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This is my first post on stackoverflow, so please don't flame me too hard if I come across like a total nitwit or if I'm unable ot make myself perfectly clear. :-)
Here's my problem: I'm trying to write a javascript function that "ties" two functions to another by checking the first one's completion and then executing the second one.
The easy solution to this obviously would be to write a meta function that calls both functions within it's scope. However, if the first function is asynchronous (specifically an AJAX call) and the second function requires the first one's result data, that simply won't work.
My idea for a solution was to give the first function a "flag", i.e. making it create a public property "this.trigger" (initialized as "0", set to "1" upon completion) once it is called; doing that should make it possible for another function to check the flag for its value ([0,1]). If the condition is met ("trigger == 1") the second function should get called.
The following is an abstract example code that I have used for testing:
<script type="text/javascript" >
/**/function cllFnc(tgt) { //!! first function
this.trigger = 0 ;
var trigger = this.trigger ;
var _tgt = document.getElementById(tgt) ; //!! changes the color of the target div to signalize the function's execution
_tgt.style.background = '#66f' ;
alert('Calling! ...') ;
setTimeout(function() { //!! in place of an AJAX call, duration 5000ms
trigger = 1 ;
},5000) ;
}
/**/function rcvFnc(tgt) { //!! second function that should get called upon the first function's completion
var _tgt = document.getElementById(tgt) ; //!! changes color of the target div to signalize the function's execution
_tgt.style.background = '#f63' ;
alert('... Someone picked up!') ;
}
/**/function callCheck(obj) {
//alert(obj.trigger ) ; //!! correctly returns initial "0"
if(obj.trigger == 1) { //!! here's the problem: trigger never receives change from function on success and thus function two never fires
alert('trigger is one') ;
return true ;
} else if(obj.trigger == 0) {
return false ;
}
}
/**/function tieExc(fncA,fncB,prms) {
if(fncA == 'cllFnc') {
var objA = new cllFnc(prms) ;
alert(typeof objA + '\n' + objA.trigger) ; //!! returns expected values "object" and "0"
}
//room for more case definitions
var myItv = window.setInterval(function() {
document.getElementById(prms).innerHTML = new Date() ; //!! displays date in target div to signalize the interval increments
var myCallCheck = new callCheck(objA) ;
if( myCallCheck == true ) {
if(fncB == 'rcvFnc') {
var objB = new rcvFnc(prms) ;
}
//room for more case definitions
window.clearInterval(myItv) ;
} else if( myCallCheck == false ) {
return ;
}
},500) ;
}
</script>
The HTML part for testing:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/strict.dtd >
<html>
<head>
<script type="text/javascript" >
<!-- see above -->
</script>
<title>
Test page
</title>
</head>
<body>
<!-- !! testing area -->
<div id='target' style='float:left ; height:6em ; width:8em ; padding:0.1em 0 0 0; font-size:5em ; text-align:center ; font-weight:bold ; color:#eee ; background:#fff;border:0.1em solid #555 ; -webkit-border-radius:0.5em ;' >
Test Div
</div>
<div style="float:left;" >
<input type="button" value="tie calls" onmousedown="tieExc('cllFnc','rcvFnc','target') ;" />
</div>
<body>
</html>
I'm pretty sure that this is some issue with javascript scope as I have checked whether the trigger gets set to "1" correctly and it does. Very likely the "checkCall()" function does not receive the updated object but instead only checks its old instance which obviously never flags completion by setting "this.trigger" to "1". If so I don't know how to address that issue.
Anyway, hope someone has an idea or experience with this particular kind of problem.
Thanks for reading!
FK
629
The code inside a function is executed when the function is invoked. It is common to use the term "call a function" instead of "invoke a function". It is also common to say "call upon a function", "start a function", or "execute a function".
There are 3 ways of writing a function in JavaScript: Function Declaration. Function Expression. Arrow Function.
It's a new feature that introduced in ES6 and is called arrow function. The left part denotes the input of a function and the right part the output of that function.
You can take advantage of a feature of JS called closure. Combine that with a very common JS pattern called "continuation passing style" and you have your solution. (Neither of these things are original to JS, but are heavily used in JS).
// a function
function foo(some_input_for_foo, callback)
{
// do some stuff to get results
callback(results); // call our callback when finished
}
// same again
function bar(some_input_for_bar, callback)
{
// do some stuff to get results
callback(results); // call our callback when finished
}
The "continuation passing style" refers to the callback. Instead of returning a value, each function calls a callback (the continuation) and gives it the results.
You can then tie the two together easily:
foo(input1, function(results1) {
bar(results1, function(results2) {
alert(results2);
});
});
The nested anonymous functions can "see" variables from the scope they live in. So there's no need to use special properties to pass information around.
Update
To clarify, in your question's code snippet, it's clear that you are thinking roughly like this:
I have a long-running asynchronous operation, so I need to know when it finishes in order to start the next operation. So I need to make that state visible as a property. Then elsewhere I can run in a loop, repeatedly examining that property to see when it changes to the "completed" state, so I know when to continue.
(And then as a complicating factor, the loop has to use setInterval to start running and clearInterval to quit, to allow other JS code to run - but it's basically a "polling loop" nevertheless).
You do not need to do that!
Instead of making your first function set a property on completion, make it call a function.
