Develop Reference

Why JavaScript code execute second part sometimes

I am draw background on canvas and also small images on that background. But sometimes, background draw on small images. Why ?
JavaScript code -
var canvasupdate = document.getElementById("myCanvas");
ctxupdate = canvasupdate.getContext("2d");
var background = new Image();
background.src = sitePath + "ATOM/chapter1/book/" + `bgimagename`;
background.onload = function() {
ctxupdate.drawImage(background, 0, 0); // set background image
};
var imageobj = new Array();
for (var d = 0; d < calloutImageArray.length; d++) // getting small images in array
{
imageobj[d] = new Image();
(function(d) {
imageobj[d].src = sitePath + "ATOM/chapter1/book/" + calloutImageArray[d];
imageobj[d].onload = function() {
ctxupdate.drawImage(imageobj[d], calloutImageArrayX[d], calloutImageArrayY[d], calloutImageArrayW[d], calloutImageArrayH[d]);
};
})(d);
}
In above code, background image code should be execute first then call out image(small image) code execute but some time execute small images code and then background image code why?

All the images are loaded asynchronously. So sometimes the small images are loaded (and drawn) before the background image. Please take a look into e.g. Network tab in Chrome in which order the resources load is done.
The simplest solution for this problem is to move the loading of small images into the callback function for load event of the background image.

The onLoad function runs asynchronously. That means JavaScript will continue to run the rest of your code and will run the callback function when the background image is loaded. That's why your second part of the code runs first. So instead try to add your code inside the onload function like this:
var canvasupdate = document.getElementById("myCanvas");
ctxupdate = canvasupdate.getContext("2d");
var background = new Image();
background.src = sitePath + "ATOM/chapter1/book/" + `bgimagename`;
background.onload = function() {
ctxupdate.drawImage(background, 0, 0); // set background image
var imageobj = new Array();
for (var d = 0; d < calloutImageArray.length; d++) // getting small images in array
{
imageobj[d] = new Image();
(function(d) {
imageobj[d].src = sitePath + "ATOM/chapter1/book/" + calloutImageArray[d];
imageobj[d].onload = function() {
ctxupdate.drawImage(imageobj[d], calloutImageArrayX[d], calloutImageArrayY[d], calloutImageArrayW[d], calloutImageArrayH[d]);
};
})(d);
}
};
that way you can be sure that the background will be set first and then your small images. Note that I didn't try your code to check if it does what you want, I just rearranged the code blocks to show you the logic and why does not run as you would expect.
Hope it helps

Image.complete is true after onload, but after onload second image not anymore

I'm using Javascript to load 2 images and drawing them on a canvas.
I use the onload event of an image to drawn the images after they are both loaded.
var loadedImagesCount = 0;
var NUM_OF_TILES=2;
window.onload=function(){
background.onload = imageOnLoad();
background.src ="background.png";
layer1 = document.getElementById("layer1");
ctx1 = layer1.getContext("2d");
character.onload = imageOnLoad();
character.src ="character.png";
layer2 = document.getElementById("layer2");
ctx2 = layer2.getContext("2d");
}
function imageOnLoad(){
loadedImagesCount++;
window.alert(''+loadedImagesCount+ ' ' + background.complete + ' ' + character.complete);
if (loadedImagesCount==NUM_OF_TILES) drawAll();
}
The imageOnLoad() is called twice (as expected). The first time is says (through an alert) that both images are loaded. The second time it says that the character image is loaded, but the background image not anymore? How is this possible? Why would it unload the image again? After a refresh this does not happen anymore, it only happens the first time you load the page.
And because the background is not loaded, when the canvas is drawn, only the character is being drawn.

This maybe because you have not declared background as a global variable outside of the window.onload function. So background is not the variable you think it is.
Check this code that works as expected and is derived from yours and notice how background and characters are "vared" outside of the window.onload function :
<html>
<script language="javascript">
var loadedImagesCount = 0;
var NUM_OF_TILES=2;
var background;
var character;
window.onload=function(){
background = document.getElementById("background");
character = document.getElementById("character");
background.onload = imageOnLoad();
background.src ="HSHOP.jpg";
character.onload = imageOnLoad();
character.src ="HSHOP 2.jpg";
}
function imageOnLoad(){
loadedImagesCount++;
window.alert(''+loadedImagesCount+ ' ' + background.completes + ' ' + character.complete);
//if (loadedImagesCount==NUM_OF_TILES) drawAll();
}
</script>
<body>
<img id="background"/>
<img id="character"/>
</body>
</html>
Quick note : As a general rule try to clean up your code as much as possible before posting it so that it is easier for us to find what's going wrong.

