Why is my value passed through NSNotifcationCenter not preserved? - objective-c

Question

I'm trying to send a CGPoint through an NSNotification like this
-(void)setPosition:(CGPoint)point
{
NSString *pointString = NSStringFromCGPoint(point);
NSDictionary *dict = [[NSDictionary alloc]
initWithObjectsAndKeys:#"p", pointString, nil];
[[NSNotificationCenter defaultCenter]
postNotificationName:#"BownceSpriteDidSetPosition"
object:self
userInfo:dict];
[super setPosition:CGPointMake(point.x, point.y)];
}
And I've implemented the observer like this
-(void) init
{
if((self = [self init])){
[[NSNotificationCenter defaultCenter]
addObserver:self selector:#selector(setViewPointCenter:)
name:#"BownceSpriteDidSetPosition"
object:nil];
// I wondered wether 'object' should be something else???
// more code etc....
}
return self
}
-(void) setViewPointCenter:(NSNotification *)notification
{
NSString * val = [[notification userInfo] objectForKey:#"p"];
CGPoint point = CGPointFromString(val);
// trying to debug
NSString debugString = [NSString stringWithFormat:#"YPOS -----> %f", point.y];
NSLog(debugString);
CGPoint centerPoint = ccp(240, 160);
viewPoint = ccpSub(centerPoint, point);
self.position = viewPoint;
}
But it seems that CGPoint is empty, or (0,0) maybe. Either way, it's not having the desired effect, and the debugString is showing point.y to be 0.0.
From all the examples I've found, it looks to me like I'm doing it all right. But obviously I'm not. Can anyone nudge me in the right direction and point out my mistake?

Answer 1

You've got your objects and keys reversed in the dictionary. It should read
NSDictionary *dict = [[NSDictionary alloc]
initWithObjectsAndKeys:pointString,#"p", nil];
Yes, it's exactly backwards of the way you would expect it to be and this bites me about every third time I create a dictionary.

Answer 2

Your problem is here:
NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:#"p", pointString, nil];
It should be:
NSDictionary *dict = [[NSDictionary alloc] initWithObjectsAndKeys:pointString, #"p", nil];
"Objects" comes before "Keys" in the selector, so you list your items as ObjectA, KeyForObjectA, ObjectB, KeyForObjectB, etc.
You're also leaking this dictionary, since you alloc/init it, but never release it (I'm assuming you're not using garbage collection).

Answer 3

In new objective-c syntax is better to use:
NSDictionary *dict = #{#"p": [NSValue valueWithCGPoint:point]};
it easier to understend and it use NSValue instead of NSString.
There is also a problem with removing observer. In your code, you only use [[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(setViewPointCenter:) name:#"BownceSpriteDidSetPosition" object:nil]; but never call [[NSNotificationCenter defaultCenter] removeObserver:self];, whitch can produce nasty crash, that would be hard to debug. I sugest you using library https://github.com/AllinMobile/AIMObservers that prevents this kind of crash. You could rewrite your code in that way:
__weak __typeof(self) weakSelf = self;
self.observer = [AIMNotificationObserver observeName:#"BownceSpriteDidSetPosition" onChange:^(NSNotification *notification) {
NSValue *valueOfPoint = [notification userInfo][#"p"];
CGPoint point = [valueOfPoint CGPointValue];
CGPoint centerPoint = ccp(240, 160);
viewPoint = ccpSub(centerPoint, point);
//use weakSelf to avoid strong reference cycles
weakSelf.position = viewPoint;
}];