Question
We're developing a custom EventEmitter inspired message system in Objective-C. For listeners to provide callbacks, should we require blocks or selectors and why?
Which would you rather use, as a developer consuming a third party library? Which seems most in line with Apple's trajectory, guidelines and practices?
Background
We're developing a brand new iOS SDK in Objective-C which other third parties will use to embed functionality into their app. A big part of our SDK will require the communication of events to listeners.
There are five patterns I know of for doing callbacks in Objective-C, three of which don't fit:
NSNotificationCenter - can't use because it doesn't guarantee the order observers will be notified and because there's no way for observers to prevent other observers from receiving the event (like stopPropagation() would in JavaScript).
Key-Value Observing - doesn't seem like a good architectural fit since what we really have is message passing, not always "state" bound.
Delegates and Data Sources - in our case, there usually will be many listeners, not a single one which could rightly be called the delegate.
And two of which that are contenders:
Selectors - under this model, callers provide a selector and a target which are collectively invoked to handle an event.
Blocks - introduced in iOS 4, blocks allow functionality to be passed around without being bound to an object like the observer/selector pattern.
This may seem like an esoteric opinion question, but I feel there is an objective "right" answer that I am simply too inexperienced in Objective-C to determine. If there's a better StackExchange site for this question, please help me by moving it there.
UPDATE #1 — April 2013
We chose blocks as the means of specifying callbacks for our event handlers. We're largely happy with this choice and don't plan to remove block-based listener support. It did have two notable drawbacks: memory management and design impedance.
Memory Management
Blocks are most easily used on the stack. Creating long-lived blocks by copying them onto the heap introduces interesting memory management issues.
Blocks which make calls to methods on the containing object implicitly boost self's reference count. Suppose you have a setter for the name property of your class, if you call name = #"foo" inside a block, the compiler treats this as [self setName:#"foo"] and retains self so that it won't be deallocated while the block is still around.
Implementing an EventEmitter means having long-lived blocks. To prevent the implicit retain, the user of the emitter needs to create a __block reference to self outside of the block, ex:
__block *YourClass this = self;
[emitter on:#"eventName" callBlock:...
[this setName:#"foo"];...
}];
The only problem with this approach is that this may be deallocated before the handler is invoked. So users must unregister their listeners when being deallocated.
Design Impedance
Experienced Objective-C developers expect to interact with libraries using familiar patterns. Delegates are a tremendously familiar pattern, and so canonical developers expect to use it.
Fortunately, the delegate pattern and block-based listeners are not mutually exclusive. Although our emitter must be able to be handle listeners from many places (having a single delegate won't work) we could still expose an interface which would allow developers to interact with the emitter as though their class was the delegate.
We haven't implemented this yet, but we probably will based on requests from users.
UPDATE #2 — October 2013
I'm no longer working on the project that spawned this question, having quite happily returned to my native land of JavaScript.
The smart developers who took over this project decided correctly to retire our custom block-based EventEmitter entirely.
The upcoming release has switched to ReactiveCocoa.
This gives them a higher level signaling pattern than our EventEmitter library previously afforded, and allows them to encapsulate state inside of signal handlers better than our block-based event handlers or class-level methods did.
Personally, I hate using delegates. Because of how objective-C is structured, It really clutters code up If I have to create a separate object / add a protocol just to be notified of one of your events, and I have to implement 5/6. For this reason, I prefer blocks.
While they (blocks) do have their disadvantages (e.x. memory management can be tricky). They are easily extendable, simple to implement, and just make sense in most situations.
While apple's design structures may use the sender-delegate method, this is only for backwards compatibility. More recent Apple APIs have been using blocks (e.x. CoreData), because they are the future of objective-c. While they can clutter code when used overboard, it also allows for simpler 'anonymous delegates', which is not possible in objective C.
In the end though, it really boils down to this:
Are you willing to abandon some older, more dated platforms in exchange for using blocks vs. a delegate? One major advantage of a delegate is that it is guaranteed to work in any version of the objc-runtime, whereas blocks are a more recent addition to the language.
As far as NSNotificationCenter/KVO is concerned, they are both useful, and have their purposes, but as a delegate, they are not intended to be used. Neither can send a result back to the sender, and for some situations, that is vital (-webView:shouldLoadRequest: for example).
