I wanted to call a function when all required images are loaded. The number of images is known in advance, so I tried attaching a function call to the onload event of each image and count the number of times it was called.
<html>
<head>
<script>
var tractor;
function Tractor()
{
this.init_graphics();
}
Tractor.prototype.init_graphics = function()
{
this.gr_max = 3;
this.load_count = 0;
this.loading_complete(); // #1 test call, works OK
this.img1 = new Image();
this.img1.onload = this.loading_complete; // #2 gets called, but gr_max = undefined, load_count = NaN
this.img1.src = "http://dl.dropbox.com/u/217824/tmp/rearwheel.gif"; //just a test image
}
Tractor.prototype.loading_complete = function()
{
this.load_count += 1;
alert("this.loading_complete, load_count = " + this.load_count + ", gr_max = " + this.gr_max);
if(this.load_count >= this.gr_max) {this.proceed();}
};
function start()
{
tractor = new Tractor();
}
</script>
</head>
<body onload="start();">
</body>
</html>
When it's just called from another function of the object (see #1), it works just as I expected. When, however, it's called from onload event (see #2), the variables become "undefined" or "NaN" or something. What's happening? What am I doing wrong? How do I make it work?
I don't remember ever creating my own objects in Javascript before, so I certainly deeply apologize for this "what's wrong with my code" kind of question. I used this article as a reference, section 1.2, mainly.
Just in case, I put the same code on http://jsfiddle.net/ffJLn/
bind the context to the callback:
this.img1.onload = this.loading_complete.bind(this);
See: http://jsfiddle.net/ffJLn/1/ (same as yours but with this addition)
Here's an explanation of how bind works in detail: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind
The basic idea is that it makes this in the bound function equal to whatever you pass as the parameter to bind.
Another option is to create a closure:
var self = this;
this.img1.onload = function() { self.loading_complete() };
Closures are functions that keep references to their context (in fact, all functions in javascript work this way). So here you are creating an anonymous function that keeps a reference to self. So this is another way to maintain context and for loading_complete to have the right this.
See: http://jsfiddle.net/ffJLn/2/ (same as yours but with the second possibility)
When #2 gets called, your this has changed. this now refers to the new Image() rather than the Tractor object.
Try changing...
this.img1.onload = this.loading_complete;
to
var that = this;
this.img1.onload = function() { that.loading_complete(); };
You can now use es6 arrow functions which provide lexical binding:
this.img1.onload = () => { this.loading_complete(); };
Related
I'd like to 'proxy' (not sure if that's the term at all) a function inside a function object for easy calling.
Given the following code
function Soldier() {
this.el = $("<div></div>").addClass('soldier');
this.pos = this.el.position; // $(".soldier").position(), or so I thought
}
In the console:
s = new Soldier();
$("#gamemap").append(s.el); // Add the soldier to the game field
s.pos === s.el.position // this returns true
s.el.position() // Returns Object {top: 0, left: 0}
s.pos() // Returns 'undefined'
What am I doing wrong in this scenario and is there an easy way to achieve my goal (s.pos() to return the result of s.el.position()) ?
I thought about s.pos = function() { return s.el.position(); } but looks a bit ugly and not apropriate. Also I'd like to add more similar functions and the library will become quite big to even load.
When you're calling s.pos(), its this context is lost.
You can simulate this behavior using call():
s.pos.call(s); // same as s.pos()
s.pos.call(s.el); // same as s.el.position()
This code is actually ok:
s.pos = function() { return s.el.position(); }
An alternative is using bind():
s.pos = s.el.position.bind(el);
You can use the prototype, that way the functions will not be created separately for every object:
Soldier.prototype.pos = function(){ return this.el.position(); }
I'd recommend to use the prototype:
Soldier.prototype.pos = function() { return this.el.position(); };
Not ugly at all, and quite performant actually.
If you want to directly assign it in the constructor, you'll need to notice that the this context of a s.pos() invocation would be wrong. You therefore would need to bind it:
…
this.pos = this.el.position.bind(this.el);
It's because the context of execution for position method has changed. If you bind the method to work inside the element context it will work.
