I have developed a music sequencer in javascript; something like this: http://stepseq.michd.me/
I have implemented loop using setInterval function in following way:
var Sequencer = {
...
// every step sequencer ...
next: function(callback) {
// restart from begin if arrive to last sequecer step
if(Sequencer.current==Sequencer.steps.length)
Sequencer.current = 0;
// play all sounds in array step contains
if(Sequencer.steps[Sequencer.current].length>0) {
var set = Sequencer.steps[Sequencer.current];
for(var i=0;i<set.length;i++){
set[i].play();
}
}
callback(Sequencer.current);
Sequencer.current++;
},
loop: function(callback) {
Sequencer.interval = $interval(function(){
Sequencer.next(callback);
}, Sequencer.time);
}
}
...
Code below works but i think that there is a better way to implement loop; infact sometimes steps goes out of time. Sequencer.time (time passed to setInterval function) is a time in millisecs (this value is the conversion of a bpm value; for example it can be 125),
Someone can suggest me a better solution?
N.B.: this is a web application angularjs based (for this reason in code above a use $interval insteed of setInterval), but i think that this point is insignificant.
Javascript timer intervals are not guaranteed to run at exactly the time you request, due to the single threaded nature of JS. What you get is a callback that is queued up to run after the interval expires, whenever the engine is free to run it.
John resig has covered this off in some depth:
http://ejohn.org/blog/how-javascript-timers-work/
http://ejohn.org/blog/analyzing-timer-performance/
And from his conclusions, which is going to be important for your app:
If a timer is blocked from immediately executing it will be delayed
until the next possible point of execution (which will be longer than
the desired delay).
I don't really have a better solution for your problem, due to these fundamental issues with timers in JS, but this may at least explain what is happening.
This article hit the top of HackerNews recently: http://highscalability.com/blog/2013/9/18/if-youre-programming-a-cell-phone-like-a-server-youre-doing.html#
In which it states:
The cell radio is one of the biggest battery drains on a phone. Every time you send data, no matter how small, the radio is powered on for up for 20-30 seconds. Every decision you make should be based on minimizing the number of times the radio powers up. Battery life can be dramatically improved by changing the way your apps handle data transfers. Users want their data now, the trick is balancing user experience with transferring data and minimizing power usage. A balance is achieved by apps carefully bundling all repeating and intermittent transfers together and then aggressively prefetching the intermittent transfers.
I would like to modify $.ajax to add an option like "doesn't need to be done right now, just do this request when another request is launched". What would be a good way to go about this?
I started with this:
(function($) {
var batches = [];
var oldAjax = $.fn.ajax;
var lastAjax = 0;
var interval = 5*60*1000; // Should be between 2-5 minutes
$.fn.extend({batchedAjax: function() {
batches.push(arguments);
}});
var runBatches = function() {
var now = new Date().getTime();
var batched;
if (lastAjax + interval < now) {
while (batched = batches.pop()) {
oldAjax.apply(null, batched);
}
}
}
setInterval(runBatches, interval);
$.fn.ajax = function() {
runBatches();
oldAjax.apply(null, arguments);
lastAjax = now;
};
})(jQuery);
I can't tell by the wording of the paper, I guess a good batch "interval" is 2-5 minutes, so I just used 5.
Is this a good implementation?
How can I make this a true modification of just the ajax method, by adding a {batchable:true} option to the method? I haven't quite figured that out either.
Does setInterval also keep the phone awake all the time? Is that a bad thing to do? Is there a better way to not do that?
Are there other things here that would cause a battery to drain faster?
Is this kind of approach even worthwhile? There are so many things going on at once in a modern smartphone, that if my app isn't using the cell, surely some other app is. Javascript can't detect if the cell is on or not, so why bother? Is it worth bothering?
I made some progress on adding the option to $.ajax, started to edit the question, and realized it's better as an answer:
(function($) {
var batches = [];
var oldAjax = $.fn.ajax;
var lastAjax = 0;
var interval = 5*60*1000; // Should be between 2-5 minutes
var runBatches = function() {
var now = new Date().getTime();
var batched;
if (lastAjax + interval < now) {
while (batched = batches.pop()) {
oldAjax.apply(null, batched);
}
}
}
setInterval(runBatches, interval);
$.fn.ajax = function(url, options) {
if (options.batchable) {
batches.push(arguments);
return;
}
runBatches();
oldAjax.apply(null, arguments);
lastAjax = now;
};
})(jQuery);
That was actually fairly straightforward. Is love to see a better answer though.