Problem solved. The onload variable needs a method without the parentheses, if you use the parentheses it will call the method immediately.

Ask for microphone on onclick event

The other day I stumbled upon with this example of a Javascript audio recorder:
http://webaudiodemos.appspot.com/AudioRecorder/index.html
Which I ended up using for implementing my own. The problem I'm having is that in this file:
var audioContext = new webkitAudioContext();
var audioInput = null,
realAudioInput = null,
inputPoint = null,
audioRecorder = null;
var rafID = null;
var analyserContext = null;
var canvasWidth, canvasHeight;
var recIndex = 0;
/* TODO:
- offer mono option
- "Monitor input" switch
*/
function saveAudio() {
audioRecorder.exportWAV( doneEncoding );
}
function drawWave( buffers ) {
var canvas = document.getElementById( "wavedisplay" );
drawBuffer( canvas.width, canvas.height, canvas.getContext('2d'), buffers[0] );
}
function doneEncoding( blob ) {
Recorder.forceDownload( blob, "myRecording" + ((recIndex<10)?"0":"") + recIndex + ".wav" );
recIndex++;
}
function toggleRecording( e ) {
if (e.classList.contains("recording")) {
// stop recording
audioRecorder.stop();
e.classList.remove("recording");
audioRecorder.getBuffers( drawWave );
} else {
// start recording
if (!audioRecorder)
return;
e.classList.add("recording");
audioRecorder.clear();
audioRecorder.record();
}
}
// this is a helper function to force mono for some interfaces that return a stereo channel for a mono source.
// it's not currently used, but probably will be in the future.
function convertToMono( input ) {
var splitter = audioContext.createChannelSplitter(2);
var merger = audioContext.createChannelMerger(2);
input.connect( splitter );
splitter.connect( merger, 0, 0 );
splitter.connect( merger, 0, 1 );
return merger;
}
function toggleMono() {
if (audioInput != realAudioInput) {
audioInput.disconnect();
realAudioInput.disconnect();
audioInput = realAudioInput;
} else {
realAudioInput.disconnect();
audioInput = convertToMono( realAudioInput );
}
audioInput.connect(inputPoint);
}
function cancelAnalyserUpdates() {
window.webkitCancelAnimationFrame( rafID );
rafID = null;
}
function updateAnalysers(time) {
if (!analyserContext) {
var canvas = document.getElementById("analyser");
canvasWidth = canvas.width;
canvasHeight = canvas.height;
analyserContext = canvas.getContext('2d');
}
// analyzer draw code here
{
var SPACING = 3;
var BAR_WIDTH = 1;
var numBars = Math.round(canvasWidth / SPACING);
var freqByteData = new Uint8Array(analyserNode.frequencyBinCount);
analyserNode.getByteFrequencyData(freqByteData);
analyserContext.clearRect(0, 0, canvasWidth, canvasHeight);
analyserContext.fillStyle = '#F6D565';
analyserContext.lineCap = 'round';
var multiplier = analyserNode.frequencyBinCount / numBars;
// Draw rectangle for each frequency bin.
for (var i = 0; i < numBars; ++i) {
var magnitude = 0;
var offset = Math.floor( i * multiplier );
// gotta sum/average the block, or we miss narrow-bandwidth spikes
for (var j = 0; j< multiplier; j++)
magnitude += freqByteData[offset + j];
magnitude = magnitude / multiplier;
var magnitude2 = freqByteData[i * multiplier];
analyserContext.fillStyle = "hsl( " + Math.round((i*360)/numBars) + ", 100%, 50%)";
analyserContext.fillRect(i * SPACING, canvasHeight, BAR_WIDTH, -magnitude);
}
}
rafID = window.webkitRequestAnimationFrame( updateAnalysers );
}
function gotStream(stream) {
// "inputPoint" is the node to connect your output recording to.
inputPoint = audioContext.createGainNode();
// Create an AudioNode from the stream.
realAudioInput = audioContext.createMediaStreamSource(stream);
audioInput = realAudioInput;
audioInput.connect(inputPoint);
// audioInput = convertToMono( input );
analyserNode = audioContext.createAnalyser();
analyserNode.fftSize = 2048;
inputPoint.connect( analyserNode );
audioRecorder = new Recorder( inputPoint );
zeroGain = audioContext.createGainNode();
zeroGain.gain.value = 0.0;
inputPoint.connect( zeroGain );
zeroGain.connect( audioContext.destination );
updateAnalysers();
}
function initAudio() {
if (!navigator.webkitGetUserMedia)
return(alert("Error: getUserMedia not supported!"));
navigator.webkitGetUserMedia({audio:true}, gotStream, function(e) {
alert('Error getting audio');
console.log(e);
});
}
window.addEventListener('load', initAudio );
As you might be able to see, the initAudio() function (the one wich ask the user for permission to use his/her microphone) is called inmediately when the page is loaded (read the last line) with this method:
window.addEventListener('load', initAudio );
Now, I have this code in the HTML:
<script type="text/javascript" >
$(function() {
$("#recbutton").on("click", function() {
$("#entrance").hide();
$("#live").fadeIn("slow");
toggleRecording(this);
$(this).toggle();
return $("#stopbutton").toggle();
});
return $("#stopbutton").on("click", function() {
audioRecorder.stop();
$(this).toggle();
$("#recbutton").toggle();
$("#live").hide();
return $("#entrance").fadeIn("slow");
});
});
</script>
And as you can see, I call the toggleRecording(this) function (the one wich starts the recording process) only after the #recbutton is pressed. Now, everything works fine with this code BUT, the user gets prompted for microphone permission as soon as the page is loaded and I want to ask them for permission to use the microphone ONLY AFTER they clicked the #recbutton Do you understand me? I tought that if I remove the last line of the first file:
window.addEventListener('load', initAudio );
and modify my embedded script like this:
<script type="text/javascript" >
$(function() {
$("#recbutton").on("click", function() {
$("#entrance").hide();
$("#live").fadeIn("slow");
initAudio();
toggleRecording(this);
$(this).toggle();
return $("#stopbutton").toggle();
});
return $("#stopbutton").on("click", function() {
audioRecorder.stop();
$(this).toggle();
$("#recbutton").toggle();
$("#live").hide();
return $("#entrance").fadeIn("slow");
});
});
</script>
I might be able to achieve what I wanted, and actually I am, the user doesn't get prompted for his/her microphone until they click the #recbutton. The problem is, the audio never get's recorded, when you try to download it, the resulting WAV it is empty.
How can I fix this?
My project's code is at: https://github.com/Jmlevick/html-recorder

