Introduction
Greetings, I have looked at some similar questions that are on this platform and none of them seem to match my problem or maybe I missed something but I will try my best to give it a try because I am new in this platform.
OverView
This audio app maps the keyboard keys to audio samples.
Problem
There is a audio delay when pressing the keys rapidly and after each press the sample sound keeps playing for 4 to 6 second before it calls another sound and doesn't kill the sound immediately after the keyboard button is released.
Code
This is what I tried:
const dump = console.log.bind(console);
const sample = new Object
({
drum: "/samples/drums/trance01/BD_Trance.wav",
clap: "/samples/drums/trance01/Clap_trance.wav",
});
const keymap = new Object
({
"KeyD": "drum",
"KeyC": "clap",
});
Object.keys(sample).forEach((smpl)=>
{
let node = document.createElement("audio");
node.id = (smpl+"Sample");
node.className = "instrument";
node.src = sample[smpl];
document.body.appendChild(node);
});
document.body.addEventListener("keydown", function keyListener(event)
{
event.preventDefault(); // kill it
event.stopPropagation(); // seal it's ashes in a capsule
event.stopImmediatePropagation(); // and hurl it into the sun!
let key = event.code;
// console.log("pressed: "+key);
let tgt = keymap[key];
if (!tgt){ dump(key+" - is unused"); return };
var intervalID = setInterval(myCallback, 500, 'Parameter 1', 'Parameter 2');
function myCallback(a, b)
{
let nde = document.getElementById(tgt+"Sample");
nde.play();
dump("play: "+tgt);
}
});
Following what happens in the logic and how elements respond in their own way (and time) is important for analysing what the issue may be.
In this case I believe this can be solved by cloning the source-node, not expecting it to play multiple instances of itself by itself (or that is what I believe should happen) - but we can force it to:
const dump = console.log.bind(console);
const sample = new Object
({
drum: "/samples/drums/trance01/BD_Trance.wav",
clap: "/samples/drums/trance01/Clap_trance.wav",
});
const keymap = new Object
({
"KeyD": "drum",
});
(Object.keys(sample)).forEach((item,indx)=>
{
let node = document.createElement("audio");
node.id = (item+"Sample");
node.className = "instrument";
node.src = sample[item];
document.body.appendChild(node);
});
function keyHandler(event)
{
if (event.ctrlKey){ return }; // whew .. that was annoying as f*ck
event.preventDefault(); // kill it
event.stopPropagation(); // seal it's ashes in a capsule
event.stopImmediatePropagation(); // and hurl it into the sun!
let key = event.code;
let tag = keymap[key];
let nde,tgt;
tgt = document.getElementById(tag+"Sample");
if (!tgt){ dump(key+" - is unused"); return };
nde = tgt.cloneNode(true);
nde.id = (tgt.id+"Clone");
nde.addEventListener("ended",function()
{
this.parentNode.removeChild(this);
});
document.body.appendChild(nde);
nde.play();
dump("playing: "+tgt);
}
document.body.addEventListener("keydown", keyHandler);
I want to sort an array, using Web Workers. But this array might receive new values over time, while the worker is still performing the sort function.
So my question is, how can I "stop" the sorting computation on the worker after receiving the new item, so it can perform the sort on the array with that item, while still keeping the sorting that was already made?
Example:
let worker = new Worker('worker.js');
let list = [10,1,5,2,14,3];
worker.postMessage({ list });
setInterval(() => worker.postMessage({ num: SOME_RANDOM_NUM, list }), 100);
worker.onmessage = event => {
list = event.data.list;
}
So lets say that, I've passed 50, the worker made some progress in the sorting before that and now I have something like this:
[1, 2, 3, 10, 5, 14, 50]. Which means the sorting stopped at index 3. So I pass this new array back to the worker, so it can continue the sorting from position 3.
How can I accomplish that, since there is no way to pause/resume a web worker?
Even though the Worker works on an other thread than the one of your main page, and can thus run continuously without blocking the UI, it still runs on a single thread.
This means that until your sort algorithm has finished, the Worker will delay the execution of the message event handler; it is as blocked as would be the main thread.
Even if you made use of an other Worker from inside this worker, the problem would be the same.
The only solution would be to use a kind of generator function as the sorter, and to yield it every now and then so that the events can get executed.
But doing this will drastically slow down your sorting algorithm.
To make it better, you could try to hook to each Event Loop, thanks to a MessageChannel object: you talk in one port and receive the message in the next Event loop. If you talk again to the other port, then you have your own hook to each Event loop.
