I'm looking for a way to save reference to two this objects in one function called after an event triggers in jQuery - this reference to the object the method is defined in (so I can use this.anotherObjectFunction()) and this reference to the object that triggered the event - so that I can use $(this).someJQueryFunction later on. The way I'd like to do it is by passing a this (function object) reference as an argument to the function. Unfortunately, the function is to be called by jQuery, not me, so it's passed as a reference, i.e.
someFunction: function()
{
...
cell.$el.on('click', 'li.multiselect-option', this.myClickFunction);
...
},
myClickFunction: function(objectReference)
{
//It should be able to call methods of that object.
objectReference.anotherFunction();
//And reference to the clicked item.
$(this).html("Don't click me anymore!!!");
}
I'm aware of the fact that I can do something like
cell.$el.on('click', 'li.multiselect-option', myFunction.bind(this));
...
myClickFunction: function(event)
{
this.anotherFunction();
$(event.currentTarget).html("Don't click me anymore!!!");
}
But this workaround doesn't really answer the question as it doesn't show how to pass additional arguments and in the future there may be a necessity to pass another (no, I don't want to register them as fields in the object).
No anonymous functions are allowed unless they can be easily removed with cell.$el.off() function that will remove them and only them (there are some other function associated with the same objects and events at the same time and they should remain intact).
UPDATE:
By no anonymous functions I mean solutions like:
var self = this;
cell.$el.on('click', 'li.multiselect-option', function() {
self.MyClickFunction(self, this);
});
They will not work because I'll have to use cell.$el.off() with the function reference (3-argument prototype) to remove this single function and only it, leaving other functions bound to both the same element and event.
Jquery .on event has option to pass the argument as parameter in event handler like this
cell.$el.on('click', 'li.multiselect-option', {arg1:'arg1' , arg2:'arg2'} , myFunction);
...
myClickFunction: function(event)
{
alert(event.data.arg1);
alert(event.data.arg2);
this.anotherFunction();
$(event.currentTarget).html("Don't click me anymore!!!");
}
Passing data to the handler
If a data argument is provided to .on() and is not null or undefined,
it is passed to the handler in the event.data property each time an
event is triggered. The data argument can be any type, but if a string
is used the selector must either be provided or explicitly passed as
null so that the data is not mistaken for a selector. Best practice is
to use a plain object so that multiple values can be passed as
properties.
or another way
cell.$el.on('click', 'li.multiselect-option', function() {
myClickFunction("hai" , "bye");
});
myClickFunction: function(arg1, arg2)
{
alert(arg1);
alert(arg2);
}
And I would also suggest a plugin-free solution to the question "how to supply a parameter to function reference"
Example
function x(a){
console.log(a);
}
setInterval('x(1)',1000);
Related
I'm not sure if this has been asked before because I don't know what it's called.
But why wouldn't a method like this work? Below is just a general example
<script>
document.getElementById('main_div').onclick=clickie(argument1,argument2);
function clickie(parameter1,parameter2){
//code here
}
</script>
The code above would work fine if the event handler was assigned without parameters, but with parameters, it doesn't work. I think I read online that to overcome this problem, you could use closures. I'm assuming it's because of the parentheses ( ) that is calling the function immediately instead of assigning it to the event?
Because you're calling the function immediately and returning the result, not referencing it.
When adding the parenthesis you call the function and pass the result back to onclick
document.getElementById('main_div').onclick = clickie(); // returns undefined
so it's actually equal to writing
document.getElementById('main_div').onclick = undefined;
which is not what you want, you want
document.getElementById('main_div').onclick = clickie;
but then you can't pass arguments, so to do that you could use an anonymous function as well
document.getElementById('main_div').onclick = function() {
clickie(argument1,argument2);
}
or use bind
document.getElementById('main_div').onclick = yourFunc.bind(this, [argument1, argument2]);
It is however generally better to use addEventListener to attach event listeners, but the same principle applies, it's either (without arguments)
document.getElementById('main_div').addEventListener('click', clickie, false);
or bind or the anonymous function to pass arguments etc.
document.getElementById('main_div').addEventListener('click', function() {
clickie(argument1,argument2);
}, false);
The easiest way is:
yourElement.onclick = yourFunc.bind(this, [arg1, arg2]);
function yourFunc (args, event) {
// here you can work with you array of the arguments 'args'
}
When you say onClick = function() {...} you are registering your function with some internal JavaScript library. So when the "click" happens, that library invokes your function.