I think the right thing to do is to implement both, use it as a client, and see what feels most natural. There are advantages to both approaches, and it really depends on the context and how you expect the SDK to be used.
The primary advantage of selectors is simple memory management--as long as the client registers and unregisters correctly, it doesn't need to worry about memory leaks. With blocks, memory management can get complex, depending on what the client does inside the block. It's also easier to unit test the callback method. Blocks can certainly be written to be testable, but it's not common practice from what I've seen.
The primary advantage of blocks is flexibility--the client can easily reference local variables without making them ivars.
So I think it just depends on the use case--there is no "objective right answer" to such a general design question.
Great writeup!
Coming from writing lots of JavaScript, event-driven programming feels way cleaner than having delegates back and forth, in my personal opinion.
Regarding the memory-managing aspect of listeners, my attempt at solving this (drawing heavily from Mike Ash's MAKVONotificationCenter), swizzles both the caller and emitter's dealloc implementation (as seen here) in order to safely remove listeners in both ways.
I'm not entirely sure how safe this approach is, but the idea is to try it 'til it breaks.
A thing about a library is, that you can only to some extend anticipate, how it will be used. so you need to provide a solution, that is as simple and open as possible — and familiar to the users.
For me all this fits best to delegation. Although you are right, that it can only have on listener (delegate), this means no limitation, as the user can write a class as delegate, that knows about all desired listeners and informs them. Of course you can provide a registering class. that will call the delegate methods on all registered objects.
Blocks are as good.
what you name selectors is called target/action and simple yet powerful.
KVO seems to be a not optimal solution for me as-well, as it would possibly weaken encapsulation, or lead to a wrog mental model of how using your library's classes.
NSNotifications are nice to inform about certain events, but the users should not be forced to use them, as they are quite informal. and your classes wont be able to know, if there is someone tuned-in.
some useful thoughts on API-Design: http://mattgemmell.com/2012/05/24/api-design/
Related
I want to have multiple observers on multiple events of a single object (1-to-N relationship).
A mechanism to achieve this task is provided by the NSNotificationCenter. The mechanism looks pretty overkill when used for my problem.
How I would do it manually without the use of NSNotificationCenter:
- (void)addDelegate:(id<DelegateProtocol>)delegate;
- (void)removeDelegate:(id<DelegateProtocol>)delegate;
to add and remove observers from my object.
- (void)someEventFired:(NSObject<NSCopying> *)eventData
{
for (id delegate in delegates) {
NSObject *data = [eventData copy];
[delegate someEventFired:data];
}
}
This mechanism is straight-forward and simple to implement without the objects having to share additional strings.
Is there an official pattern for 1-to-N delegates (like C# events) in an iOS framework besides the NSNotificationCenter?
When should the NSNotificationCenter be used and when not?
When should an implementation like the one I am suggesting here be used and when not?
By convention, delegates should probably only be used for 1:1 relationships. If you really need 1:N relationships for this type of functionality, you have two options:
As you mentioned, NSNotificationCenter.
Key-Value Observing (also known as KVO).
KVO is appropriate if you only care about when a particular property of an object changes. Otherwise, you should really just consider using NSNotificationCenter. You can even be notified only when a specific object posts that notification by passing that object into the addObserver:selector:name:object: method.
Apple uses NSNotification in similar scenarios (like the notifications defined for UITextField, including UITextFieldTextDidBeginEditingNotification, UITextFieldTextDidChangeNotification, and UITextFieldTextDidEndEditingNotification).
using notifications is broadcasting: 1 sender just sends an information and who ever tuned in, receives it. Petty much like a radio station, there is no channel back (lets for the moment forget about telephones)
delegation is something different. Th object, that asks a deleagte to do something, usually needs a result of that request, there fore delegation is a 1-to-1 communication, that is always initiated by the object, not the delegate (while the object can have methods that can be called to inform the object to initiate the communication, ie [tableView reloadData]).
So if the sender needs to get data back, it is delegation. If the sender doesn't care about anything after broadcasting, go with notifications.
