.
Since it's probably just a string as far as PHP cares, you could create it as PHP DOM object and insert it properly (although this could be laborious). Instead, maybe place it at the very end of the page, just before the closing body tags. (This is the preferred position for js anyway - dead last, after all the other elements on the page have loaded and are available to the DOM.)
HTH :)
I have put together a script which is very much like the flickr photostream feature. Two thumbnails next to each other, and when you click the next or prev links the next (or previous) two images slide in. Cool!
Currently when the page loads it loads the two images. The first time nxt / prv is used then the next two images or previous two are loaded via ajax, with the id of the first image being passed in the url and the HTML for the two new images returned and displayed via ajax.
simple enough, but it got me to thinking, on a slow connection, or heavy server load then the two images, although relatively small thumbnails could still take a while to load, and the nice things with sliding panes is the fact that the hidden data slides in quickly and smoothly preferbly without a loading delay.
So I was wondering from a performance and good practice point of view which option is best, this is what I can think of for now, open to suggestions.
1, call each set of images via JSON (its supposed to be fast?)
2,load all the possible images into a json file and pull in the details that way - although browser will still have to load image. Plus sometimes there might be 4 images, other times there could be upto 1000!
3, Load 10 images via php into a Json or other file, and load all 10 images into the browser hiding the 8 which are not on show, and always showing the middle two. Problem here is that each time someone clicks, the file has to reload the first and last images, which still takes up time, although i suppose the middle images will have all been cached via the browser by now though. But still there is a loading time.
4, Is it possible to have a json image with all the image details (regardless of numbers) and use no 3 above to load 10 of those images, is it possible to use ajax to only read 10 lines and keep a pointer of the last one it read, so the json file can be loaded fast, short refresh and images either side are cached via the browser!!
Hope thats clear, any suggestions on how you would handle this?
To preload an image from Javascript, you don't need to do anything that sounds like AJAX or JSON. All you need is this:
var img = new Image();
img.src = "http://example.com/new/image.jpg";
The browser will quite happily load the image in the background, even though it's not displayed anywhere. Then, when you update the src field of another (displayed) image tag, the browser will immediately show the part of the image it's already loaded (hopefully all of it).
Fetching JSON Via Ajax will just slow you down.
You're better off using inline JSON and generated Javascript.
<?php
$images = array( "foo.jpg","bar.jpg" );
?>
<script type="text/javascript">
jQuery(function($){
var images = ( <?php echo json_encode($images); ?> );
/* Creating A Hidden box to preload the images into */
var imgbox = document.createElement("div");
$(imgbox).css({display: 'none'});
/* Possible Alternative trick */
/* $(imgbox).css({height:1px; width: 1px; overflow: hidden;}); */
$('body').append(imgbox);
$.each( images , function( ind, item )
{
#Injecting images into the bottom hidden div.
var img = document.createElement("img");
$(img).src("/some/prefix/" + item );
$(imgbox).append(img);
});
});
</script>
In the case where you want to concurrently preload a larger number of resources, a little ajax can solve the problem for you. Just make sure the caching headers are such that the browser will use the resources on the next request. In the following example, I load up to four resources concurrently.
<script>
var urls = [
"a.png",
"b.png",
"c.png",
"d.png",
"e.png",
"f.png"
];
var currentStep = 0;
function loadResources()
{
if(currentStep<urls.length){
var url = urls[currentStep];
var req = GetXmlHttpObject();
update('loading ' + url);
req.open("GET", url, true);
req.onreadystatechange = getUpdateState(req, url);
req.send(null);
} else {
window.location = 'done.htm';
}
}
function getUpdateState(req, url) {
return function() {
var state = req.readyState;
if (state != 4) { return; }
req = null;
setTimeout('loadResources()', 0);
}
}
</script>
<!-- This will queue up to four concurrent requests. Modern browsers will request up to eight images at time -->
<body onload="javascript: loadResources(); loadResources(); loadResources(); loadResources();">
Why not use text and replace the text with a picture code (works in php really nice with ajax up to 500 pictures and more)?