Now imagine you're the author of that library and someone registered their function with it. How would you know how many parameters to pass to the function? How would you know know what kind of parameters to pass in?
clickie(argument1,argument2)
This means to invoke the function and return its return value.
clickie
This simply is a reference to the function (doesn't invoke/execute it)
To bind an event to a element, you need to use either the attachEvent or addEventListener method. For example.
/* Non IE*/
document.getElementById('main_div').addEventListener('click', function () {}, false);
/* IE */
document.getElementById('main_div').attachEvent('onclick', function () {});
A function name followed by parentheses is interpreted as a function call or the start of a function declaration. The a onclick property needs to be set to a function object. A function declaration is a statement, and is not itself a function. It doesn't return a reference to the function. Instead it has the side effect of creating a variable in the global scope that refers to a new function object.
function clickie(param) { return true; }
creates a global variable named clickie that refers to a function object. One could then assign that object as an event handler like so: element.onclick = clickie;. An anonymous function declaration (often confused with a closure; for the difference see Closure vs Anonymous function (difference?)) does return a function object and can be assigned to a property as an event handler, as follows:
element.onclick = function(event) { return true; };
But this doesn't work:
element.onclick = function clickie(event) { return true;};
Why? Because function clickie(event) { return true;} is a statement, not a function. It doesn't return anything. So there is nothing to be assigned to the onclick property. Hope this helps.
I'm learning to develop Windows 8 style applications with the help of a book. The chapter I'm reading focuses on HTML, CSS and JavaScript languages for developing. The application displays in a ListView the images you have in the My Pictures Folder and deletes them when the user clicks or taps an image. Here is the code that implements the deletion of an image in the ListView:
var lv = document.getElementById('lv');
lv.addEventListener('iteminvoked', function (eventObj) {
eventObj.detail.itemPromise.then(function (listViewItem) {
var binding = files.dataSource.createListBinding();
binding.fromIndex(listViewItem.index).then(function (dataItem) {
var key = dataItem.key;
files.dataSource.remove(key);
binding.release();
});
});
});
My question is, where does the eventObj parameter of the anonymous function in the addEventListener method gets its value? I have found a similar question asked here: Passing arguments in anonymous functions in JavaScript, but i cannot fully understand it. I searched the documentation for addEventListener on MSDN but it just says it takes an event handler function, but it doesn't say anything about the parameters. Thanks in advance.
It's rather simple: whatever function internally calls that callback passes the arguments. See, addEventListener tells the executing Javascript engine to call the callback function you specify whenever an event occurs. The javascript engine saves your anonymous function in some variable - and cann call it later on using that exact variable, passing any number of arguments.
To illustrate it, consider something like this the internal function that handels events (purlely fictional, just to illustrate how it could be done):
var callbacks = [];
function addEventListener(newEvent, newCallback) {
callbacks.push({event : newEvent, callback : newCallback});
}
function handleEvent (someEvent) {
for (var i = 0 ; i < callbacks.length ; i++ ) {
if (callbacks[i].event == someEvent.name) {
callbacks[i].callback(someEvent);
}
}
}
Some more explanation:
As javascript is a so-called "functional language", functions are just values of variables.
function someFunc () {}
is actually just some kind of shortcut (technically it's not, but it does the same thing) for
var someFunc = function () {}
This having said, it's of cours possible to associate multiple names with one function:
var someFunc = function () {}
var sameFunc = someFunc;
var stillSame = somefunc;
var alsoSame = stillSame;
and you can call that function using any of those names, including passing arguments of course:
var someFunc = function (arg) { alert(arg); }
var sameFunc = someFunc;
sameFunc("It worx");
You can even call a function without ever naming it:
(function () {alert("test")})();<
or
(function (arg) { alert(arg); })("test")
Using this concept to perversion finally leads (long way to go however) to things like the y-combinator.