To make this absolutely clear, let's refactor your original code:
function cllFnc(tgt) { //!! first function
this.trigger = 0 ;
var trigger = this.trigger ;
var _tgt = document.getElementById(tgt) ; //!! changes the color...
_tgt.style.background = '#66f' ;
alert('Calling! ...') ;
setTimeout(function() { //!! in place of an AJAX call, duration 5000ms
trigger = 1 ;
},5000) ;
}
[Update 2: By the way, there's a bug there. You copy the current value of the trigger property into a new local variable called trigger. Then at the end you assign 1 to that local variable. No one else is going to be able to see that. Local variables are private to a function. But you don't need to do any of this anyway, so keep reading...]
All we have to do is tell that function what to call when it's done, and get rid of the property-setting:
function cllFnc(tgt, finishedFunction) { //!! first function
var _tgt = document.getElementById(tgt) ; //!! changes the color...
_tgt.style.background = '#66f' ;
alert('Calling! ...') ;
setTimeout(function() { //!! in place of an AJAX call, duration 5000ms
finishedFunction(); // <-------- call function instead of set property
},5000) ;
}
There's now no need for your "call-check" or your special tieExc helper. You can easily tie two functions together with very little code.
var mySpan = "#myspan";
cllFnc(mySpan, function() { rcvFnc(mySpan); });
Another advantage of this is that we can pass different parameters to the second function. With your approach, the same parameters are passed to both.
For example, the first function might do a couple of calls to an AJAX service (using jQuery for brevity):
function getCustomerBillAmount(name, callback) {
$.get("/ajax/getCustomerIdByName/" + name, function(id) {
$.get("/ajax/getCustomerBillAmountById/" + id), callback);
});
}
Here, callback accepts the customer bill amount, and the AJAX get call passes the received value to the function we pass it, so the callback is already compatible and so can directly act as the callback for the second AJAX call. So this is itself an example of tying two asynchronous calls together in sequence and wrapping them in what appears (from the outside) to be a single asynchronous function.
Then we can chain this with another operation:
function displayBillAmount(amount) {
$("#billAmount").text(amount);
}
getCustomerBillAmount("Simpson, Homer J.", displayBillAmount);
Or we could (again) have used an anonymous function:
getCustomerBillAmount("Simpson, Homer J.", function(amount) {
$("#billAmount").text(amount);
});
So by chaining function calls like this, each step can pass information forward to the next step as soon as it is available.
By making functions execute a callback when they're done, you are freed from any limitations to how each functions works internally. It can do AJAX calls, timers, whatever. As long as the "continuation" callback is passed forward, there can be any number of layers of asynchronous work.
Basically, in an asynchronous system, if you ever find yourself writing a loop to check a variable and find out if it has changed state, then something has gone wrong somewhere. Instead there should be a way to supply a function that will be called when the state changes.
Update 3
I see elsewhere in comments you mention that the actual problem is caching results, so all my work explaining this was a waste of time. This is the kind of thing you should put in the question.
Update 4
More recently I've written a short blog post on the subject of caching asynchronous call results in JavaScript.
(end of update 4)
Another way to share results is to provide a way for one callback to "broadcast" or "publish" to several subscribers:
function pubsub() {
var subscribers = [];
return {
subscribe: function(s) {
subscribers.push(s);
},
publish: function(arg1, arg2, arg3, arg4) {
for (var n = 0; n < subscribers.length; n++) {
subscribers[n](arg1, arg2, arg3, arg4);
}
}
};
}
So:
finished = pubsub();
// subscribe as many times as you want:
finished.subscribe(function(msg) {
alert(msg);
});
finished.subscribe(function(msg) {
window.title = msg;
});
finished.subscribe(function(msg) {
sendMail("[email protected]", "finished", msg);
});
Then let some slow operation publish its results:
lookupTaxRecords("Homer J. Simpson", finished.publish);
When that one call finishes, it will now call all three subscribers.
160
The definitive answer to this "call me when you're ready" problem is a callback. A callback is basically a function that you assign to an object property (like "onload"). When object state changes, the function is called. For example, this function makes an ajax request to the given url and screams when it's complete:
function ajax(url) {
var req = new XMLHttpRequest();
req.open('GET', url, true);
req.onreadystatechange = function (aEvt) {
if(req.readyState == 4)
alert("Ready!")
}
req.send(null);
}
Of course, this is not flexible enough, because we presumably want different actions for different ajax calls. Fortunately, javascript is a functional language, so we can simply pass the required action as a parameter:
function ajax(url, action) {
var req = new XMLHttpRequest();
req.open('GET', url, true);
req.onreadystatechange = function (aEvt) {
if(req.readyState == 4)
action(req.responseText);
}
req.send(null);
}
This second function can be used like this:
ajax("http://...", function(text) {
do something with ajax response
});
As per comments, here an example how to use ajax within an object
function someObj()
{
this.someVar = 1234;
this.ajaxCall = function(url) {
var req = new XMLHttpRequest();
req.open('GET', url, true);
var me = this; // <-- "close" this
req.onreadystatechange = function () {
if(req.readyState == 4) {
// save data...
me.data = req.responseText;
// ...and/or process it right away
me.process(req.responseText);
}
}
req.send(null);
}
this.process = function(data) {
alert(this.someVar); // we didn't lost the context
alert(data); // and we've got data!
}
}
o = new someObj;
o.ajaxCall("http://....");
The idea is to "close" (aliased) "this" in the event handler, so that it can be passed further.
20
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