I am attempting to build a webapp on a Chromebook, I need it to read RFID card serial numbers with an ACR122U NFC. I am using chrome-nfc.
I am reading cards happily, but I do not know how to fire an event when a card is presented.
Are there any events in chrome-nfc I can use to know when a card has been presented to the reader?
EDIT: I have been trying to use chrome.nfc.wait_for_tag, but it does not behave as I would expect.
// With a card on the reader
chrome.nfc.wait_for_tag(device, 10000, function(tag_type, tag_id){
var CSN = new Uint32Array(tag_id)[0];
console.log ( "CSN: " + CSN );
});
[DEBUG] acr122_set_timeout(round up to 1275 secs)
DEBUG: InListPassiveTarget SENS_REQ(ATQA)=0x4, SEL_RES(SAK)=0x8
DEBUG: tag_id: B6CA9B6B
DEBUG: found Mifare Classic 1K (106k type A)
[DEBUG] nfc.wait_for_passive_target: mifare_classic with ID: B6CA9B6B
CSN: 1805372086
// with no card on the reader
chrome.nfc.wait_for_tag(device, 10000, function(tag_type, tag_id){
var CSN = new Uint32Array(tag_id)[0];
console.log ( "CSN: " + CSN );
});
[DEBUG] acr122_set_timeout(round up to 1275 secs)
DEBUG: found 0 target, tg=144
Both return the results as above immediately, it does not seem to matter what number I use for a timeout...
If I call the function with no card on the reader, and then immediately put the card on the reader after function call, I get no output in the console.
I'm not familiar with chrome-nfc, but taking a shot in the dark by reverse engineering the source, it looks like you would want to use the wait_for_tag method, like:
chrome.nfc.wait_for_tag(device, 3000, function(tag_type, tag_id) {
// Do your magic here.
});
...Where device is your reader, 3000 is the maximum time to wait (in ms), and replacing // Do your magic here. with your desired logic. If it times out, both tag_type and tag_id will be null.
If you wanted to wait indefinitely, you could just recursively call a function with the above code. Example:
function waitAllDay(device) {
chrome.nfc.wait_for_tag(device, 1000, function(tag_type, tag_id) {
if(tag_type !== null && tag_id !== null)
{
// Do your magic here.
}
waitAllDay(device);
});
}
That's assuming you want it to continue waiting even after a tag has been presented. Wrap the waitAllDay(device); in an else if you want it to stop once a tag is read.
UPDATE: It seems the wait_for_tag method does not work as intended, so I'm proposing a second solution. I'm leaving the existing solution in place in case the method is fixed by the developers of chrome-nfc.
Another thing to try is to use chrome.nfc.read, passing in a timeout option, inside a window.setInterval.
var timer = window.setInterval(function () {
chrome.nfc.read(device, { timeout: 1000 }, function(type, ndef) {
if(!!type && !!ndef) {
// Do your magic here.
// Uncomment the next line if you want it to stop once found.
// window.clearInterval(timer);
}
});
}, 1000);
Be sure and call window.clearInterval(timer) whenever you want it to stop watching for tags.
While I do not consider this a proper solution; here is a workaround I am using for the time being.
function listen_for_tag(callback, listen_timeout){
var poll_delay = 400; //ms
var listen_loop = null;
if(!listen_timeout){
listen_timeout = 99999999;
}
function check_for_tag(){
if(listen_timeout < 0) {
clearInterval(listen_loop);
console.log("we didnt find a tag. finished");
}
chrome.nfc.wait_for_tag(dev_manager.devs[0].clients[0], 10, function(tag_type, tag_id){
console.log ( "FOUND A TAG!!" );
clearInterval(listen_loop);
// handle the callback (call it now)
var C = callback;
if (C) {
callback = null;
window.setTimeout(function() {
C(tag_type, tag_id);
}, 0);
}
});
listen_timeout -= poll_delay;
}
listen_loop = setInterval(check_for_tag, poll_delay);
}
I have a requirement where I need to poll the database via ajax from js to check for a status. If the status is "active" then the polling should stop and an alert should popup "case is now active". The js should check the db every 2 seconds until the db status returns "active." Can you provide an elegant js routine for this? Here's some general js to show what I want to do:
function ReportAsActivePoll()
{
for(var i=0; i<10; i++)
{
setTimeout(StatusIsActive,(i*2000));
if(statusIsActive)
{
ReportAsActive();
break;
}
}
}
var statusIsActive = false;
function StatusIsActive(case)
{
statusIsActive = GetStatusFromDB(case) == "active";
}
function ReportAsActive()
{
alert("case is now active")
}
A few notes:
I know the code above is not correct. It's just for illustrative purposes.