Event handlers may be attached to various objects including DOM
elements, document, the window object, etc. When an event occurs, an
event object is created and passed sequentially to the event
listeners.
Source: https://developer.mozilla.org/en-US/docs/Web/API/Event
An event listener or event handler can be an anonymous function or named function, it really doesn’t matter. The point is that it’s the event interface that defines the event object that is passed to the handler.
To find out exactly the event property from the event you are using, please refer to the windows docs: http://msdn.microsoft.com/en-us/library/windows/apps/br211827.aspx
The arguments recieved by the event listener are sent from the dispatchEvent, i.e. when the event dispatched it passes an event object to your handler.
Refer to this documentation on how to create and dispatch the event. The event object can vary in structure to convey information to the eventhandler to execute necessary steps. So in your case when you do lv.dispatchEvent(newevent) this sends an newevent as eventObj to your event handler.
Keep in mind there can be multiple eventhandlers listening to an event so the browser maintains a stack for the eventlisteners running them sequentially with each of them passed eventObj.
Anonymous function is no different from a named function. In JavaScript functions are first-class objects meaning regular objects. So you can pass them like regular objects(numbers,strings) without having to name them. Only thing is reuse becomes an issue.
What you need to understand this code is to rewrite it a bit:
var lv = document.getElementById('lv'),
invokeHandler = function (eventObj) {
var promiseFullfilled = function (listViewItem) {
var binding = files.dataSource.createListBinding(),
anotherPromiseFullfilled = function (dataItem) {
var key = dataItem.key;
files.dataSource.remove(key);
binding.release();
};
binding.fromIndex(listViewItem.index).then(anotherPromiseFullfilled);
};
eventObj.detail.itemPromise.then(promiseFullfilled);
};
lv.addEventListener('iteminvoked', invokeHandler);
This code works just the same, however it is now obvious that addEventListener or then actually do not know anything about the callback functions they are passed with. They can, however, use Function.prototype.call or Function.prototype.apply to apply arguments:
// This is PSEUDOCODE, event model actually works in a totally different way
HTMLElement.prototype.addEventListener = function(eventType, callback, bubbles) {
// callbacks is some internal collection for this specific element, probably available via a closure, looks something like:
// {
// 'someEventType': [callback1, callback2],
// 'someOtherEvent': [callback1, callback3, callback4]
// }
callbacks[eventType].push(callback);
}
// This is called whenever an event is triggered on an element
HTMLElement.prototype.dispatchEvent = function(event) {
callbacks[event.type].forEach( function(callback) {
return callback.call(this, event); // the callback is called with 'this' set to the element DOM object, and 'event' is the first argument
});
// then it can bubble or cancel depending on the event type and callback results
}
it is a CustomEvent, and all the process is like that:
//you add a anonymous function to a specific listener
lv.addEventListener('iteminvoked', function (eventObj) {
console.log(eventObj===myEvent);
});
//somewhere in your code a CustomEvent gets created based on "iteminvoked" key
var myEvent = new CustomEvent("iteminvoked", {
itemInfo: {
name: "yourItem"
},
bubbles: true,
cancelable: false
});
//somewhere when an item gets invoked this code raise the `iteminvoked` trigger
lv.dispatchEvent(myEvent);
all the functions that are passed as a listener are stored based on the key, something like:
var observers = {
"iteminvoked" : [f1, f2],
//other keys
}
it doesn't have anything to do with not having name, the function object is stored in the some kind of array. and dispatchEvent goes thru the array and invokes all the functions, and pass the myEvent as their parameter. It is a Observer pattern, implemented in javascript, I have implemented it once in my own javascript library like:
var lv = /*your element*/;
if(observers["iteminvoked"]){
for(var i=0;i<observables["iteminvoked"].length;i++){
var func = observables["iteminvoked"][i];
var o = func.call(lv, myEvent);
//this line is to support return false
if(o!==undefined && o===false) break;
}
}
as you can see it is dispatchEvent resplonsiblity to invoke all the observers, and your function no matter it has name or not gets invoked with the lv as the this context and myEvent as the parameter.