No, your problem is that getUserMedia() has an asynchronous callback (gotMedia()); you need to have the rest of your code logic in the startbutton call (the toggleRecording bit, in particular) inside that callback, because right now it's getting executed before getUserMedia returns (and sets up the audio nodes).

I found an elegant & easy solution for this (or at least I see it that way):
What I did was toss "main.js" and "recorder.js" inside a getScript call that is executed only when a certain button (#button1) is clicked by the user... These scripts do not get loaded with the webpage itself until the button it's pressed, but we need some more nifty tricks to make it work the way I described and wanted above:
in main.js, I changed:
window.addEventListener('load', initAudio );
for:
window.addEventListener('click', initAudio );
So when the scripts are loaded into the page with getScript the "main.js" file now listens for a click event in the webpage to ask the user for the microphone. Next, I had to create a hidden button (#button2) on the page wich is fakely clicked by jQuery exactly right after the scripts are loaded on the page, so it triggers the "ask for microphone permisson" event and then, just below that line of code wich generates the fake click I added:
window.removeEventListener("click", initAudio, false);
so the "workflow" for this trick ends up as follows:
User presses a button wich loads the necesary js files into the page with getScript, it's worth mentioning that now the "main.js" file listens for a click event on the window instead of a load one.
We have a hidden button wich is "fakely clicked" by jQuery just in the moment you click the first one, so it triggers the permisson event for the user.
Once this event is triggered, the click event listener is removed from the window, so it never fires the "ask for permisson" event again when the user clicks anywhere on the page.
And basically that's all folks! :) now when the user goes into the page he/she never get asked for microphone permisson until they click a "Rec" button on the page just as I wanted. With one click of the user we do 3 things in jQuery, but for the user it seems like nothing happened other that the "microphone permisson message" appearing on the screen instantly right after they click the "Rec" Button.

