iOS implemention of "window.setTimeout" with JavascriptCore

Question

I am using JavaScriptCore library inside iOS application and I am trying to implement setTimeout function.

setTimeout(func, period)

After application is launched, the JSC engine with global context is created and two functions are added to that context:

_JSContext = JSGlobalContextCreate(NULL);

[self mapName:"iosSetTimeout" toFunction:_setTimeout];
[self mapName:"iosLog" toFunction:_log];

Here is native implementation that is mapping global JS function with desired name to static objective C function:

- (void) mapName:(const char*)name toFunction:(JSObjectCallAsFunctionCallback)func
{
  JSStringRef nameRef = JSStringCreateWithUTF8CString(name);
  JSObjectRef funcRef = JSObjectMakeFunctionWithCallback(_JSContext, nameRef, func);
  JSObjectSetProperty(_JSContext, JSContextGetGlobalObject(_JSContext), nameRef, funcRef, kJSPropertyAttributeNone, NULL);
  JSStringRelease(nameRef);
}

And here is the implementation of objective C setTimeout function:

JSValueRef _setTimeout(JSContextRef ctx,
                     JSObjectRef function,
                     JSObjectRef thisObject,
                     size_t argumentCount,
                     const JSValueRef arguments[],
                     JSValueRef* exception)
{
  if(argumentCount == 2)
  {
    JSEngine *jsEngine = [JSEngine shared];
    jsEngine.timeoutCtx =  ctx;
    jsEngine.timeoutFunc = (JSObjectRef)arguments[0];
    [jsEngine performSelector:@selector(onTimeout) withObject:nil afterDelay:5];
  }
  return JSValueMakeNull(ctx);
}

Function that should be called on jsEngine after some delay:

- (void) onTimeout
{
  JSValueRef excp = NULL;
  JSObjectCallAsFunction(timeoutCtx, timeoutFunc, NULL, 0, 0, &excp);
  if (excp) {
    JSStringRef exceptionArg = JSValueToStringCopy([self JSContext], excp, NULL);
    NSString* exceptionRes = (__bridge_transfer NSString*)JSStringCopyCFString(kCFAllocatorDefault, exceptionArg);  
    JSStringRelease(exceptionArg);
    NSLog(@"[JSC] JavaScript exception: %@", exceptionRes);
  }
}

Native function for javascript evaluation:

- (NSString *)evaluate:(NSString *)script
{
    if (!script) {
        NSLog(@"[JSC] JS String is empty!");
        return nil;
    }


    JSStringRef scriptJS = JSStringCreateWithUTF8CString([script UTF8String]);
    JSValueRef exception = NULL;

    JSValueRef result = JSEvaluateScript([self JSContext], scriptJS, NULL, NULL, 0, &exception);
    NSString *res = nil;

    if (!result) {
        if (exception) {
            JSStringRef exceptionArg = JSValueToStringCopy([self JSContext], exception, NULL);
            NSString* exceptionRes = (__bridge_transfer NSString*)JSStringCopyCFString(kCFAllocatorDefault, exceptionArg);

            JSStringRelease(exceptionArg);
            NSLog(@"[JSC] JavaScript exception: %@", exceptionRes);
        }

        NSLog(@"[JSC] No result returned");
    } else {
        JSStringRef jstrArg = JSValueToStringCopy([self JSContext], result, NULL);
        res = (__bridge_transfer NSString*)JSStringCopyCFString(kCFAllocatorDefault, jstrArg);

        JSStringRelease(jstrArg);
    }

    JSStringRelease(scriptJS);

    return res;
}

After that whole setup, the JSC engine should evaluate this:

[jsEngine evaluate:@"iosSetTimeout(function(){iosLog('timeout done')}, 5000)"];

The JS execution calls the native _setTimeout, and after five seconds, the native onTimeout is called and crash happens in JSObjectCallAsFunction. The timeoutCtx becomes invalid. Sounds like timeout function context is local and during the time period garbage collector deletes that context in JSC side.

The interesting thing is also, if _setTimeout function is changed in order to call JSObjectCllAsFunction immediately, without waiting for timeout, then it works as expected.

How to prevent automatic context deletion in such asynchronous callbacks?

Answer 1

For registered iOS developer, take a look at the new video about javascript core from wwdc 2013 called "Integrating JavaScript into Native Apps". You will find there the solution for newest iOS version.

My alternative solution, for the current iOS version, was to make a global array in JSC for storing objects that should be protected from garbage collector. So, you have control to pop variable from array when it is not needed any more.