Now, the best would be to run a good batch in every of these Event loop, but for demo, I'll call only one instance of our generator function (that I borrowed from this Q/A)
const worker = new Worker(getWorkerURL());
worker.onmessage = draw;
onclick = e => worker.postMessage(0x0000FF/0xFFFFFF); // add a red pixel
// every frame we request the current state from Worker
function requestFrame() {
worker.postMessage('gimme a frame');
requestAnimationFrame(requestFrame);
}
requestFrame();
// drawing part
const ctx = canvas.getContext('2d');
const img = ctx.createImageData(50, 50);
const data = new Uint32Array(img.data.buffer);
ctx.imageSmoothingEnabled = false;
function draw(evt) {
// converts 0&1 to black and white pixels
const list = evt.data;
list.forEach((bool, i) =>
data[i] = (bool * 0xFFFFFF) + 0xFF000000
);
ctx.setTransform(1,0,0,1,0,0);
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.putImageData(img,0,0);
// draw bigger
ctx.scale(5,5);
ctx.drawImage(canvas, 0,0);
}
function getWorkerURL() {
const script = document.querySelector('[type="worker-script"]');
const blob = new Blob([script.textContent]);
return URL.createObjectURL(blob);
}
body{
background: ivory;
}
<script type="worker-script">
// our list
const list = Array.from({length: 2500}).map(_=>+(Math.random()>.5));
// our sorter generator
let sorter = bubbleSort(list);
let done = false;
/* inner messaging channel */
const msg_channel = new MessageChannel();
// Hook to every Event loop
msg_channel.port2.onmessage = e => {
// procede next step in sorting algo
// could be a few thousands in a loop
const state = sorter.next();
// while running
if(!state.done) {
msg_channel.port1.postMessage('');
done = false;
}
else {
done = true;
}
}
msg_channel.port1.postMessage("");
/* outer messaging channel (from main) */
self.onmessage = e => {
if(e.data === "gimme a frame") {
self.postMessage(list);
}
else {
list.push(e.data);
if(done) { // restart the sorter
sorter = bubbleSort(list);
msg_channel.port1.postMessage('');
}
}
};
function* bubbleSort(a) { // * is magic
var swapped;
do {
swapped = false;
for (var i = 0; i < a.length - 1; i++) {
if (a[i] > a[i + 1]) {
var temp = a[i];
a[i] = a[i + 1];
a[i + 1] = temp;
swapped = true;
yield swapped; // pause here
}
}
} while (swapped);
}
</script>
<pre> click to add red pixels</pre>
<canvas id="canvas" width="250" height="250"></canvas>
Note that the same can be achieved with an async function, which may be more practical in some cases:
const worker = new Worker(getWorkerURL());
worker.onmessage = draw;
onclick = e => worker.postMessage(0x0000FF/0xFFFFFF); // add a red pixel
// every frame we request the current state from Worker
function requestFrame() {
worker.postMessage('gimme a frame');
requestAnimationFrame(requestFrame);
}
requestFrame();
// drawing part
const ctx = canvas.getContext('2d');
const img = ctx.createImageData(50, 50);
const data = new Uint32Array(img.data.buffer);
ctx.imageSmoothingEnabled = false;
function draw(evt) {
// converts 0&1 to black and white pixels
const list = evt.data;
list.forEach((bool, i) =>
data[i] = (bool * 0xFFFFFF) + 0xFF000000
);
ctx.setTransform(1,0,0,1,0,0);
ctx.clearRect(0,0,canvas.width,canvas.height);
ctx.putImageData(img,0,0);
// draw bigger
ctx.scale(5,5);
ctx.drawImage(canvas, 0,0);
}
function getWorkerURL() {
const script = document.querySelector('[type="worker-script"]');
const blob = new Blob([script.textContent]);
return URL.createObjectURL(blob);
}
body{
background: ivory;
}
<script type="worker-script">
// our list
const list = Array.from({length: 2500}).map(_=>+(Math.random()>.5));
// our sorter generator
let done = false;
/* outer messaging channel (from main) */
self.onmessage = e => {
if(e.data === "gimme a frame") {
self.postMessage(list);
}
else {
list.push(e.data);
if(done) { // restart the sorter
bubbleSort(list);
}
}
};
async function bubbleSort(a) { // async is magic
var swapped;
do {
swapped = false;
for (var i = 0; i < a.length - 1; i++) {
if (a[i] > a[i + 1]) {
const temp = a[i];
a[i] = a[i + 1];
a[i + 1] = temp;
swapped = true;
}
if( i % 50 === 0 ) { // by batches of 50?
await waitNextTask(); // pause here
}
}
} while (swapped);
done = true;
}
function waitNextTask() {
return new Promise( (resolve) => {
const channel = waitNextTask.channel ||= new MessageChannel();
channel.port1.addEventListener("message", (evt) => resolve(), { once: true });
channel.port2.postMessage("");
channel.port1.start();
});
}
bubbleSort(list);
</script>
<pre> click to add red pixels</pre>
<canvas id="canvas" width="250" height="250"></canvas>
There are two decent options.