2 or more async requests in javascript to load image or do anything else

Single request and response model at one time do not utilizes full network/internet bandwidth, thus resulting in low performance. (benchmark is of half speed utilization)
how to make this code use 2 or 3 or more async requests instead of one.(ajax)
or do i need multi threading? and is it possible in javascript?
(this is for making a video out of an ip )
every time the image changes on request. and yes i need to be async with multiple fetch requests (not single as i explained above) or you recomend threads?
<html>
<head> <script language="JavaScript">
// Global vars
img = 'http://pastebin.com/i/t.gif';
timeout = 1000;
next = 0;
function onLoad( ) {
setTimeout( 'reloadImage( )', timeout );
}
// Reloader
function reloadImage( ) {
next = ( new Date( ) ).getTime( ) + timeout;
document.images.dv.src = img + "?" + next;
}
</script>
</head>
<body>
<img src="img" name="dv" onLoad="onLoad( )">
</body>
</html>
and
<html>
<head>
</head>
<body>
<div id="container"></div>
<script language="JavaScript">
var canLoad = true;
var container = document.getElementById("container");
var image = document.createElement("img");
image.onload = function() {
canLoad = true;
console.log("Image reloaded.");
}
var imageUrl = "http://url/snapshot.jpg";
var fps = 2;
container.appendChild(image);
function loadImage() {
if (canLoad) {
canLoad = false;
var str = new Date().getTime();
image.setAttribute("src", imageUrl + "?" + str);
console.log("Reloaded now.");
} else {
console.log("Can't reload now.");
}
}
setInterval(loadImage, fps); // 30 fps
</script>
</body>
</html>

Not actually tested, and I think it'll very likely to cause a "stack overflow" eventually (if you directly implement it), but you may still give it a look:
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
(function(){
var img="/*url*/";
var interval=50;
var pointer=0;
function showImg(image,idx)
{
if(idx<=pointer) return;
document.body.replaceChild(image,document.getElementsByTagName("img")[0]);
pointer=idx;
preload();
}
function preload()
{
var cache=null,idx=0;;
for(var i=0;i<5;i++)
{
idx=Date.now()+interval*(i+1);
cache=new Image();
cache.onload=(function(ele,idx){return function(){showImg(ele,idx);};})(cache,idx);
cache.src=img+"?"+idx;
}
}
window.onload=function(){
document.getElementsByTagName("img")[0].onload=preload;
document.getElementsByTagName("img")[0].src="/*initial url*/";
};
})();
</script>
</head>
<body>
<img />
</body>
</html>
What it does:
When the initial image loads, preload() is called;
When preload() is called, it creates 5 image cache, and each attach its onload event to showImg();
When showImg() is called, it checks whether the current index is behind current pointer, and if it does, replace the current image with this new one, and call preload();
Back to 2.
If you really going to implement this, increase interval and decrease i<5. Also, a caching/queuing mechanic to check how many images in cache/queue before loading the next queue would be nice.
Also, notice that I didn't use getElementById to get the image, because there will be no stable ID.

Asset.images slow? How do I execute functions so they don't freeze the browser?

