Correct management of addObserverForName:object:queue:usingBlock: - ios

Question

I'm still new to blocks in objective-c and wondering if I have this psuedo code correct. I'm not sure if it's enough to just remove the observer or if i have to call removeObserver:name:object:
-(void) scan {
Scanner *scanner = [[Scanner alloc] init];
id scanComplete = [[NSNotificationCenter defaultCenter] addObserverForName:#"ScanComplete"
object:scanner
queue:nil
usingBlock:^(NSNotification *notification){
/*
do something
*/
[[NSNotificationCenter defaultCenter] removeObserver:scanComplete];
[scanner release];
}];
[scanner startScan];
}
Update: I'm receiving intermittent EXC_BAD_ACCESS from this block, so this can't be right.

Answer 1

Declare the scanComplete variable before defining the block itself.
The reason why you need to do this is because you're trying to access a variable that doesn't exist within the block at the time of definition since the variable itself has not been assigned yet.
What is EXC_BAD_ACCESS? Well, it's an exception that is thrown when you try to access a reference that doesn't exist. So that is exactly the case in your example.
So if you declare the variable before the block itself, then it should work:
-(void) scan {
Scanner *scanner = [[Scanner alloc] init];
__block id scanComplete;
scanComplete = [[NSNotificationCenter defaultCenter] addObserverForName:#"ScanComplete"
object:scanner
queue:nil
usingBlock:^(NSNotification *notification){
/*
do something
*/
[[NSNotificationCenter defaultCenter] removeObserver:scanComplete];
[scanner release];
}];
[scanner startScan];
}

Answer 2

You should not unregister in the register block. Instead, store the token returned from addObserverForName (in this case, your scanComplete) as an instance variable or in a collection that is an instance variable, and unregister later when you're about to go out of existence (e.g. in dealloc). What I do is keep an NSMutableSet called observers. So:
id ob = [[NSNotificationCenter defaultCenter]
addObserverForName:#"whatever" object:nil queue:nil
usingBlock:^(NSNotification *note) {
// ... whatever ...
}];
[self->observers addObject:ob];
And then later:
for (id ob in self->observers)
[[NSNotificationCenter defaultCenter] removeObserver:ob];
self->observers = nil;

Answer 3

Apple Document about this method:
The following example shows how you can register to receive locale change notifications.
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
self.localeChangeObserver = [center addObserverForName:NSCurrentLocaleDidChangeNotification object:nil
queue:mainQueue usingBlock:^(NSNotification *note) {
NSLog(#"The user's locale changed to: %#", [[NSLocale currentLocale] localeIdentifier]);
}];
To unregister observations, you pass the object returned by this method to removeObserver:. You must invoke removeObserver: or removeObserver:name:object: before any object specified by addObserverForName:object:queue:usingBlock: is deallocated.
NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center removeObserver:self.localeChangeObserver];

Answer 4

The scope of the block doesn't have permission to release the scanner object. If you're not using garbage collection, removing the release and making the scanner autorelease ([[[Scanner alloc] init] autorelease]) should do the trick.
You should also be able to safely move the call to removeObserver outside of the block.
For the case of EXC_BAD_ACCESS: Entering bt in the console window after the application crashes will give you a backtrace, and should inform you where the error occurred.