Does setInterval also keep the phone awake all the time? Is that a bad thing to do? Is there a better way to not do that?
From an iPhone 4, iOS 6.1.0 Safari environment:
A wrote an app with a countdown timer that updated an element's text on one-second intervals. The DOM tree had about medium complexity. The app was a relatively-simple calculator that didn't do any AJAX. However, I always had a sneaking suspicion that those once-per-second reflows were killing me. My battery sure seemed to deplete rather quickly, whenever I left it turned-on on a table, with Safari on the app's webpage.
And there were only two timeouts in that app. Now, I don't have any quantifiable proof that the timeouts were draining my battery, but losing about 10% every 45 minutes from this dopey calculator was a little unnerving. (Who knows though, maybe it was the backlight.)
On that note: You may want to build a test app that does AJAX on intervals, other things on intervals, etc, and compare how each function drains your battery under similar conditions. Getting a controlled environment might be tricky, but if there is a big enough difference in drain, then even "imperfect" testing conditions will yield noticeable-enough results for you to draw a conclusion.
However, I found out an interesting thing about how iOS 6.1.0 Safari handles timeouts:
The timeouts don't run their callbacks if you turn off the screen.
Consequentially, long-term timeouts will "miss their mark."
If my app's timer was to display the correct time (even after I closed and reopened the screen), then I couldn't go the easy route and do secondsLeft -= 1. If I turned off the screen, then the secondsLeft (relative to my starting time) would have been "behind," and thus incorrect. (The setTimeout callback did not run while the screen was turned off.)
The solution was that I had to recalculate timeLeft = fortyMinutes - (new Date().getTime() - startTime) on each interval.
Also, the timer in my app was supposed to change from green, to lime, to yellow, to red, as it got closer to expiry. Since, at this point, I was worried about the efficiency of my interval-code, I suspected that it would be better to "schedule" my color changes for their appropriate time (lime: 20 minutes after starting time, yellow: 30 mins, red: 35) (this seemed preferable to a quadruple-inequality-check on every interval, which would be futile 99% of the time).
However, if I scheduled such a color change, and my phone's screen was turned off at the target time, then that color change would never happen.
The solution was to check, on each interval, if the time elapsed since the last 1-second timer update had been ">= 2 seconds". (This way, the app could know if my phone had had its screen turned off; it was able to realize when it had "fallen behind.") At that point, if necessary, I would "forcibly" apply a color change and schedule the next one.
(Needless to say, I later removed the color-changer...)
So, I believe this confirms my claim that
iOS 6.1.0 Safari does not execute setTimeout callback functions if the screen is turned off.
So keep this in mind when "scheduling" your AJAX calls, because you will probably be affected by this behavior as well.
And, using my proposition, I can answer your question:
At least for iOS, we know that setTimeout sleeps while the screen is off.
Thus setTimeout won't give your phone "nightmares" ("keep it awake").
Is this kind of approach even worthwhile? There are so many things going on at once in a modern smartphone, that if my app isn't using the cell, surely some other app is. Javascript can't detect if the cell is on or not, so why bother? Is it worth bothering?
If you can get this implementation to work correctly then it seems like it would be worthwhile.
You will incur latency for every AJAX request you make, which will slow down your app to some degree. (Latency is the bane of page loading time, after all.) So you will definitely achieve some gain by "bundling" requests. Extending $.ajax such that you can "batch" requests will definitely have some merit.
The article you've linked clearly focuses on optimizing power consumption for apps (yes, the weather widget example is horrifying). Actively using a browser is, by definition, a foreground task; plus something like ApplicationCache is already available to reduce the need for network requests. You can then programmatically update the cache as required and avoid DIY.