The code above will call StatusIsActive 10 times. I would like the calls to stop/break/discontinue after status is active. However, I think polling requires to queue up all the calls ahead of time so I'm not sure how to achieve this.
Use setInterval() and clearInterval() for simplicity. Like so:
<script type="text/javascript">
function checkStatus(theCase) {
var intervalId = window.setInterval(function() {
if (getStatusFromDb(theCase) == 'active') {
clearInterval(intervalId)
reportAsActive()
}
}, 2000)
}
function reportAsActive()
{
alert("case is now active")
}
var tmpCounter = 0
function getStatusFromDb(theCase)
{
if (tmpCounter++ == 4) return "active"
}
checkStatus('case 123')
</script>
You should also consider making functions start with a lowercase letter, because that is the normal JS convention. By choosing another style, you risk having case-sensitive errors that are annoying to track down.
You need to use setInterval instead of your setTimeout and when you received a valid response you have to remove this interval with clearInterval.
So you need to do something like this
var intervalID = window.setInterval(function(){
var resFromYourDB = ...; // get your result via ajax
if (resFromYourDB['active']){
window.clearInterval(intervalID);
// do you alert
}
}, 2000)
This way it will be polling your server till it will get active as a response and not a predefined amount of time as with setTimeout. Also when it will get this response it will properely stops.
I have a button which runs a long running function when it's clicked. Now, while the function is running, I want to change the button text, but I'm having problems in some browsers like Firefox, IE.
html:
<button id="mybutt" class="buttonEnabled" onclick="longrunningfunction();"><span id="myspan">do some work</span></button>
javascript:
function longrunningfunction() {
document.getElementById("myspan").innerHTML = "doing some work";
document.getElementById("mybutt").disabled = true;
document.getElementById("mybutt").className = "buttonDisabled";
//long running task here
document.getElementById("myspan").innerHTML = "done";
}
Now this has problems in firefox and IE, ( in chrome it works ok )
So I thought to put it into a settimeout:
function longrunningfunction() {
document.getElementById("myspan").innerHTML = "doing some work";
document.getElementById("mybutt").disabled = true;
document.getElementById("mybutt").className = "buttonDisabled";
setTimeout(function() {
//long running task here
document.getElementById("myspan").innerHTML = "done";
}, 0);
}
but this doesn't work either for firefox! the button gets disabled, changes colour ( due to the application of the new css ) but the text does not change.
I have to set the time to 50ms instead of just 0ms, in order to make it work ( change the button text ). Now I find this stupid at least. I can understand if it would work with just a 0ms delay, but what would happen in a slower computer? maybe firefox would need 100ms there in the settimeout? it sounds rather stupid. I tried many times, 1ms, 10ms, 20ms...no it won't refresh it. only with 50ms.
So I followed the advice in this topic:
Forcing a DOM refresh in Internet explorer after javascript dom manipulation
so I tried:
function longrunningfunction() {
document.getElementById("myspan").innerHTML = "doing some work";
var a = document.getElementById("mybutt").offsetTop; //force refresh
//long running task here
document.getElementById("myspan").innerHTML = "done";
}
but it doesn't work ( FIREFOX 21). Then i tried:
function longrunningfunction() {
document.getElementById("myspan").innerHTML = "doing some work";
document.getElementById("mybutt").disabled = true;
document.getElementById("mybutt").className = "buttonDisabled";
var a = document.getElementById("mybutt").offsetTop; //force refresh
var b = document.getElementById("myspan").offsetTop; //force refresh
var c = document.getElementById("mybutt").clientHeight; //force refresh
var d = document.getElementById("myspan").clientHeight; //force refresh
setTimeout(function() {
//long running task here
document.getElementById("myspan").innerHTML = "done";
}, 0);
}
I even tried clientHeight instead of offsetTop but nothing. the DOM does not get refreshed.
