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How to get a reference to a prototype's parent


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#1 Winged   Members   -  Reputation: 129

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Posted 28 April 2014 - 01:22 PM

When I need to pass a function with parameters as a parameter to another function I use a partial function application function, which looks like this:

function wrapFunction(func, params) { // 'params' should be an array
	return function() {
		return func.apply(this, params);
	};
}

This works fine when passing a function like alert(arg), which is not a prototype of any object (as far as I'm convinced). But as soon as I need to pass a prototype function of an object, for example camera.moveTowards(arg1, arg2, ...), argument 'this' in wrapFunction() needs to be replaced by 'camera'. Is there any way to get a reference to a prototype's parent and pass it in place of 'this'?

 

PS: I don't want to add another argument to wrapFunction, which I would declare every time (like wrapFunction(camera, camera.moveTowards, [arg1, arg2, ...])), because it messes up the readability.


Edited by Winged, 28 April 2014 - 01:24 PM.


#2 DareDeveloper   GDNet+   -  Reputation: 1018

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Posted 28 April 2014 - 02:26 PM

I think you can not get the scope from the function reference itself ...
that is what I think I read not long ago and I think it is also what this article says:
http://stackoverflow.com/questions/1550782/how-to-get-the-this-scope-of-a-javascript-anonymous-function
 

Please leave the scope burden to the calling code, they can always use the 'bind' method or something similar.


I have a feeling that the better answer would be how the problem you are trying to solve should usually be solved.
Unfortunately I don't know that. It might help to think about what the places where the functions are invoked have in common ... and what the places where the functions are defined have in common.
Maybe you can work with some kind of data structure that wraps the scope and the function pointer to help with readability.



Readability is a matter of taste. Working with another wrapper makes it clear what happens, but I don't suppose you like that more than the original call to wrapFunction(...):
 
function buildCallback(func, scope) {
    var callback = new Object();
    callback.wrappedFunction = func;
    callback.scope = scope;
    return callback;
}

function wrapFunctionInvocation(callback, params) { // 'params' should be an array
    return function() {
        return callback.wrappedFunction.apply(callback.scope, params);
    };
}

// ... so that you can call the invocation like this:
var moveTowardsCallback = buildCallback(camera.moveTowards, camera);
var moveTowardsFunctionInvokation = wrapFunctionInvocation(moveTowardsCallback, params);

Not even sure if that would work, btw.

Edited by DareDeveloper, 28 April 2014 - 02:27 PM.

Given enough eyeballs, all mysteries are shallow.

MeAndVR


#3 Winged   Members   -  Reputation: 129

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Posted 01 May 2014 - 12:57 AM

The method you have provided, DareDeveloper, still requires to pass the function's 'parent', which is what I'm trying to avoid, because I feel that there should be a simple way to get the reference's origin.



#4 DareDeveloper   GDNet+   -  Reputation: 1018

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Posted 03 May 2014 - 02:13 PM

Yeah, thought that the better readability is not good enough. But like I said, I think there is no way to do that.

All you can hope for is finding a good place (generic, convenient and mostly hidden functionality) for logic that builds an object with the reference to the scope.

 

Btw. you can read about JavaScript Closures ... they might be on topic but I do not get them:

 

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures

http://msdn.microsoft.com/en-us/magazine/ff696765.aspx


Edited by DareDeveloper, 03 May 2014 - 02:24 PM.

Given enough eyeballs, all mysteries are shallow.

MeAndVR


#5 Prototype   Members   -  Reputation: 1677

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Posted 03 May 2014 - 02:57 PM

I don't know if this applies to your problem, but I often find myself copying 'this' before defining an anonymous function. I'm not a Javascript guru so I'm not sure if this is the right way to do it, but it's the only way I know of.

function bar() {
    that = this;
    foo = function() {
        // bar.this is not in scope, but 'that' is.
    }
}

HTH



#6 tanzanite7   Members   -  Reputation: 1408

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Posted 03 May 2014 - 04:41 PM

How about:

 

wrapFunction(camera.moveTowards.bind(camera), args).

 

// in wrapFunction it would be called as: func.apply(func, args) ... why do you have "this" there in your example?

 

Bind: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind


Edited by tanzanite7, 03 May 2014 - 04:45 PM.


#7 Winged   Members   -  Reputation: 129

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Posted 29 May 2014 - 02:35 AM

I decided to totally abandon the wrapFunction idea 'cause warping a function just like this:

function(args) { doSomething(args) }

works flawlessly, it's easier to type, easier to understand and generally it's more suitable for me ;]






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