Key Value Observe static NSDictionary on facade class - ios

Question

I have a ServiceFacade class with class methods for communicating with backend services.
On that ServiceFacade class I have a static method that returns NSMutableDictionary in which I keep current ServiceFacade's downloading operations.
I want to observer changes on this NSMutableDictionary either in AppDelegate or any other place. The app delegate seems not to respond to
- (void)addObserver:(NSObject *)anObserver
forKeyPath:(NSString *)keyPath
options:(NSKeyValueObservingOptions)options
context:(void *)context{
}
Method for returning NSMutableDictionary:
+(NSMutableDictionary *)downloadOperations
{
if (_downloadOperations)
{
return _downloadOperations;
}
[_downloadOperations addObserver:[AppDelegate sharedAppDelegate] forKeyPath:#"downloadOperationsDict" options:0 context:NULL];
_downloadOperations = [NSMutableDictionary dictionary];
return _downloadOperations;
}
Any idea ?

Answer 1

There is no way to observe NSMutableDictionary changes. But there are 2 workarounds
1) Subclass NSMutableDictionary and trigger Notifications on setObject:forKey and removeObjectForKey.
2) Wrap your _downloadOperations write/remove operations and trigger notifications there.
I suggest you to use 2) variant, as subclassing NSMutableDictionary is not so easy.
So 2) variant will be like this.
Add these 2 methods to class ServiceFacade
- (void)setDownloadObject:(id)aObj forKey:(id<NSCopying>)aKey
{
[self.downloadOperations setObject:aObj forKey:aKey];
[[NSNotificationCenter defaultCenter] postNotificationName:#"DownloadOperationsChanged" object:self
userInfo:self.downloadOperations];
}
- (id)removeDownloadObjectForKey:(id<NSCopying>)aKey
{
[[self.downloadOperations] removeObjectForKey:aKey];
[[NSNotificationCenter defaultCenter] postNotificationName:#"DownloadOperationsChanged" object:self
userInfo:self.downloadOperations];
}
After this, you need to add and remove objects from that dictionary via this 2 methods. And you can also subscribe for changes
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(downloadOperationsChanged:)
name:#"DownloadOperationsChanged"
object:nil];
return YES;
}
- (void)downloadOperationsChanged:(NSNotification *)aNotification
{
NSLog(#"Operations : %#", aNotification);
}