JS Fiddle
function Soldier() {
this.el = $("<div></div>").addClass('soldier');
this.pos = this.el.position.bind(this.el);
}
var s = new Soldier();
$("#gamemap").append(s.el);
console.log(s.pos());
Here is a simplified snippet from some code I wrote for managing tablet gestures on canvas elements
first a function that accepts an element and a dictionary of callbacks and register the events plus adding other features like 'hold' gestures:
function registerStageGestures(stage, callbacks, recieverArg) {
stage.inhold = false;
stage.timer = null;
var touchduration = 1000;
var reciever = recieverArg || window;
stage.onLongTouch = function(e) {
if (stage.timer) clearTimeout(stage.timer);
stage.inhold = true;
if (callbacks.touchholdstart) callbacks.touchholdstart.call(reciever, e);
};
stage.getContent().addEventListener('touchstart', function(e) {
e.preventDefault();
calcTouchEventData(e);
stage.timer = setTimeout(function() {
stage.onLongTouch(e);
}, touchduration);
if (callbacks.touchstart) callbacks.touchholdstart.call(reciever, e);
});
stage.getContent().addEventListener('touchmove', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdmove) callbacks.touchholdmove.call(reciever, e);
} else {
if (callbacks.touchmove) callbacks.touchmove.call(reciever, e);
}
});
stage.getContent().addEventListener('touchend', function(e) {
e.preventDefault();
if (stage.timer) clearTimeout(stage.timer);
if (stage.inhold) {
if (callbacks.touchholdend) callbacks.touchholdend.call(reciever, e);
} else {
if (callbacks.touchend) callbacks.touchend.call(reciever, e);
}
stage.inhold = false;
});
}
later I call registerStageGestures on a few elements (represented by 'View' objects) in the same page. Something like:
function View() {
var self=this;
..
function InitView() {
...
registerStageGestures(kineticStage, {
touchstart: function(e) {
// do something
},
touchmove: function(e) {
// do something
},
touchendunction(e) {
// do something
},
touchholdstart: function(e) {
// do something
},
touchholdmove: function(e) {
// do something
},
touchholdend: function(e) {
// do something
},
}, self);
Everything works fine, however I'm left wondering about two things in the implementation of registerStageGestures:
First, is it necessary to make inhold, timer and onLongTouch members of the stage ? or will closures make everything works well if they are local vars in registerStageGestures ?
Second, is it necessary to call the callbacks with '.call(receiver,' syntax ? I'm doing this to make sure the callback code will run in the context of the View but I'm not sure if it's needed ?
any input is much appreciated
Thanks!
First, is it necessary to make inhold, timer and onLongTouch members
of the stage ? or will closures make everything works well if they are
local vars in registerStageGestures ?
As far as registerStageGestures() is concerned, var inhold, var timer and function onLongTouch(e) {...}. would suffice. The mechanism by which an inner function has automatic access to its outer function's members is known as "closure". You would only need to set stage.inhold, stage.timer and stage.onLongTouch if some other piece of code needs access to these settings as properties of stage.
Second, is it necessary to call the callbacks with '.call(receiver,'
syntax ? I'm doing this to make sure the callback code will run in the
context of the View but I'm not sure if it's needed ?
Possibly, depending on how those callbacks are written. .call() and .apply() are sometimes used when calling functions that use this internally. In both cases, the first parameter passed defines the object to be interpreted as this. Thus, javascript gives you the means of defining general purpose methods with no a priori assumption about the object to which those methods will apply when called. Similarly, you can call a method of an object in such a way that it acts on another object.
EDIT:
For completeness, please note that even in the absence of this in a function, .apply() can be very useful as it allows multiple parameters to be specified as elements of a single array, eg the ubiquitous jQuery.when.apply(null, arrayOfPromises)...
There are some simple answers, here.
First, closure:
Closure basically says that whatever is defined inside of a function, has access to the rest of that function's contents.
And all of those contents are guaranteed to stay alive (out of the trash), until there are no more objects left, which ere created inside.
A simple test:
var testClosure = function () {
var name = "Bob",
recallName = function () { return name; };
return { getName : recallName };
};
var test = testClosure();
console.log(test.getName()); // Bob
So anything that was created inside can be accessed by any function which was also created inside (or created inside of a function created in a function[, ...], inside).
var closure_2x = function () {
var name = "Bob",
innerScope = function () {
console.log(name);
return function () {
console.log("Still " + name);
}
};
return innerScope;
};
var inner_func = closure_2x();
var even_deeper = inner_func(); // "Bob"
even_deeper(); // "Still Bob"
This applies not only to variables/objects/functions created inside, but also to function arguments passed inside.
The arguments have no access to the inner-workings(unless passed to methods/callbacks), but the inner-workings will remember the arguments.
So as long as your functions are being created in the same scope as your values (or a child-scope), there's access.
.call is trickier.
You know what it does (replaces this inside of the function with the object you pass it)...
...but why and when, in this case are harder.
var Person = function (name, age) {
this.age = age;
this.getAge = function () {
return this.age;
};
};
var bob = new Person("Bob", 32);
This looks pretty normal.
Honestly, this could look a lot like Java or C# with a couple of tweaks.
bob.getAge(); // 32
Works like Java or C#, too.
doSomething.then(bob.getAge);
? Buh ?