Can someone offer a reliable solution preferrably non-hacky ?
thanks in advance!
as suggested here i also tried
$('#parentOfElementToBeRedrawn').hide().show();
to no avail
Force DOM redraw/refresh on Chrome/Mac
TL;DR:
looking for a RELIABLE cross-browser method to have a forced DOM refresh WITHOUT the use of setTimeout (preferred solution due to different time intervals needed depending on the type of long running code, browser, computer speed and setTimeout requires anywhere from 50 to 100ms depending on situation)
jsfiddle: http://jsfiddle.net/WsmUh/5/
Webpages are updated based on a single thread controller, and half the browsers don't update the DOM or styling until your JS execution halts, giving computational control back to the browser. That means if you set some element.style.[...] = ... it won't kick in until your code finishes running (either completely, or because the browser sees you're doing something that lets it intercept processing for a few ms).
You have two problems: 1) your button has a <span> in it. Remove that, just set .innerHTML on the button itself. But this isn't the real problem of course. 2) you're running very long operations, and you should think very hard about why, and after answering the why, how:
If you're running a long computational job, cut it up into timeout callbacks (or, in 2019, await/async - see note at the end of this anser). Your examples don't show what your "long job" actually is (a spin loop doesn't count) but you have several options depending on the browsers you take, with one GIANT booknote: don't run long jobs in JavaScript, period. JavaScript is a single threaded environment by specification, so any operation you want to do should be able to complete in milliseconds. If it can't, you're literally doing something wrong.
If you need to calculate difficult things, offload it to the server with an AJAX operation (universal across browsers, often giving you a) faster processing for that operation and b) a good 30 seconds of time that you can asynchronously not-wait for the result to be returned) or use a webworker background thread (very much NOT universal).
If your calculation takes long but not absurdly so, refactor your code so that you perform parts, with timeout breathing space:
function doLongCalculation(callbackFunction) {
var partialResult = {};
// part of the work, filling partialResult
setTimeout(function(){ doSecondBit(partialResult, callbackFunction); }, 10);
}
function doSecondBit(partialResult, callbackFunction) {
// more 'part of the work', filling partialResult
setTimeout(function(){ finishUp(partialResult, callbackFunction); }, 10);
}
function finishUp(partialResult, callbackFunction) {
var result;
// do last bits, forming final result
callbackFunction(result);
}
A long calculation can almost always be refactored into several steps, either because you're performing several steps, or because you're running the same computation a million times, and can cut it up into batches. If you have (exaggerated) this:
var resuls = [];
for(var i=0; i<1000000; i++) {
// computation is performed here
if(...) results.push(...);
}
then you can trivially cut this up into a timeout-relaxed function with a callback
function runBatch(start, end, terminal, results, callback) {
var i;
for(var i=start; i<end; i++) {
// computation is performed here
if(...) results.push(...); }
if(i>=terminal) {
callback(results);
} else {
var inc = end-start;
setTimeout(function() {
runBatch(start+inc, end+inc, terminal, results, callback);
},10);
}
}
function dealWithResults(results) {
...
}
function doLongComputation() {
runBatch(0,1000,1000000,[],dealWithResults);
}
TL;DR: don't run long computations, but if you have to, make the server do the work for you and just use an asynchronous AJAX call. The server can do the work faster, and your page won't block.
The JS examples of how to deal with long computations in JS at the client are only here to explain how you might deal with this problem if you don't have the option to do AJAX calls, which 99.99% of the time will not be the case.
edit
also note that your bounty description is a classic case of The XY problem
2019 edit
In modern JS the await/async concept vastly improves upon timeout callbacks, so use those instead. Any await lets the browser know that it can safely run scheduled updates, so you write your code in a "structured as if it's synchronous" way, but you mark your functions as async, and then you await their output them whenever you call them:
async doLongCalculation() {
let firstbit = await doFirstBit();
let secondbit = await doSecondBit(firstbit);
let result = await finishUp(secondbit);
return result;
}
async doFirstBit() {
//...
}
async doSecondBit...
...
SOLVED IT!! No setTimeout()!!!