Sceptical side note: if you are using jQuery as part of your HTML5 app (perhaps wrapped in Sencha or similar), perhaps the mobile app framework has more to do with request optimization than the code itself. I have no proof whatsoever, but goddammit this sounds about right :)
How can I make this a true modification of just the ajax method, by
adding a {batchable:true} option to the method? I haven't quite
figured that out either.
A perfectly valid approach but to me this sounds like duck punching gone wrong. I wouldn't. Even if you correctly default batchable to false, personally I would rather use a facade (perhaps even in its own namespace?)
var gQuery = {}; //gQuery = green jQuery, patent pending :)
gQuery.ajax = function(options,callback){
//your own .ajax with blackjack and hooking timeouts, ultimately just calling
$.ajax(options);
}
Does setInterval also keep the phone awake all the time? Is that a
bad thing to do? Is there a better way to not do that?
Native implementations of setInterval and setTimeout are very similar afaik; think of the latter not firing while the website is in the background for online banking inactivity prompts; when a page is not in the foreground its execution is basically halted. If an API is available for such "deferrals" (the article mentions of some relevant iOS7 capabilities) then it's likely a preferable approach, otherwise I see no reason to avoid setInterval.
Are there other things here that would cause a battery to drain
faster?
I'd speculate that any heavy load would (from calculating pi to pretty 3d transitions perhaps). But this sounds like premature optimization to me and reminds me of an e-reader with battery-saving mode that turned the LCD screen completely off :)
Is this kind of approach even worthwhile? There are so many things
going on at once in a modern smartphone, that if my app isn't using
the cell, surely some other app is. Javascript can't detect if the
cell is on or not, so why bother? Is it worth bothering?
The article pointed out a weather app being unreasonably greedy, and that would concern me. It seems to be a development oversight though more than anything else, as in fetching data more often than it's really needed. In an ideal world, this should be nicely handled on OS level, otherwise you'd end up with an array of competing workarounds. IMO: don't bother until highscalability posts another article telling you to :)
Here is my version:
(function($) {
var batches = [],
ajax = $.fn.ajax,
interval = 5*60*1000, // Should be between 2-5 minutes
timeout = setTimeout($.fn.ajax, interval);
$.fn.ajax=function(url, options) {
var batched, returns;
if(typeof url === "string") {
batches.push(arguments);
if(options.batchable) {
return;
}
}
while (batched = batches.shift()) {
returns = ajax.apply(null, batched);
}
clearTimeout(timeout);
timeout = setTimeout($.fn.ajax, interval);
return returns;
}
})(jQuery);
I think this version has the following main advantages:
If there is a non-batchable ajax call, the connection is used to send all batches. This Resets the timer.
Returns the expected return value on direct ajax calls
A direct processing of the batches can be triggered by calling $.fn.ajax() without parameters
As far as hacking the $.ajax method, I would :
try to also preserve the Promise mechanism provided by $.ajax,
take advantage of one of the global ajax events to trigger ajax calls,
maybe add a timer, to have the batch being called anyways in case no "immediate" $.ajax call is made,
give a new name to this function (in my code : $.batchAjax) and keep the orginal $.ajax.
Here is my go :
(function ($) {
var queue = [],
timerID = 0;
function ajaxQueue(url, settings) {
// cutom deferred used to forward the $.ajax' promise
var dfd = new $.Deferred();
// when called, this function executes the $.ajax call
function call() {
$.ajax(url, settings)
.done(function () {
dfd.resolveWith(this, arguments);
})
.fail(function () {
dfd.rejectWith(this, arguments);
});
}
// set a global timer, which will trigger the dequeuing in case no ajax call is ever made ...
if (timerID === 0) {
timerID = window.setTimeout(ajaxCallOne, 5000);
}
// enqueue this function, for later use
queue.push(call);
// return the promise
return dfd.promise();
}
function ajaxCallOne() {
window.clearTimeout(timerID);
timerID = 0;
if (queue.length > 0) {
f = queue.pop();
// async call : wait for the current ajax events
//to be processed before triggering a new one ...
setTimeout(f, 0);
}
}
// use the two functions :
$(document).bind('ajaxSend', ajaxCallOne);
// or :
//$(document).bind('ajaxComplete', ajaxCallOne);
$.batchAjax = ajaxQueue;
}(jQuery));
In this example, the hard coded delay fo 5 seconds defeats the purpose of "if less than 20 seconds between calls, it drains the battery". You can put a bigger one (5 minutes ?), or remove it altogether - it all depends on your app really.
fiddle
Regarding the general question "How do I write a web app which doesn't burn a phone's battery in 5 minutes ?" : it will take more than one magic arrow to deal with that one. It is a whole set of design decisions you will have to take, which really depends on your app.