I was trying the following:
f.addEventListener('submit',(function(frm){
var func = (function(e){somefunction(e,frm);})(e);
})(f),false);
But this is failing. I want to pass the form (f) as a static reference and the dynamic event object to the named function 'somefunction'.
What I have above isnt working, what is the right syntax for passing both?
The issue is that each of the functions is being called right away, with undefined actually being passed to addEventListener().
You'll want to instead return one of the functions without its calling parenthesis so the event can call it later:
f.addEventListener('submit', (function (frm) {
return function (e) {
someFunction(e, frm);
};
})(f), false);
Though, with event bindings, you may not necessarily need the closure, as the <form> will be the context (this) of the function passed:
f.addEventListener('submit', someFunction, false);
function someFunction(e) {
var frm = this;
// ...
}
not saure exactly what you are trying to do but, to looks like you are trying to manually pass in the form via the event handler. Instead save a reference and just refer to it in the handler such as
f.addEventListener('submit',function(){
var func = function(e){
somefunction(e,f);
};
},false);
you shouldn't need the self executing functions unless I am missing your intent here
I am binding a select of a custom control to a function. I just want to clarify when I bind something like this
auditFileUpload.bind("select", uploadSelectfunction);
I know I can use the following function
uploadSelectfunction(e)
{
}
how is e getting passed? is it the events? or the object itself?
if I had a function like this, how would I bind it?
uploadSelectfunction(e, datatype)
{
}
auditFileUpload.bind("select", function() { uploadSelectfunction(events, "CSV" });
I tried events, it doesn't work..
sugesstions? clarifications?
The events parameter is simply the first parameter passed to the event handler/method. Just pass that on:
auditFileUpload.bind("select", function(events) { uploadSelectfunction(events, "CSV"
In the original version uploadSelectfunction is simply a pointer to a function that happens to take one argument. That events argument is defined inside the select event.
If you put an inline method instead (as you have done) you need to accept the events parameter, and pass it on to your code.
I've got a function, like this:
menu[0].onclick = function() {
filters.reset_all();
clients.get();
}
So it's called when user clicks on the first menu element. Now I need to call it form the other place and what I've done is this:
if (li.onclick) { //li and menu[0] above are the same
li.onclick.call();
}
and it works well.
But now I need to pass some params to onclick function. I've tried this .call(some_param); but arguments array in onclick is empty.
What am I doing wrong?
edit: changed into this:
menu[0].onclick = function(arg) {
console.log(arg);
filters.reset_all();
clients.get();
}
and
li.onclick.call(li,param);
still nothing
The first argument to .call() is the value for the this pointer. The 2nd, 3rd, etc... arguments get passed to the function. You only need to use .call() if you're explicitly trying to set the this pointer. Otherwise, you can just directly call the function li.onclick().
Event handlers are generally called by the system and when that happens, they are passed the event object as the first argument. If you want a function available that takes different arguments, you should create your own function for that and not use the event handling function. Your event handler can also call this other function if desired.
menu[0].onclick = function() {
myFunc("aaa");
}
function myFunc(arg1) {
// do whatever here
filters.reset_all();
clients.get();
}
if (li.onclick) {
myFunc("bbb");
}
it should be
call(object, param1, param2, param3...);
In your case you can write
li.onclick.call(li, param1);
The first parameter of call() is the context (in this case this) the rest of the parameters go into the arguments array in order. If you use apply() instead of call then you have just two parameters: apply(this, arguments_array)