Tested in Chrome 27.0.1453, Firefox 21.0, Internet 9.0.8112
$("#btn").on("mousedown",function(){
$('#btn').html('working');}).on('mouseup', longFunc);
function longFunc(){
//Do your long running work here...
for (i = 1; i<1003332300; i++) {}
//And on finish....
$('#btn').html('done');
}
DEMO HERE!
As of 2019 one uses double requesAnimationFrame to skip a frame instead of creating a race condition using setTimeout.
function doRun() {
document.getElementById('app').innerHTML = 'Processing JS...';
requestAnimationFrame(() =>
requestAnimationFrame(function(){
//blocks render
confirm('Heavy load done')
document.getElementById('app').innerHTML = 'Processing JS... done';
}))
}
doRun()
<div id="app"></div>
As an usage example think of calculating pi using Monte Carlo in an endless loop:
using for loop to mock while(true) - as this breaks the page
function* piMonteCarlo(r = 5, yield_cycle = 10000){
let total = 0, hits = 0, x=0, y=0, rsqrd = Math.pow(r, 2);
while(true){
total++;
if(total === Number.MAX_SAFE_INTEGER){
break;
}
x = Math.random() * r * 2 - r;
y = Math.random() * r * 2 - r;
(Math.pow(x,2) + Math.pow(y,2) < rsqrd) && hits++;
if(total % yield_cycle === 0){
yield 4 * hits / total
}
}
}
let pi_gen = piMonteCarlo(5, 1000), pi = 3;
for(let i = 0; i < 1000; i++){
// mocks
// while(true){
// basically last value will be rendered only
pi = pi_gen.next().value
console.log(pi)
document.getElementById('app').innerHTML = "PI: " + pi
}
<div id="app"></div>
And now think about using requestAnimationFrame for updates in between ;)
function* piMonteCarlo(r = 5, yield_cycle = 10000){
let total = 0, hits = 0, x=0, y=0, rsqrd = Math.pow(r, 2);
while(true){
total++;
if(total === Number.MAX_SAFE_INTEGER){
break;
}
x = Math.random() * r * 2 - r;
y = Math.random() * r * 2 - r;
(Math.pow(x,2) + Math.pow(y,2) < rsqrd) && hits++;
if(total % yield_cycle === 0){
yield 4 * hits / total
}
}
}
let pi_gen = piMonteCarlo(5, 1000), pi = 3;
function rAFLoop(calculate){
return new Promise(resolve => {
requestAnimationFrame( () => {
requestAnimationFrame(() => {
typeof calculate === "function" && calculate()
resolve()
})
})
})
}
let stopped = false
async function piDOM(){
while(stopped==false){
await rAFLoop(() => {
pi = pi_gen.next().value
console.log(pi)
document.getElementById('app').innerHTML = "PI: " + pi
})
}
}
function stop(){
stopped = true;
}
function start(){
if(stopped){
stopped = false
piDOM()
}
}
piDOM()
<div id="app"></div>
<button onclick="stop()">Stop</button>
<button onclick="start()">start</button>
As described in the "Script taking too long and heavy jobs" section of Events and timing in-depth (an interesting reading, by the way):
[...] split the job into parts which get scheduled after each other. [...] Then there is a “free time” for the browser to respond between parts. It is can render and react on other events. Both the visitor and the browser are happy.
I am sure that there are many times in which a task cannot be splitted into smaller tasks, or fragments. But I am sure that there will be many other times in which this is possible too! :-)
Some refactoring is needed in the example provided. You could create a function to do a piece of the work you have to do. It could begin like this:
function doHeavyWork(start) {
var total = 1000000000;
var fragment = 1000000;
var end = start + fragment;
// Do heavy work
for (var i = start; i < end; i++) {
//
}
Once the work is finished, function should determine if next work piece must be done, or if execution has finished:
if (end == total) {
// If we reached the end, stop and change status
document.getElementById("btn").innerHTML = "done!";
} else {
// Otherwise, process next fragment
setTimeout(function() {
doHeavyWork(end);
}, 0);
}
}
Your main dowork() function would be like this:
function dowork() {
// Set "working" status
document.getElementById("btn").innerHTML = "working";
// Start heavy process
doHeavyWork(0);
}
Full working code at http://jsfiddle.net/WsmUh/19/ (seems to behave gently on Firefox).
If you don't want to use setTimeout then you are left with WebWorker - this will require HTML5 enabled browsers however.
