I am looking for a pub/sub mechanism that behaves like a promise but can resolve multiple times, and behaves like an event except if you subscribe after a notification has happened it triggers with the most recent value.
I am aware of notify, but deferred.notify is order-sensitive, so in that way it behaves just like an event. eg:
d.notify('notify before'); // Not observed :(
d.promise.progress(function(data){ console.log(data) });
d.notify('notify after'); // Observed
setTimeout(function(){ d.notify('notify much later') }, 100); // Also Observed
fiddle: http://jsfiddle.net/foLhag3b/
The notification system I'd like is a good fit for a UI component that should update to reflect the state of the data behind it. In these cases, you don't want to care about whether the data has arrived yet or not, and you want updates when they come in.
Maybe this is similar to Immediate mode UIs, but is distinct because it is message based.
The state of the art for message based UI updating, as far as I'm aware, is something which uses a promise or callback to initialize, then also binds an update event:
myUIComponent.gotData(model.data);
model.onUpdate(myUIComponent.gotData); // doing two things is 2x teh workz :(
I don't want to have to do both. I don't think anyone should have to, the use case is common enough to abstract.
model.whenever(myUIComponent.gotData); // yay one intention === one line of code
I could build a mechanism to do what I want, but I wanted to see if a pub/sub mechanism like this already exists. A lot of smart people have done a lot in CS and I figure this concept must exist, I just am looking for the name of it.
To be clear, I'm not looking to change an entire framework, say to Angular or React. I'm looking only for a pub/sub mechanism. Preferably an implementation of a known concept with a snazzy name like notifr or lemme-kno or touch-base.
You'll want to have a look at (functional) reactive programming. The concept you are looking for is known as a Behavior or Signal in FRP. It models the change of a value over time, and can be inspected at any time (continuously holds a value, in contrast to a stream that discretely fires events).
var ui = state.map(render); // whenever state updates, so does ui with render() result
Some popular libraries in JS are Bacon and Rx, which use their own terminology however: you'll find properties and observables.
Related
The most confusing thing in this API is for me the reason why use it. I know ReactJS and RxJS and I'm used to the concept when view reacts to data change. So watching changes to DOM, which happens definitely after some mutations to data, I can't see much sense in it. So my question is when (not) use it?
You're thinking of the problem with a situation where you are already one step ahead. If you are using React/RxJS then the actual value of MutationObserver will most likely be very small.
Even within this, however, there is a clear possibility to leverage this. Suppose you are attempting to use a library within your React application that is not explicitly built for it, and modifies the DOM in some way, but want to extend this further or capture something from it. The best example for this would be augmenting FancyGrid further.
Currently, in a component, you would invoke such a library in componentDidMount, the same way the component above is built. However, this is simply fire-and-forget - you don't know when it is done executing, you don't even know what is happening on the "outside".
Enter MutationObserver. With it, before binding such a library to an element, you can use an observer to be notified of when elements are created, track them, and track property changes. The simplest use case for this would be to make a spinner above a (particularly time-consuming on load) grid.
I know these types of question come up fairly often, but I need help with a wait-like mechanism in JavaScript. I know setTimeout-based solutions are going to come up, but I'm not sure how to pull it off in my case.
I'm writing an API that uses a WebSocket internally. There's a connect() method that sets up the WebSocket, and I need to make it not return until after the WebSocket is set up. I'd like it to return a value for whether or not the connection was successful, but that's not the main problem.
The issue I'm hitting is that after a user calls connect(), they may call another method that relies on the WebSocket to be properly set up. If it's called too early, an error is thrown stating that the object is not usable.
My current solution is setting a "connected" flag when I've determined a successful connection and in each method checking for it in each method. If it's not connected, I add the method call to a queue that is ran through by the same code that sets the flag. This works, but it introduces that style of code all over my methods and also seems misleading from the user-perspective, since the call of those functions is deferred. Also, if there is other user code that relies on those calls being completed before it gets to them, it won't behave as expected.
I've been racking my brain with how to handle this case. The easiest solution is to just find a way to block returning from connect until after the WebSocket is set up, but that's not really the JavaScript way. The other option was to make them provide the rest of their code in a callback, but that seems like a weird thing to do in this case. Maybe I'm over-thinking it?
Edit: To better illustrate my problem, here's a example of what the user could do:
var client = new Client(options);
client.connect();
client.getServerStatus();
The getServerStatus() method would be using the WebSocket internally. If the WebSocket is not set up yet, the user will get that not usable error.