Option 1: Worker.terminate()
The first is just to kill your existing web worker and start a new one. For that you can use Worker.terminate().
The terminate() method of the Worker interface immediately terminates the Worker. This does not offer the worker an opportunity to finish its operations; it is simply stopped at once.
The only downsides of this approach are:
You lose all worker state. If you had to copy a load of data into it for the request you have to do it all again.
It involves thread creation and destruction, which isn't as slow as most people think but if you terminate web workers a lot it might cause issues.
If neither of those are an issue it is probably the easiest option.
In my case I have lots of state. My worker is rendering part of an image, and when the user pans to a different area I want it to stop what it is doing and start rendering the new area. But the data needed to render the image is pretty huge.
In your case you have the state of your (presumably huge) list that you don't want to use.
Option 2: Yielding
The second option is basically to do cooperative multitasking. You run your computation as normal, but every now and then you pause (yield) and say "should I stop?", like this (this is for some nonsense calculation, not sorting).
let requestId = 0;
onmessage = event => {
++requestId;
sortAndSendData(requestId, event.data);
}
function sortAndSendData(thisRequestId, data) {
let isSorted = false;
let total = 0;
while (data !== 0) {
// Do a little bit of computation.
total += data;
--data;
// Check if we are still the current request ID.
if (thisRequestId !== requestId) {
// Data was changed. Cancel this sort.
return;
}
}
postMessage(total);
}
This won't work though because sortAndSendData() runs to completion and blocks the web worker's event loop. We need some way to yield just before thisRequestId !== requestId. Unfortunately Javascript doesn't quite have a yield method. It does have async/await so we might try this:
let requestId = 0;
onmessage = event => {
console.log("Got event", event);
++requestId;
sortAndSendData(requestId, event.data);
}
async function sortAndSendData(thisRequestId, data) {
let isSorted = false;
let total = 0;
while (data !== 0) {
// Do a little bit of computation.
total += data;
--data;
await Promise.resolve();
// Check if we are still the current request ID.
if (thisRequestId !== requestId) {
console.log("Cancelled!");
// Data was changed. Cancel this sort.
return;
}
}
postMessage(total);
}
Unfortunately it doesn't work. I think it's because async/await executes things eagerly using "microtasks", which get executed before pending "macrotasks" (our web worker message) if possible.
We need to force our await to become a macrotask, which you can do using setTimeout(0):
let requestId = 0;
onmessage = event => {
console.log("Got event", event);
++requestId;
sortAndSendData(requestId, event.data);
}
function yieldToMacrotasks() {
return new Promise((resolve) => setTimeout(resolve));
}
async function sortAndSendData(thisRequestId, data) {
let isSorted = false;
let total = 0;
while (data !== 0) {
// Do a little bit of computation.
total += data;
--data;
await yieldToMacrotasks();
// Check if we are still the current request ID.
if (thisRequestId !== requestId) {
console.log("Cancelled!");
// Data was changed. Cancel this sort.
return;
}
}
postMessage(total);
}
This works! However it is extremely slow. await yieldToMacrotasks() takes approximately 4 ms on my machine with Chrome! This is because browsers set a minimum timeout on setTimeout(0) of something like 1 or 4 ms (the actual minimum seems to be complicated).
Fortunately another user pointed me to a quicker way. Basically sending a message on another MessageChannel also yields to the event loop, but isn't subject to the minimum delay like setTimeout(0) is. This code works and each loop only takes ~0.04 ms which should be fine.
let currentTask = {
cancelled: false,
}
onmessage = event => {
currentTask.cancelled = true;
currentTask = {
cancelled: false,
};
performComputation(currentTask, event.data);
}
async function performComputation(task, data) {
let total = 0;
let promiseResolver;
const channel = new MessageChannel();
channel.port2.onmessage = event => {
promiseResolver();
};
while (data !== 0) {
// Do a little bit of computation.
total += data;
--data;
// Yield to the event loop.
const promise = new Promise(resolve => {
promiseResolver = resolve;
});
channel.port1.postMessage(null);
await promise;
// Check if this task has been superceded by another one.
if (task.cancelled) {
return;
}
}
// Return the result.
postMessage(total);
}
I'm not totally happy about it - it relies on postMessage() events being processed in FIFO order, which I doubt is guaranteed. I suspect you could rewrite the code to make it work even if that isn't true.