This is one way you can use them -
Define your HTML and an inline script (you don't have to use inline script, you can just as well give an url to an existing separate JS file):
<input id="start" type="button" value="Start" />
<div id="status">Preparing worker...</div>
<script type="javascript/worker">
postMessage('Worker is ready...');
onmessage = function(e) {
if (e.data === 'start') {
//simulate heavy work..
var max = 500000000;
for (var i = 0; i < max; i++) {
if ((i % 100000) === 0) postMessage('Progress: ' + (i / max * 100).toFixed(0) + '%');
}
postMessage('Done!');
}
};
</script>
For the inline script we mark it with type javascript/worker.
In the regular Javascript file -
The function that converts the inline script to a Blob-url that can be passed to a WebWorker. Note that this might note work in IE and you will have to use a regular file:
function getInlineJS() {
var js = document.querySelector('[type="javascript/worker"]').textContent;
var blob = new Blob([js], {
"type": "text\/plain"
});
return URL.createObjectURL(blob);
}
Setup worker:
var ww = new Worker(getInlineJS());
Receive messages (or commands) from the WebWorker:
ww.onmessage = function (e) {
var msg = e.data;
document.getElementById('status').innerHTML = msg;
if (msg === 'Done!') {
alert('Next');
}
};
We kick off with a button-click in this demo:
document.getElementById('start').addEventListener('click', start, false);
function start() {
ww.postMessage('start');
}
Working example here:
http://jsfiddle.net/AbdiasSoftware/Ls4XJ/
As you can see the user-interface is updated (with progress in this example) even if we're using a busy-loop on the worker. This was tested with an Atom based (slow) computer.
If you don't want or can't use WebWorker you have to use setTimeout.
This is because this is the only way (beside from setInterval) that allow you to queue up an event. As you noticed you will need to give it a few milliseconds as this will give the UI engine "room to breeth" so-to-speak. As JS is single-threaded you cannot queue up events other ways (requestAnimationFrame will not work well in this context).
Hope this helps.
Update: I don't think in the long term that you can be sure of avoiding Firefox's aggressive avoidance of DOM updates without using a timeout. If you want to force a redraw / DOM update, there are tricks available, like adjusting the offset of elements, or doing hide() then show(), etc., but there is nothing very pretty available, and after a while when those tricks get abused and slow down user experience, then browsers get updated to ignore those tricks. See this article and the linked articles beside it for some examples: Force DOM redraw/refresh on Chrome/Mac
The other answers look like they have the basic elements needed, but I thought it would be worthwhile to mention that my practice is to wrap all interactive DOM-changing functions in a "dispatch" function which handles the necessary pauses needed to get around the fact that Firefox is extremely aggressive in avoiding DOM updates in order to score well on benchmarks (and to be responsive to users while browsing the internet).
I looked at your JSFiddle and customized a dispatch function the one that many of my programs rely on. I think it is self-explanatory, and you can just paste it into your existing JS Fiddle to see how it works:
$("#btn").on("click", function() { dispatch(this, dowork, 'working', 'done!'); });
function dispatch(me, work, working, done) {
/* work function, working message HTML string, done message HTML string */
/* only designed for a <button></button> element */
var pause = 50, old;
if (!me || me.tagName.toLowerCase() != 'button' || me.innerHTML == working) return;
old = me.innerHTML;
me.innerHTML = working;
setTimeout(function() {
work();
me.innerHTML = done;
setTimeout(function() { me.innerHTML = old; }, 1500);
}, pause);
}
function dowork() {
for (var i = 1; i<1000000000; i++) {
//
}
}
Note: the dispatching function also blocks calls from happening at the same time, because it can seriously confuse users if status updates from multiple clicks are happening together.