You will have to arbitrate between loading as much data as possible in one go (and possibly send data which won't be used) vs fetching what you need (and possibly send many small individual requests).
Some parameters to take into account are :
volume of data (you don't want to drain your clients data plan either ...),
server load,
how much can be cached,
importance of being "up to date" (5 minutes delay for a chat app won't work),
frequency of client updates (a network game will probably require lots of updates from the client, a news app probably less ...).
One rather general suggestion : you can add a "live update" checkbox, and store its state client side. When unchecked, the client should hit a "refresh" button to download new data.
Here is my go, it somewhat grew out of what #Joe Frambach posted but I wanted the following additions:
retain the jXHR and error/success callbacks if they were provided
Debounce identical requests (by url and options match) while still triggering the callbacks or jqXHRs provided for EACH call
Use AjaxSettings to make configuration easier
Don't have each non batched ajax flush the batch, those should be separate processes IMO, but thus supply an option to force a batch flush as well.
Either way, this sucker would mostly likely be better done as a separate plugin rather than overriding and affecting the default .ajax function... enjoy:
(function($) {
$.ajaxSetup({
batchInterval: 5*60*1000,
flushBatch: false,
batchable: false,
batchDebounce: true
});
var batchRun = 0;
var batches = {};
var oldAjax = $.fn.ajax;
var queueBatch = function(url, options) {
var match = false;
var dfd = new $.Deferred();
batches[url] = batches[url] || [];
if(options.batchDebounce || $.ajaxSettings.batchDebounce) {
if(!options.success && !options.error) {
$.each(batches[url], function(index, batchedAjax) {
if($.param(batchedAjax.options) == $.param(options)) {
match = index;
return false;
}
});
}
if(match === false) {
batches[url].push({options:options, dfds:[dfd]});
} else {
batches[url][match].dfds.push(dfd);
}
} else {
batches[url].push({options:options, dfds:[dfd]);
}
return dfd.promise();
}
var runBatches = function() {
$.each(batches, function(url, batchedOptions) {
$.each(batchedOptions, function(index, batchedAjax) {
oldAjax.apply(null, url, batchedAjax.options).then(
function(data, textStatus, jqXHR) {
var args = arguments;
$.each(batchedAjax.dfds, function(index, dfd) {
dfd.resolve(args);
});
}, function(jqXHR, textStatus, errorThrown) {
var args = arguments;
$.each(batchedAjax.dfds, function(index, dfd) {
dfd.reject(args);
});
}
)
});
});
batches = {};
batchRun = new Date.getTime();
}
setInterval(runBatches, $.ajaxSettings.batchInterval);
$.fn.ajax = function(url, options) {
if (options.batchable) {
var xhr = queueBatch(url, options);
if((new Date.getTime()) - batchRun >= options.batchInterval) {
runBatches();
}
return xhr;
}
if (options.flushBatch) {
runBatches();
}
return oldAjax.call(null, url, options);
};
})(jQuery);
Is there a way to know when the browser is actively running requestAnimationFrame?
For example when I switch tabs from one that was running requestAnimationFrame, the function stops getting executed, when I switch back it continues, what is the best way to deal with this?
To detect if requestAnimationFrame is running 100% you can check:
window.addEventListener('blur', function() {
//not running full
}, false);
and
window.addEventListener('focus', function() {
//running optimal (if used)
}, false);
this can be used as we know requestAnimationFrame reduces trigger rate (in most browsers) when window (tab) is not the active one (IF being used - it depends on the code actually using the requestAnimationFrame).