You can do it with some trick – with the help of setTimeout function interrupting. For example it is not possible without an addition thread to execute 2 functions parallel, but with setTimeout function interrupting trick we can do it like follows:
Example of parallel execution of functions
var count_0 = 0,
count_1 = 0;
function func_0()
{
if(count_0 < 3)
setTimeout(func_0, 0);//the same: setTimeout(func_0);
console.log('count_0 = '+count_0);
count_0++
}
function func_1()
{
if(count_1 < 3)
setTimeout(func_1, 0);
console.log('count_1 = '+count_1)
count_1++
}
func_0();
func_1();
You will get this output:
count_0 = 0
count_1 = 0
count_0 = 1
count_1 = 1
count_0 = 2
count_1 = 2
count_0 = 3
count_1 = 3
Why is it possible? Because the setTimeout function needs some time to be executed. And this time is even enought for the execution of some part from your following code.
Solution for you
For this case you have to write your own array sort function (or you can also use the following function from me) because we can not interrupt the native sort function. And in this your own function you have to use this setTimeout function interrupting trick. And you can receive your message event notification.
In the following example I have the interrupting in the half length of my array, and you can change it if you want.
Example with custom sort function interrupting
var numbers = [4, 2, 1, 3, 5];
// this is my bubble sort function with interruption
/**
* Sorting an array. You will get the same, but sorted array.
* #param {array[]} arr – array to sort
* #param {number} dir – if dir = -1 you will get an array like [5,4,3,2,1]
* and if dir = 1 in opposite direction like [1,2,3,4,5]
* #param {number} passCount – it is used only for setTimeout interrupting trick.
*/
function sortNumbersWithInterruption(arr, dir, passCount)
{
var passes = passCount || arr.length,
halfOfArrayLength = (arr.length / 2) | 0; // for ex. 2.5 | 0 = 2
// Why we need while loop: some values are on
// the end of array and we have to change their
// positions until they move to the first place of array.
while(passes--)
{
if(!passCount && passes == halfOfArrayLength)
{
// if you want you can also not write the following line for full break of sorting
setTimeout(function(){sortNumbersWithInterruption(arr, dir, passes)}, 0);
/*
You can do here all what you want. Place 1
*/
break
}
for(var i = 0; i < arr.length - 1; i++)
{
var a = arr[i],
b = arr[i+1];
if((a - b) * dir > 0)
{
arr[i] = b;
arr[i+1] = a;
}
}
console.log('array is: ' + arr.join());
}
if(passCount)
console.log('END sring is: ' + arr.join());
}
sortNumbersWithInterruption(numbers, -1); //without passCount parameter
/*
You can do here all what you want. Place 2
*/
console.log('The execution is here now!');
You will get this output:
array is: 4,2,3,5,1
array is: 4,3,5,2,1
The execution is here now!
array is: 4,5,3,2,1
array is: 5,4,3,2,1
END sring is: 5,4,3,2,1
You can do it with insertion sort (kind of).
Here is the idea:
Start your worker with an internal empty array (empty array is sorted obviously)
Your worker receives only elements not the entire array
Your worker insert any received element right in correct position into the array
Every n seconds, the worker raises a message with the current array if it has changed after the last event. (If you prefer, you can send the array on every insertion, but is more efficient to buffer somehow)
Eventually, you get the entire array, if any item is added, you will receive the updated array to.
NOTE: Because your array is always sorted, you can insert in correct position using binary search. This is very efficient.
I think the case comes down to careful management of postMessage calls and amount of data passed to be processed at a time. Was dealing with problem of this kind - think about not sending all new data into the function at once but rather creating your own queue and when small enough portion of the task has been acomplished by webworker thread send a message back to the main thread and decide to send the next portion, wait or quit.