Fake an ajax request
function longrunningfunction() {
document.getElementById("myspan").innerHTML = "doing some work";
document.getElementById("mybutt").disabled = true;
document.getElementById("mybutt").className = "buttonDisabled";
$.ajax({
url: "/",
complete: function () {
//long running task here
document.getElementById("myspan").innerHTML = "done";
}
});}
Try this
function longRunningTask(){
// Do the task here
document.getElementById("mybutt").value = "done";
}
function longrunningfunction() {
document.getElementById("mybutt").value = "doing some work";
setTimeout(function() {
longRunningTask();
}, 1);
}
Some browsers don't handle onclick html attribute good. It's better to use that event on js object. Like this:
<button id="mybutt" class="buttonEnabled">
<span id="myspan">do some work</span>
</button>
<script type="text/javascript">
window.onload = function(){
butt = document.getElementById("mybutt");
span = document.getElementById("myspan");
butt.onclick = function () {
span.innerHTML = "doing some work";
butt.disabled = true;
butt.className = "buttonDisabled";
//long running task here
span.innerHTML = "done";
};
};
</script>
I made a fiddle with working example http://jsfiddle.net/BZWbH/2/
Have you tried adding listener to "onmousedown" to change the button text and click event for longrunning function.
Slightly modified your code at jsfiddle and:
$("#btn").on("click", dowork);
function dowork() {
document.getElementById("btn").innerHTML = "working";
setTimeout(function() {
for (var i = 1; i<1000000000; i++) {
//
}
document.getElementById("btn").innerHTML = "done!";
}, 100);
}
Timeout set to more reasonable value 100ms did the trick for me. Try it.
Try adjusting the latency to find the best value.
DOM buffer also exists in default browser on android,
long running javascript only flush DOM buffer once,
use setTimeout(..., 50) to solve it.
I have adapted Estradiaz's double animation frame method for async/await:
async function waitForDisplayUpdate() {
await waitForNextAnimationFrame();
await waitForNextAnimationFrame();
}
function waitForNextAnimationFrame() {
return new Promise((resolve) => {
window.requestAnimationFrame(() => resolve());
});
}
async function main() {
const startTime = performance.now();
for (let i = 1; i <= 5; i++) {
setStatus("Step " + i);
await waitForDisplayUpdate();
wasteCpuTime(1000);
}
const elapsedTime = Math.round(performance.now() - startTime);
setStatus(`Completed in ${elapsedTime} ms`);
}
function wasteCpuTime(ms) {
const startTime = performance.now();
while (performance.now() < startTime + ms) {
if (Math.random() == 0) {
console.log("A very rare event has happened.");
}
}
}
function setStatus(s) {
document.getElementById("status").textContent = s;
}
document.addEventListener("DOMContentLoaded", main);
Status: <span id="status">Start</span>
I'm working with some pretty old code and the following is being used to monitor session status. If the user is inactive for X minutes (determined by check_session.php), they are logged out.
The server side stuff works fine. Actually, the existing javascript appears to work OK as well, but looks like it needs cleaning up.
Here's the existing javascript:
function checkSessionStatus()
{
session_http.open('GET', '/check_session.php', true);
session_http.onreadystatechange = handleSessionHttpResponse;
session_http.send(null);
}
function handleSessionHttpResponse()
{
if (session_http.readyState == 4)
{
results = session_http.responseText;
if (results == 'inactive')
{
window.location='/logout.php';
document.getElementById('session_divbox').innerHTML = results;
}
}
}
function get_session_HTTPObject()
{
var xml_session_http;
if (!xml_session_http && typeof XMLHttpRequest != 'undefined')
{
try
{
xml_session_http = new XMLHttpRequest();
}
catch (e)
{
xml_session_http = false;
}
}
return xml_session_http;
}
var session_http = get_session_HTTPObject();
function init_page_header()
{
window.setInterval( 'checkSessionStatus();', 30000);
}
This seems incredibly long for what it is doing.
I am still learning jquery and am able to do some basic ajax calls like this one, which places a returned value in a div:
$(document).ready(function()
{
$('#users_online').load('/show_users_online.php');
var refreshId = setInterval(function()
{
$('#users_online').load('/show_users_online.php');
}, 2000);
$.ajaxSetup({ cache: false });
});
The issue with the first bit of code is that it returns a value of 'inactive', which is then acted on by the client (window redirect).
Is it possible to do this in Jquery without winding up with dozens of lines of code? I may already know how to do this and am not seeing the forest for the trees -- some guidance here is appreciated.
Even if its very vampiric question style, should look like
$.get('/check_session.php', function( data ) {
if( data === 'inactive' ) {
window.location='/logout.php';
document.getElementById('session_divbox').innerHTML = data;
}
});