Temporarily storing Javascript object in Objective-C

[Ian Beck]

Hey there,

I know this is possible, but I'm having trouble figuring out
specifically what I need to do to make it happen. I have a Javascript
object:

var myObject = {
"something": "stuff",
"awesomeness": function() { /* do something */ }
}

And I need to store it temporarily in Objective-C by doing something
like this in Javascript:

myObjCObject.setJSObject(myObject);
// ... time passes ...
var thatObject = myObjCObject.jsObject;
thatObject.awesomeness(); // etc.

The trouble is that I am not sure what type of object/reference I'm
getting in Objective-C, or how best to preserve and then return it. So
far my attempts using JSValueRef and JSValueProtect have lead to non-
descriptive crashes or the object getting lost between storage and
retrieval. Unfortunately, Google has not been helpful in alleviating
my ignorance, so any advice is appreciated.

Ian

[Patrick Geiller]

You must use JSValueRefAndContextRef to pass a Javascript object to ObjC, then call JSValueProtect with the global context.

// valueAndContext gives the js object and its associated context (the current call stack).
// This context might be soon destroyed, therefore we must use the main context to protect the value
- (id)setJSObject:(JSValueRefAndContextRef)valueAndContext {
// Get the main context
JSContextRef mainContext = [[JSCocoa controllerFromContext:valueAndContext.ctx] ctx];

// Protect the object
JSValueProtect(mainContext, valueAndContext.value);
}

// Use JSValueRefAndContextRef also when returning your object to js
- (JSValueRefAndContextRef)jsObject
{
JSValueRefAndContextRef returnValue = { NULL, NULL };
returnValue.value = /* your previously stored object */
return returnValue;
}

> --
> JSCocoa: http://inexdo.com/JSCocoa
> Source: http://github.com/parmanoir/jscocoa/tree/master
> Docs: http://code.google.com/p/jscocoa/
> Group: http://groups.google.com/group/jscocoa
> Unsubscribe: jscocoa+u...@googlegroups.com

[Ian Beck]

Thanks Patrick! I forgot to try JSValueRefAndContextRef. This is the
Objective-C I am trying based on your code:

- (void)setJSObject:(JSValueRefAndContextRef)jsObject {

// Get the main context
JSContextRef mainContext = [[JSCocoa

controllerFromContext:jsObject.ctx] ctx];
// In .h file defined as: JSValueRef _jsObject;
_jsObject = jsObject.value;
JSValueProtect(mainContext, _jsObject);

}

- (JSValueRefAndContextRef)jsObject {
JSValueRefAndContextRef returnValue = {NULL, NULL};

returnValue.value = _jsObject;
return returnValue;
}

However, this does not work. I am not getting any errors when calling
myObject.setJSObject({"foo": "bar}) in Javascript, but then this:

var storedObject = myObject.jsObject;
log(storedObject.foo);

Results in an error ('Result of expression "storedObject" [undefined]
is not an object') and a segmentation fault that crashes the program.

Is there any obvious reason why this would happen (some oversight on
my part, for instance), or should I make a testing project and try to
debug from there?

Ian

[Patrick Geiller]

I don't know :(

I've had JSValueProtect fail while protecting values from webViews and ended up using a global array that stored the values. Where JSValueProtect fails, the array is a root variable of the context and JavascriptCore does keep it around. Look for __gcprotect in JSCocoaPrivateObject.

[Ian Beck]

Damn. Okay, I'll try the global array, or else find a workaround that
doesn't require storing the object in Objective-C.

Ian