In Your case, e.g. one time You get 9000 new items, next 100k - maybe create a queue/buffer that adds next 10k new elements each time webworker is done processing last data change.
const someWorker = new Worker('abc.js');
var processingLock = false;
var queue = [];
function newDataAction(arr = null) {
if (arr != null) {
queue = queue.concat(arr);
}
if (!processingLock) {
processingLock = true;
var data = [];
for (let i = 0; i < 10000 && queue.length > 0; i++) {
data.push(queue.pop());
}
worker.postMessage(data);
}
}
someWorker.addEventListener('message', function(e) {
if (e.data == 'finished-last-task') {
processingLock = false;
if (queue.length > 0) {
newDataAction();
}
}
});
Worked through many sorting algorithms and I don't see how sending new data into an sorting algorithm with partially sorted array makes much difference in terms of compuation time from sorting them both sequentially and performing a merge.
I am wondering if an id of a div is 1 and someone changed the id to 34, when someone clicks on it the attr('id') still equals to 1 even after change instead of 34. Apologize for my english thank you.
$(document).on('click','.edit_btn:eq(0)',function(){
var itm_id = $(this).siblings('input').attr('id');
alert(itm_id);
});
Want the id to equal to 1 instead of 34 even after someone changes it with DOM
You could use a MutationObserver to see if the attribute changes, then change it back. Make sure when you change it back, it knows to ignore that change. Or you will lag the browser out. Note, that this could be disabled by any monkey with dev tools.
Don't rely on the client.
let i = 0;
const MutationObserver = window.MutationObserver || window.WebKitMutationObserver || window.MozMutationObserver;
let e = document.querySelector('#test');
setInterval(() => {
i++;
// Change
e.setAttribute('id', i);
}, 1000); // Loop Sleep
let observer = new MutationObserver((mutations) => {
mutations.forEach((mutation) => {
if (mutation.type === "attributes") {
if (e.id !== 'test') {
let previous = e.id;
// Revert
e.setAttribute('id', 'test');
console.log('Changed to test from', previous)
}
}
});
});
observer.observe(e, {
attributes: true
});
<div id="test">Test</div>
I'm writing a function using the mutation observer with jQuery to register changes to the DOM, specifically when a new node is added so i can change its content:
$("SELeCTOR GOOD" ).click(function(){
var targetNode = $(this).find('.content').get(0);
var config = { attributes: true, childList: true, subtree: true, attributeOldValue: true };
// Callback function to execute when mutations are observed
var callback = function(mutationsList) {
for(var mutation of mutationsList) {
if (mutation.type == 'childList') {
var courses = $(targetNode).find('.courses').get(0);
$(courses).find('.coursebox.clearfix').each(function( index,item ) {
var attacherURL = $(item).find('.coursename a').attr('href');
var moreInfoURL = '<a class="btn btn-outline-primary pull-right" href="'+attacherURL+'">More info</a>';
var oldHTML = $(item).find('div.moreinfo').html();
var newHTML = moreInfoURL + oldHTML;
//this following line is supposed to replace the html, but it creates an infinite loop
$(item).find('div.moreinfo').html(newHTML);
//end
});
}
else if (mutation.type == 'attributes') {
console.log('The ' + mutation.attributeName + ' attribute was modified.');
}
}
};
I've tried append/prepend as well, but everything creates the same infinite loop. As usual, any help is greatly appreciated.
Regards
Well, your modification causes another mutation, which results in you modifying it again, causing an infinite loop. A simple solution is to add a class to your element to mark it as already processed, and ignore already processed nodes (nodes that have that class). Another is just check if the dev.morinfo alreade has moreInfoURL inside it, and ignore if it already does
I'm trying to monitor the DOM element on a third-party website. The element is non-existent until a countdown timer reaches, then it is created.
I've had some success playing around with: document.getElementsByClassName('countdown ng-hide').length
when it changes from 0 to 1 I want to effect a function.
How would I do this? I've tried using a Mutation Observer but it won't let me observe a null node.
Thanks!
EDIT: This is what I've got so far.
var timesDone = 0;
var songID = 0;
function clickit(xsongID) {
if(document.getElementsByClassName('lottery-countdown ng-hide').length == 1) {
document.getElementsByClassName('media submission ng-scope')[xsongID].click(); songName = document.getElementsByClassName('media-title submission-name ng-binding')[xsongID].outerHTML; timesDone++; }
}
setInterval(clickit, 29900, songID);
I did this recently by setting up an Interval function like this :
var timesTest = 0;
var checkExists = setInterval(function() {
if ($('.yourClassElement').length) {
// ok element found : do your stuff and clear the Interval...
// stuff...
clearInterval(checkExists);
timesTest = 0;
}
timesTest++;
// I won't let this run more than 5 seconds, so :
if (timesTest*100 > 5000)
clearInterval(checkExists);
}, 100);