We've now passed Bob's method into a function, as a function, all by itself.
var doug = { age : 28 };
doug.getAge = bob.getAge;
Now we've given doug a reference to directly use bobs methid -- not a copy, but a pointer to the actual method.
doug.getAge(); // 28
Well, that's odd.
What about what came out of passing it in as a callback?
var test = bob.getAge;
test(); // undefined
The reason for this, is, as you said, about context...
But the specific reason is because this inside of a function in JS isn't pre-compiled, or stored...
this is worked out on the fly, every time the function is called.
If you call
obj.method();
this === obj;
If you call
a.b.c.d();
this === a.b.c;
If you call
var test = bob.getAge;
test();
...?
this is equal to window.
In "strict mode" this doesn't happen (you get errors really quickly).
test.call(bob); //32
Balance restored!
Mostly...
There are still a few catches.
var outerScope = function () {
console.log(this.age);
var inner = function () {
console.log("Still " + this.age);
};
inner();
};
outerScope.call(bob);
// "32"
// "Still undefined"
This makes sense, when you think about it...
We know that if a function figures out this at the moment it's called -- scope has nothing to do with it...
...and we didn't add inner to an object...
this.inner = inner;
this.inner();
would have worked just fine (but now you just messed with an external object)...
So inner saw this as window.
The solution would either be to use .call, or .apply, or to use function-scoping and/or closure
var person = this,
inner = function () { console.log(person.age); };
The rabbit hole goes deeper, but my phone is dying...
function DialogWindow(contents, clickEvent){
// given a contents we can create a DialogWindow of that content
this.contents = contents;
this.domElement = $(".hideMe");
this.test = function(){
console.log(this.contents);
}
$("div").click(this.test); //<-note this spot
/*
This function simply returns the html representation of the contents
This function should be called withen WindowPage so that it can fully figure out what
Arguments:
container: the container that containes the WindowPage that this is apart of
lanes: the number of lanes on the page. Defined as: lanes = (number of columns in WindowPage) * 3 + 1
*/
this.toHtml = function(container, lanes){
var dialogWidth = container.width()/(lanes/2);
var dialogSep = container.width()/lanes;
var htmlWindow = jQuery('<div/>', {
id: "dialogWindow"
}).css("width", dialogWidth).css("height", dialogWidth).css("position","absolute");
jQuery(this.contents.toHtml()).appendTo(htmlWindow);
this.domElement = htmlWindow;
return htmlWindow;
}
}
My goal is to get a click of htmlWindow, to execute a function of DialogWindow. However whenever I do this all of DialogWindows properties return undefined. If I replace the line:
$("div").click(this.test);
with
$("div").click(this.test());
Then the function test() fires imediatelly and works (i.e. prints this.contents to the console). However if I leave that line as it was (i.e. I wait for a click to make the test() function fire) then it prints undefined to the console.
It is because this inside test does not point to DialogWindow object, it points to the clicked dom element
One solution is to pass a custom execution proxy to the event callback using $.proxy()
this.test = function(){
console.log(this.contents);
}
$("div").click($.proxy(this.test, this)); //<-note this spot
Another popular solution is to use a closure variable
var self = this
this.test = function(){
console.log(self.contents);
}
$("div").click(this.test); //<-note this spot
In this scenario I'll prefer to use the former method
Background: I am trying to edit a zen cart horizontal pop out menu to make the popout open inline within the menu. The problem I am having is that I am struggling to get my head around the javascript/jquery that came with it.
Without posting the whole thing the structure of the code is something like this:
(declare some vars)
//some functions like this:
function funcname(obj) {
//do something
}
//then one big master function like this:
function bigfunc(arg1, arg2, arg3, arg4, arg5) {
//declare some vars based on this
this.varname1=varname1;
this.varname2=varname2;
//declare some functions inside the big function
this.innerfunc1= function() {
//do stuff
}
this.innerfunc2= function() {
//do stuff
}
}//end of big function
//then goes on to declare init function
function initfunc(){
//this creates new bigfunc(arg1 arg2 arg3...) for each main menu item
}
//finally calls init function with
window.onload = initfunc();
Now on to my confusion -
1) firstly for clarification, am I correct in thinking based on all the this's floating about in bigfunc() and the fact that it is called with new bigfunc() that this is creating an object?