I recently downloaded a nice mootools plugin to provide a rating system for search results on my site: MooStarRating.
It works quite well, but it is very slow to initialize. Here are the execution times I have logged (for pulling back 50 search results).
======== starrating ========
init: 0.06ms 50
img: 5.40ms 50
str: 0.54ms 50
each: 3.04ms 50
inject: 0.86ms 50
end: 1.52ms 50
subtotal: 11.42ms 50
-----------------
total: 571.00ms
Here is the initialize function these logs refer to (just for reference):
initialize: function (options) {
lstart("starrating");
// Setup options
this.setOptions(options);
// Fix image folder
if ((this.options.imageFolder.length != 0) && (this.options.imageFolder.substr(-1) != "/"))
this.options.imageFolder += "/";
// Hover image as full if none specified
if (this.options.imageHover == null) this.options.imageHover = this.options.imageFull;
lrec("init");
// Preload images
try {
Asset.images([
this.options.imageEmpty,
this.options.imageFull,
this.options.imageHover
]);
} catch (e) { };
lrec("img");
// Build radio selector
var formQuery = this.options.form;
this.options.form = $(formQuery);
if (!this.options.form) this.options.form = $$('form[name=' + formQuery + "]")[0];
if (this.options.form) {
var uniqueId = 'star_' + String.uniqueID();
this.options.form.addClass(uniqueId);
this.options.selector += 'form.' + uniqueId + ' ';
}
this.options.selector += 'input[type=radio][name=' + this.options.radios + "]";
// Loop elements
var i = 0;
var me = this;
var lastElement = null;
var count = $$(this.options.selector).length;
var width = this.options.width.toInt();
var widthOdd = width;
var height = this.options.height.toInt();
if (this.options.half) {
width = (width / 2).toInt();
widthOdd = widthOdd - width;
}
lrec("str");
$$(this.options.selector).each(function (item) {
// Add item to radio list
this.radios[i] = item;
if (item.get('checked')) this.currentIndex = i;
// If disabled, whole star rating control is disabled
if (item.get('disabled')) this.options.disabled = true;
// Hide and replace
item.setStyle('display', 'none');
this.stars[i] = new Element('a').addClass(this.options.linksClass);
this.stars[i].store('ratingIndex', i);
this.stars[i].setStyles({
'background-image': 'url("' + this.options.imageEmpty + '")',
'background-repeat': 'no-repeat',
'display': 'inline-block',
'width': ((this.options.half && (i % 2)) ? widthOdd : width),
'height': height
});
if (this.options.half)
this.stars[i].setStyle('background-position', ((i % 2) ? '-' + width + 'px 0' : '0 0'));
this.stars[i].addEvents({
'mouseenter': function () { me.starEnter(this.retrieve('ratingIndex')); },
'mouseleave': function () { me.starLeave(); }
});
// Tip
if (this.options.tip) {
var title = this.options.tip;
title = title.replace('[VALUE]', item.get('value'));
title = title.replace('[COUNT]', count);
if (this.options.tipTarget) this.stars[i].store('ratingTip', title);
else this.stars[i].setProperty('title', title);
}
// Click event
var that = this;
this.stars[i].addEvent('click', function () {
if (!that.options.disabled) {
me.setCurrentIndex(this.retrieve('ratingIndex'));
me.fireEvent('click', me.getValue());
}
});
// Go on
lastElement = item;
i++;
}, this);
lrec("each");
// Inject items
$$(this.stars).each(function (star, index) {
star.inject(lastElement, 'after');
lastElement = star;
}, this);
lrec("inject");
// Enable / disable
if (this.options.disabled) this.disable(); else this.enable();
// Fill stars
this.fillStars();
lrec("end");
return this;
},
So, the slowest part of the function is this:
// Preload images
try {
Asset.images([
this.options.imageEmpty,
this.options.imageFull,
this.options.imageHover
]);
} catch (e) { };
Which is strange. What does Asset.images do? Does the script block until these images have been loaded by the browser? Is there a way to preload images that runs faster?
How can I make the scripts on my page execute faster? It is a big problem for them to take 800ms to execute, but 200ms is still quite bad. At the moment, my search results all pop into existence at once. Is it possible to make it so that individual search results are created separately, so that they don't block the browser while being created? Similarly, is it possible to do this for the individual components of the search results, such as the MooStarRating plugin?

no. Asset.images is non-blocking as each of these gets loaded separately and a singular event is being dispatched when all done.
the speed for loading will be dependent on the browser but it be will multi-threaded to whatever capability it has for parallel downloading from the same host.
https://github.com/mootools/mootools-more/blob/master/Source/Utilities/Assets.js#L115-129
immediately, it returns an Element collection with the PROMISE of elements that may still be downloading. that's fine - you can use it to inject els, attach events, classes etc - you just cannot read image properties yet like width, height.
each individual image has it's own onload that fires an onProgress and when all done, an onComplete for the lot - i would advise you to enable that, remove the try/catch block and see which image is creating a delay. you certainly don't need to wait for anything from Asset.images to come back.
you also seem to be using it as a 'prime the cache' method, more than anything - as you are NOT really saving a reference into the class instance. your 'each' iteration can probably be optimised so it uses half the time if objects and functions are cached and so are references. probably more if you can use event delegation.
To answer your questions about not freezing the browser due to the single-threaded nature of javascript, you defer the code via setTimeout (or Function.delay in mootools) with a timer set to 0 or a 10ms due to browser interpretations. You also write the function to to exec a callback when done, in which you can pass the function result, if any (think ajax!).