Fix the bug about the circular reference between jsCanvas and jsContext

[finscn]

fix #398

test - case :

var w = window.innerWidth;
var h = window.innerHeight;
var canvas = document.getElementById('canvas');
canvas.width = w;
canvas.height = h;
var context = canvas.getContext('2d');


function createCanvas(width, height) {
    var canvas = document.createElement("canvas");
    canvas.width = width;
    canvas.height = height;
    canvas.retinaResolutionEnabled = false;
    return canvas;
}

function createTempContexts(count) {
    count = count || 10
    for (var i = 0; i < count; i++) {
        var temp = createCanvas(1000, 1000);
        var ctx = temp.getContext("2d");
    }
}


var globalContextList = [];

function createGlobalCanvases(count) {
    count = count || 10
    for (var i = 0; i < count; i++) {
        var canvas = createCanvas(500, 500);
        var ctx = canvas.getContext("2d");
        globalContextList.push(ctx);
    }
}

createGlobalCanvases(40);
createTempContexts(40);

var pool = [];
setInterval(function() {
    // for eating memory
    var longString = (new Array(1000)).join("X")
    pool.push(longString);

    // Expected Result:
    // When recive memroy warning ,
    // The canvas of ctx be created in createTempContexts should be release (size 1000*1000).
    // BUT the canvas of ctx (in globalContextList) should NOT be release (size 500*500).

}, 20);

[finscn]

I read an document( a keynote of WWDC 2013, pdf http://devstreaming.apple.com/videos/wwdc/2013/615xax5xpcdns8jyhaiszkz2p/615/615.pdf?dl=1 , video http://devstreaming.apple.com/videos/wwdc/2013/615xax5xpcdns8jyhaiszkz2p/615/615-SD.mov?dl=1 ) about circular reference and garbage collection of JSBinging in iOS.
It says that if a circular reference is between a Objective-C Object and a JS Object , the situation is too complex .
So I try to move the circular reference bewteen Canvas and Context to JS Context.

The core of this pull request is :

• Don't protect jsCanvas and jsCanvasContext in EJBingingXXXXXX
• Create the circular reference bwtween jsObjects via JSObjectSetProperty

- (void)linkCanvasAndContext{
    JSStringRef canvasName = JSStringCreateWithUTF8CString("canvas");
    JSObjectSetProperty(scriptView.jsGlobalContext, jsCanvasContext, canvasName, jsObject, kJSPropertyAttributeReadOnly, NULL);
    JSStringRelease(canvasName);

    JSStringRef contextName = JSStringCreateWithUTF8CString("__context");
    JSObjectSetProperty(scriptView.jsGlobalContext, jsObject, contextName, jsCanvasContext, kJSPropertyAttributeDontEnum, NULL);
    JSStringRelease(contextName);
}

A question :
How to let the attribute named "__context" can't be access ?

.

[finscn]

the sytleObject in EJBingingCanvas may also use this way ?

[finscn]

@phoboslab , Could you reproduce this issue ?
If you couldn't , I will create a project for this issue.

[phoboslab]

I understand the issue, but I'm not very fond of the workaround using JS. I'll see if I can fix it in Obj-C land.

[phoboslab]

Closing the PR. I'm leaving the related issue #398 open and will fix it eventually when I find the time.

[finscn]

@phoboslab , Could you tell me How to fix it ? If you are busy, I could try to do

[finscn]

This is very important for me, please.