问题
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?
回答1:
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>回答2:
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
回答3:
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.
来源：https://stackoverflow.com/questions/54478195/how-to-allow-web-workers-to-receive-new-data-while-it-still-performing-computati