2)My current problem is with one of the functions inside bigfunc() which looks like this:
this.slideChildMenu = function() {
var divref = this.children[0].div;
var ulref = this.children[0].ul;
var maxwidth = this.children[0].width;
var nextWidth;
if (this.isMouseOnMe || this.isMouseOnChild()) {
nextWidth = divref.offsetWidth + slideSpeed_out;
if (nextWidth >= maxwidth) {
this.finishOpeningChild(divref, ulref, maxwidth);
} else {
ulref.style.left = nextWidth - maxwidth + "px";
divref.style.width = nextWidth + "px";
setTimeout("slideChildMenu('" + this.getId() + "')", slideTimeout_out);
}
}
Now my plan is to alter this to use jquery show to open the element so I tried this:
this.slideChildMenu = function() {
var divref = this.children[0].div;
var ulref = this.children[0].ul;
if (this.isMouseOnMe || this.isMouseOnChild()) {
$(divref).show(function(){
this.finishOpeningChild(divref, ulref);
});
}
}
But I am getting this-> TypeError: this.finishOpeningChild is not a function
Now, there is a lot of other stuff going on in this js so I wouldnt dream of asking someone on here to do my work for me, but I am hoping that if someone can explain to me why this function is not a function I may be able to work the rest out.
NOTE: I thought this was to do with the scope of "this" but the value of this appears to be exactly the same in both versions of the code.
I know this is a long one but your help is greatly appreciated.
The value of this in a function is called the "context" in which the function runs. In general, whenever you pass a callback function as an argument (as you do with $(divref).show(function() {...})), the function can run the callback in whatever context it wants. In this case, the jQuery show function chooses to run its callback in the context of the element being animated.
However, you want access to the value of this at the time the anonymous callback function is defined, rather than when it is run. The solution here is to store the outer value of this in a variable (traditionally called self) which is included in the scope of the newly-defined function:
this.slideChildMenu = function() {
//...
var self = this;
$(divref).show(function(){
self.finishOpeningChild(divref, ulref);
});
}
I am thinking that the jQuery selector has changed the scope of this.
In your example $(this); would refer to object being animated per jQuery api docs:
If supplied, the callback is fired once the animation is complete. This can be useful for stringing different animations together in sequence. The callback is not sent any arguments, but this is set to the DOM element being animated. If multiple elements are animated, it is important to note that the callback is executed once per matched element, not once for the animation as a whole.
If the object in question is instantiated you can call it with dot notation without using this like bigFunc.finishOpeningChild(divref, ulref);
You're probably a little confused about scope, it's not always easy keeping track, but doing something more like this:
var site = {
init: function(elm) {
self=site;
self.master.funcname2(self.varname1, elm); //call function in master
},
funcname: function(obj) {
//do something
},
varname1: 'some string',
varname2: 3+4,
master: function() {
this.varname3 = sin(30);
this.funcname2 = function(stuff, element) {
site.funcname(element); //call function in 'site'
var sinus = site.master.varname3; //get variable
}
}
}
window.onload = function() {
var elm = document.getElementById('elementID');
site.init(elm); //call init function
}
usually makes it a little easier to keep track.
How can I pass parameters to a function declared like something = function(){};
window.prototype.initInterface = function(){
this.mainPane = document.createElement('div');
this.mainPane.style.border="5px solid grey";
this.mainPane.style.margin="0px";
this.mainPane.style.width="420px";
this.mainPane.style.height="600px";
this.exitButton = document.createElement('input');
this.exitButton.setAttribute("type", "button");
this.exitButton.setAttribute("value", "exit");
this.exitButton.onclick = function(){
document.body.removeChild(this.mainPane);
};
this.mainPane.appendChild(this.exitButton);
document.body.appendChild(this.mainPane);
}
When the user presses the exit button I want to remove the mainPane from the body of the html page.
this.exitButton.onclick = function(this.mainPage){
document.body.removeChild(this.mainPane);
};
Does not work
How can I do this?
For your exitButton.onclick function to have access to variables you create in the enveloping initInterface function you want a to create a closure in the exitButton.onclick function by returning a function that performs the action you want and passing that the variable.
exitButton.onclick = function () {
return (function() {
document.body.removeChild(mainPane);
})(mainPane);
};
Read more on how closures work here and here and see a working example fiddle.
Alternatively, you forget about closures and walk up the DOM from the button which triggers the event to your mainPane
exitButton.onclick = function() {
// in here "this" is the object that triggered the event, exitButton
document.body.removeChild(this.parentNode);
}
As an aside, window.prototype does not exist if you are doing this in a browser; window is the object at the top of prototype chain in browser scripting. You want just window.initInterface = function () {} which is the exact same thing as function initInterface() {} because everything you do in javascript in the browser becomes a property of window.
This function is the function w/o function name. It could only be used once and you may not easy to find out what parameters should be passed.
You can create another function like ：
function go(a1){}
And call it like window.prototype.initInterface = go(a1);
Or you can get some DOM parameters in this unnamed function by using functions like getDocumentById("DOM ID") etc.