Todays Javascript does not really work like that unfortunately. In the future (ECMA6) there may be new language features that address this issue more directly. However for now you are stuck with the currently accepted method of handling asynchronous events, which is limited to callbacks. You may also want to explore 'promises' to handle 'callback hell' however you will need a library for this.
And yes it does seem strange to have callbacks everywhere, especially for someone new to web programming, however it is really the only way to go about it at this stage (assuming you want a cross-browser friendly solution).
"Wait" is almost the keyword you are looking for. Actually, it's yield that does this. See e.g. MDN's documentation.
There's a connect() method that sets up the WebSocket, and I need to make it not return until after the WebSocket is set up
That isn't going to happen unless you rewrite the javascript execution engine.
Either the code trying to send data will need to check the socket state (I'd go with encapsulating the socket in a object, supplying a method which sets a member variable on the open/close events and poll the state of that member variable from the external code). Alternatively you could add messages and call backs to a queue and process the queue when the socket connects.
I know enough jQuery/JavaScript to be dangerous. I have a JSON array that I'm interacting with using two different elements (a calendar and a table, to be precise). Is there an event handler (or any other way) I could bind to so that the table would refresh when the JSON changes?
Basic programming, parse the json (=string) into a javascript object or array. (you probably have already done that.) Use an implementation of the observer patern.
I suggest taking a good look at #Adam Merrifield 's interesting links.
Most of the time using getters and setter where you can fire a custom event (or call a callback method) inside a setter is the key in this.
KnockoutJS is a good framework to help you do such binding. It also uses the observable - observer/subscriber pattern.
using timers is not a really good idea.. little to much overhead. (doing stuff also when nothing gets changed. And you will always hop x ms behind (depending on the polling frequency).
You might want to consider Knockout.JS
It allows bi-directional mapping, so a change to your model should reflect on your view and vice/versa.
http://knockoutjs.com/documentation/json-data.html
However, it might be late stages of your dev cycle, but something to consider.
I'm getting in to a situation where I have several interacting widgets (on a web UI), all of whom can be in multiple different states, and whose behavior depends on others the others. I'm running in to situations where, for example, a set of data gets sorted twice, or the data gets displayed before it's sorted, rather than the other way around. It's a little bit of a wack-a-mole problem, where I think I've simplified things and gotten it working, only to find out I've broken things somewhere else.
I have functions that do things like:
widgetAFunction
load data into widget B
tell widget B to sort the data
tell widget B to display the data
My love of code reuse makes me want to do something like write a loadData function in widget A that goes something like this:
widgetBLoadDataFunction
update data
sort the data
refresh the view
So that all widgetA has to do is call one function on widgetB. But then there are cases where I just want to sort the data, without updating the data, so I write:
widgetBSortFunction
sort the data
refresh the view
And then maybe I want a filter function
widgetBFilterFunction
filter the data
refresh the view
And maybe I want to be update the data but not sort it, so I have
widgetBNoSortLoadDataFunction
update data
refresh the view
It doesn't seem that complex, but I wind up with these really long, very brittle chains of function calls, or a bunch of very similar calls. As Martin Fowler would say, the code is getting a little smelly.
So, what other alternatives do I have? I did something on a recent project where I did a state machine kind of thing, where I registered a bunch of functions with a set of conditions, or states which would trigger their execution. That worked somewhat well, and I'm thinking that approach might be good to use again.
Does anyone know what I'm talking about here, and even better, can anyone point me toward some patterns that will help me get my head around this better?
What you need is a finite state machine implementation. Basically every finite state machine needs:
Events that the program responds to
States where the program waits between events
Transitions between states in response to events
Actions taken during transitions
Variables that hold values needed by actions between events
A good article from IBM teachs you a way of implementing it by means of Javascript.
Edit: Here is a FSM builder, so you don't have to build your own.
Fernando already mentioned FSMs, and gave good info and links. :)
In addition, I'll add that your classes should already incorporate enough state so that you're not worried about sorting twice, etc. I.e., widgetB.sort() should check if it's been sorted since last update and just return if so. There's practically no downside to doing this, and it can improve performance (and also guard consistency).
I have seen this link: Implementing Mutual Exclusion in JavaScript.
On the other hand, I have read that there are no threads in javascript, but what exactly does that mean?
When events occur, where in the code can they interrupt?
And if there are no threads in JS, do I need to use mutexes in JS or not?
Specifically, I am wondering about the effects of using functions called by setTimeout() and XmlHttpRequest's onreadystatechange on globally accessible variables.