If you want it to run constantly you can insert a mechanism such as this:
var isActiveTab = true; //update with the events above
function myLoop() {
//my cool stuff here
if (isActiveTab) {
requestAnimationFrame(myLoop);
} else {
setTimeout(myLoop, 16); //force a rate (vblank sync not necessary
//when display isn't updated
}
}
Note that the reduction in rate for requestAnimationFrame is not part of the standard and is a browser specific implementation.
When you again come back to the tab with animation,It must be working fine(If thats the case--following is your answer!!!)
This is what RAF made for.To optimize performance.
SetInterval and Settimeout can be used instead for creating animations, But they cannot interact with the browser and eventually end up hogging up the cpu and the performance is also quite slow.
But your question is really not a question.This is actually a trick used by RAF to better your overall animation experience.
There are several articles which explains RAF.
http://creativejs.com/resources/requestanimationframe/
Just An Optimization TRICK--No need to worry about it
A solution I used in a project for canvas repainting. It's not 100% accurate but it works for out of focus users
// This will run when user is inactive
let = handleVisibilityChange = () => {
if (document.hidden) {
setTimeout(() => {
updateYourStuff();
handleVisibilityChange();
}, 1000);
}
};
// Listen if user is active or inactive
document.addEventListener("visibilitychange", handleVisibilityChange, false);
// Your loop when user is active
function myLoop() {
updateYourStuff();
requestAnimationFrame(myLoop);
}
If you need to know at what time a frame was painted, you can call requestPostAnimationFrame (google canary) or use a polyfill for it.
I'm procesing a kind of "big" JSON object around of 4000 elements passing for different methods, and I would like to update a div tag with a text showing the process.
But for some strange reason (just tested in Firefox and Chrome), they don't update the DOM object with the text.
$("#estatusBar").text(_item.Centro_de_trabajo);
Both prefer to continue calculating all the data and other process without and dont take the time for update the text. But if I just code an Alert("") in the loop and then in chrome I click on the "selected box" saying ignore all other alerts, chrome suddenly starts updating the text.
So I was thinking if I can "pause" the calculation with some kind of code to stop and update the DOM element and then continue making the other process?
Is this possible or what is an alternative to this strange behavior?
-- EDIT --
this is the code of the loop
$.each(plantillas, function(_index, _item){
updateBar(_item.Centro_de_trabajo);
calculateItem(_item,_index);
a.push("<div class='blockee'><ul>"+ /*temp.join("")*/ t(_item) +"</ul></div>");
});
No you cannot do what alert does. This limitation is really annoying in some cases but if your problem is just a progress for a single long computation then the solution is simple.
Instead of doing alll the records in one single loop break the computation in "small enough" chunks and then do something like
function doit()
{
processBlockOfRecords();
updateProgressBar();
if (!finished()) {
setTimeout(doit, 0);
}
}
setTimeout(doit, 0);
With this approach is also simple to add an "abort" button to stop the computation.
In your example the loop is
$.each(plantillas, function(_index, _item){
updateBar(_item.Centro_de_trabajo);
calculateItem(_item,_index);
a.push("<div class='blockee'><ul>"+ /*temp.join("")*/ t(_item) +"</ul></div>");
});
so the computation could be split with (untested)
function processRecords(plantillas, completion_callback) {
var processed = 0;
var result = [];
function doit() {
// Process up to 20 records at a time
for (var i=0; i<20 && processed<plantillas.length; i++) {
calculateItem(plantillas[processed], processed);
result.push("<div class='blockee'><ul>" +
t(plantillas[processed]) +
"</ul></div>");
processed++;
}
// Progress bar update
updateProgress(processed, plantillas.length);
if (processed < plantillas.length) {
// Not finished, schedule another block
setTimeout(doit, 0);
} else {
// Processing complete... inform caller
if (completion_callback) completion_callback(result);
}
}
// Schedule computation start
setTimeout(doit, 0);
}
You can try using web workers, to defer the calculation to the background, and this should help you out. Web workers were designed to do this very thing (push large calculations to a background thread). However, this may not work in all browsers. Your main concern is IE, and only IE 10 supports it:
Do you want some version of wait or pause or sleep or similar in javascript is that it?
The only way is with
window.setTimeout()
You can pause any function with it.
