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Alpha_ProgDes

Why does this work?

6 posts in this topic

/**
* The animation loop. Calls the requestAnimationFrame shim to
* optimize the game loop and draws all game objects. This
* function must be a gobal function and cannot be within an
* object.
*/
function animate() {
    requestAnimFrame( animate );
    game.background.draw();
}
/**
* requestAnim shim layer by Paul Irish
* Finds the first API that works to optimize the animation loop,
* otherwise defaults to setTimeout().
*/
window.requestAnimFrame = (function(){
    return window.requestAnimationFrame ||
            window.webkitRequestAnimationFrame ||
            window.mozRequestAnimationFrame ||
            window.oRequestAnimationFrame ||
            window.msRequestAnimationFrame ||
            function(/* function */ callback, /* DOMElement */ element){
                window.setTimeout(callback, 1000 / 60);
            };
})();

Shouldn't the call to animate result in an infinite recursive loop with the draw() being unable to be called?

 

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Where do you see a recursive call?

 

The return chains checks if methods exist and if they all fail finally calls window.setTimeout. Either window.setTimeout or any of the other functions before will put a regular call to callback (in this case the animate function) in some event queue to be called later.

Edited by Endurion
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As far as I understand it:

 

1.) window.requestAnimFrame is statically initialized with the first match found during looking up a couple of (browser specific) setter for a timed callback.

 

2.) Function animate() sets self as the function to be called when the next 1/60 second has gone, and when this setter returns the background is rendered. I don't see why this should not work. 

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Well I just don't understand how it is working in the first place. It's why I asked. I see a function passing itself to another function. Which seems recursive to me.

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requestAnimationFrame doesn't call animate imminently. I simply adds it to a list of functions that will be called before the next screen redraw.

 

The effect is, theoretically, the function animation gets called over and over infinitely. It isn't recursive, however because requestAnimationFrame doesn't directly call animate, instead it schedules it to be called. This keeps it from entering into an infinite recursive loop resulting in a stack overflow.

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Well I just don't understand how it is working in the first place. It's why I asked. I see a function passing itself to another function. Which seems recursive to me.


The concepts to look up, if you're not familiar with them, are 'closures' and 'callbacks'. JavaScript innately supports first-class functions, meaning that you can pass a function itself (and not just the result of a function) as a parameter into another function. This is used to implement callbacks. JavaScript functions also implicitly support acting as closures, allowing a JavaScript function to reference local variables from an enclosing scope even long after that enclosing scope is gone.
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Just to add to what everyone else said, check the difference that "()" makes:

function animate() {
    anotherFunction( animate ); //no () after animate
    moreCode();
}

function animate2() {
    anotherFunction( animate2() ); //() after animate2
    moreCode();
}

In the first case, the order of the instructions are:

- animate function starts

- anotherFunction is called with a reference to animate function as the first parameter

- moreCode is called

- animate function ends

 

In the second case:

- animate2 function starts

- animate2 is called, because the return value must be passed to anotherFunction as the first parameter

- animate2 function starts again

- infinite loop (anotherFunction and moreCode are never called)

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