Javascript is defined as a reentrant language which means there is no threading exposed to the user, there may be threads in the implementation. Functions like setTimeout() and asynchronous callbacks need to wait for the script engine to sleep before they're able to run.
That means that everything that happens in an event must be finished before the next event will be processed.
That being said, you may need a mutex if your code does something where it expects a value not to change between when the asynchronous event was fired and when the callback was called.
For example if you have a data structure where you click one button and it sends an XmlHttpRequest which calls a callback the changes the data structure in a destructive way, and you have another button that changes the same data structure directly, between when the event was fired and when the call back was executed the user could have clicked and updated the data structure before the callback which could then lose the value.
While you could create a race condition like that it's very easy to prevent that in your code since each function will be atomic. It would be a lot of work and take some odd coding patterns to create the race condition in fact.
The answers to this question are a bit outdated though correct at the time they were given. And still correct if looking at a client-side javascript application that does NOT use webworkers.
Articles on web-workers:
multithreading in javascript using webworkers
Mozilla on webworkers
This clearly shows that javascript via web-workers has multithreading capabilities. As concerning to the question are mutexes needed in javascript? I am unsure of this. But this stackoverflow post seems relevant:
Mutual Exclusion for N Asynchronous Threads
Yes, mutexes can be required in Javascript when accessing resources that are shared between tabs/windows, like localStorage.
For example, if a user has two tabs open, simple code like the following is unsafe:
function appendToList(item) {
var list = localStorage["myKey"];
if (list) {
list += "," + item;
}
else {
list = item;
}
localStorage["myKey"] = list;
}
Between the time that the localStorage item is 'got' and 'set', another tab could have modified the value. It's generally unlikely, but possible - you'd need to judge for yourself the likelihood and risk associated with any contention in your particular circumstances.
See the following articles for a more detail:
Wait, Don't Touch That: Mutual Exclusion Locks & JavaScript - Medium Engineering
JavaScript concurrency and locking the HTML5 localStorage - Benjamin Dumke-von der Eh, Stackoverflow
As #william points out,
you may need a mutex if your code does something where it expects a
value not to change between when the asynchronous event was fired and
when the callback was called.
This can be generalised further - if your code does something where it expects exclusive control of a resource until an asynchronous request resolves, you may need a mutex.
A simple example is where you have a button that fires an ajax call to create a record in the back end. You might need a bit of code to protect you from trigger happy users clicking away and thereby creating multiple records. there are a number of approaches to this problem (e.g. disable the button, enable on ajax success). You could also use a simple lock:
var save_lock = false;
$('#save_button').click(function(){
if(!save_lock){
//lock
save_lock=true;
$.ajax({
success:function()
//unlock
save_lock = false;
}
});
}
}
I'm not sure if that's the best approach and I would be interested to see how others handle mutual exclusion in javascript, but as far as i'm aware that's a simple mutex and it is handy.
JavaScript is single threaded... though Chrome may be a new beast (I think it is also single threaded, but each tab has it's own JavaScript thread... I haven't looked into it in detail, so don't quote me there).
However, one thing you DO need to worry about is how your JavaScript will handle multiple ajax requests coming back in not the same order you send them. So, all you really need to worry about is make sure your ajax calls are handled in a way that they won't step on eachother's feet if the results come back in a different order than you sent them.
This goes for timeouts too...
When JavaScript grows multithreading, then maybe worry about mutexes and the like....
JavaScript, the language, can be as multithreaded as you want, but browser embeddings of the javascript engine only runs one callback (onload, onfocus, <script>, etc...) at a time (per tab, presumably). William's suggestion of using a Mutex for changes between registering and receiving a callback should not be taken too literally because of this, as you wouldn't want to block in the intervening callback since the callback that will unlock it will be blocked behind the current callback! (Wow, English sucks for talking about threading.) In this case, you probably want to do something along the lines of redispatching the current event if a flag is set, either literally or with the likes of setTimeout().
If you are using a different embedding of JS, and that executes multiple threads at once, it can get a bit more dicey, but due to the way JS can use callbacks so easily and locks objects on property access explicit locking is not nearly as necessary. However, I would be surprised if an embedding designed for general code (eg, game scripting) that used multi threading didn't also give some explicit locking primitives as well.
Sorry for the wall of text!
Events are signaled, but JavaScript execution is still single-threaded.
My understanding is that when event is signaled the engine stops what it is executing at the moment to run event handler. After the handler is finished, script execution is resumed. If event handler changed some shared variables then resumed code will see these changes appearing "out of the blue".
If you want to "protect" shared data, simple boolean flag should be sufficient.