Check this post too might help:
Sleep/Pause/Wait in Javascript
I would like to thread some Javascript code while both the main process and thread are free to update the browser UI.
For example:
function StartStuff() {
StartThreadedCode();
// do more work and update the UI while StartThreadedCode() does its work
}
function StartThreadedCode() {
// do stuff (do work and update the UI)
}
Is it possible?
There are two main ways to achieve "multithreading" in Javascript. The first way is a cross-browser solution, that would also work in older browsers, but is more complicated to implement.
The idea behind it is that you give the UI some time to update every once in awhile. Since there's no synchronous sleep function in Javascript, the only way to achieve this is to use setTimeout (or setInterval with a little bit more complicate logic) to delay the execution of every loop of your complex calculations. This would give the browser some time to update the UI between loops, giving the visual effect of multiple things happening simultaneously. A few ms should be more than enough for the UI to reflect the latest changes.
It has it's drawbacks of course, and can be quite difficult to implement if there are multiple actions the user might want to do while the background calculations are being performed. Also it can drastically slow down the whole background calculation, since it's delayed a few ms now and then. In specific cases, however, it does the trick and performs well.
The second option would be to use web workers, that are basically Javascript scripts running independently in the background, like a thread. It's much easier to implement, you only have to worry about messaging between main code and background workers, so your whole application isn't affected as much. You can read about using them from the link posted by Mic https://developer.mozilla.org/en/Using_web_workers. The greatest drawback of web workers is their support by browsers, which you can see at http://caniuse.com/#search=worker There's no possible workaround for IE <9 or mobile browsers that truly simulate the effect, so there's not much you can do about those browsers, but then again, the benefits of modern browsers might outweigh poor IE support. This, of course, depends on your application.
Edit: Im not sure whether I explained the first concept clearly enough, so I decided to add a small example. The following code is functionally equivalent to:
for (var counter = 0; counter < 10; counter++) {
console.log(counter);
}
But instead of logging 0-9 in quick succession, it delays 1s before executing the next iteration of the loop.
var counter = 0;
// A single iteration of your calculation function
// log the current value of counter as an example
// then wait before doing the next iteration
function printCounter() {
console.log(counter);
counter++;
if (counter < 10)
setTimeout(printCounter, 1000);
}
// Start the loop
printCounter();
As of 2009 (FF 3.5/Gecko 1.9.1) a new Web API was added that is called Web Workers. It works also on Chrome 4+, Opera 10.6+ and IE10+.
The worker is basically a background thread that runs in a separate process.
The communication between the master process (eg. your UI's main thread) and the slave process (the background thread, the worker) is established with the aid of a generic PostMessage/onmessage function where you can exchange whatever data you like between the two parties.
It is worth mentioning that every single worker is assigned to a different core. For instance by creating 4 different workers (that do a long-running computation) on a quad-processor you are going to see all the 4 CPU-cores like 100% while the main-script is still idling and thus responding to your UI events (look at this example).
A basic example:
main-script.js
if ("function" !== typeof window.Worker) {
throw "Your browser doesn't support Web Workers";
}
var thread = new Worker("background-thread.js");
thread.onmessage = function(e) {
console.log("[A] : I received a " + e.data + " :-P");
};
thread.onerror = function(e) {
console.log(e);
};
console.log("[A] : I'm sending a PING :-)");
thread.postMessage("PING");
background-thread.js
onmessage = function(e) {
console.log("[B] : I receveid a " + e.data + " :-)");
console.log("[B] : I will respond with a PONG ;-)");
postMessage("PONG");
};
The above example should produce the following output at your browser's console:
[A] : I'm sending a PING :-)
[B] : I receveid a PING :-)
[B] : I will respond with a PONG ;-)
[A] : I received a PONG :-P
So happy PING-ing to your background script!
Javascript is single threaded language, but you can do some tricks to imitate multithreading:
http://www.nczonline.net/blog/2009/08/11/timed-array-processing-in-javascript/
http://www.nczonline.net/blog/2011/09/19/script-yielding-with-setimmediate/
http://www.nczonline.net/blog/2011/05/03/better-javascript-animations-with-requestanimationframe/