If you run into the situation, that you need delegation, but several objects should implement the protocol. you should have 1 delegate, that hold references to the other objects and calls the methods on the senders behalf — or you could go with blocks.
NSNotificationCenter is not overkill for what you are suggesting, it is exactly the right solution. It prevents the observed object having to know or care about its observers, making your code more loosely coupled and cleaner.
Sharing strings for notification names is trivial and they can be defined in either a shared constants file or in the header of the observed object, if your observers need to import this header to do their jobs.
Your proposed solution is neither simpler than using NSNotificationCenter nor is it thread safe.
To make your solution thread safe, you would need to provide a mechanism to prevent the delegates array from changing while the event dispatch for loop is running.
Your solution also requires that you maintain the delegates array in your class. With the NotificationCenter you can simply use the default center and you don't need to implement the add/remove methods in your class. Instead, instances can register themselves to receive notifications as they see best fit (selector/block, queue, source). Your source class doesn't have to worry about those details. It only needs to register itself as a source of notifications of a specified type. Using blocks to handle notifications is really convenient.
An alternative to the notification center is to use Key-Value-Observing if that meets the needs of your use case.
Ultimately, the mechanism you decide to use depends on how best it applies to your specific use case.
A 1-to-N delegate relationship doesn't make sense. Have a look at
- (NSView *)tableView:(NSTableView *)tableView viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row
for example. What if this object really had n delegates? How should it decide which of the n views it gets back from all its delegates should be used? Delegates are exactly this 1-to-1 principle.
The NSNotificationCenter is the right approach. Simply use
addObserver:selector:name:object:
respectively
postNotification:
This is definitely not too much code. And it's very easy for you as the center handles all calls.
You don't want to use NSNotificationCenter for anything other than system-wide events (e.g. the appearance of the keyboard or some similar event). The reason is that it is completely not type-safe, can make everything dependent on everything and that you get no compile time checks or usage search results anymore.
KVO in my opinion should not be used to observe changes outside of the object you're listening to since it has similar down sides (no compile time checks, crashes if you don't remove listeners properly or register them twice).
The addDelegate/removeDelegate pattern that you pose is completely the right path in my opinion since that has the advantage of maintaining type-safety and compiler checks and makes dependencies explicit. The only problem is that Apple doesn't supply an out-of-the-box solution for this pattern, since you need a collection type that weakly retains its elements to avoid retain cycles.
However, see code from my BMCommons framework which solves this problem neatly using BMNullableArray and macros. See the BMCore.h header for a definition of those macros:
BM_LISTENER_METHOD_DECLARATION(protocol)
BM_LISTENER_METHOD_IMPLEMENTATION(protocol)
The implementation ensures that the same listener will never be added twice and also that listeners are weakly retained, not causing any crash even if they forget to deregister themselves upon deallocation (although I prefer to catch this condition with an assert since it is a programming mistake).
I say NSNotificationCenter should ALWAYS be used, over the delegate model, except in situations where you query a delegate on information (e.g. -webView:shouldLoadRequest:). It is more stable, easier to implement, and results in cleaner code then trying to use a delegate. The other alternative is blocks, which can be good, but they can be a pain when it comes to memory-managment.
In the end, it's up to you, but I think that NSNotificationCenter is the best way to go in almost any situation, if only for the multiple observer functionality.
I have many objects reference the same class of stored data. In previous programs, I've used singletons, but am trying to abandon that practice and only use them as a last resort when necessary, mainly due to the bad reputation they have (and indeed I've abused them in the past).
But I'm wondering just how much of an advantage my new technique is. I'm simply creating weak references to the same set of data so a bunch of classes point to the same memory to pull data as needed. Such as:
#property (nonatomic, assign) MyDataClass*mydata;
In a custom init of the class, I pass a reference as a method parameter, then the property assigns to this reference.
Is this a valid, acceptable way to do things? I'm having trouble finding much of an organizational advantage to doing this over using singletons.
The pattern you use is just fine, eventually this pattern is used in all standard C++ programs without reference counting or other advanced memory management tools. The only thing you have to ensure that your object hierarchy strictly respects the weakness of the reference i.e. the dependency of the object having the reference towards the object that is behind that reference. In other words, you always have to ensure that the owner of the reference is deleted before the reference and you have to do ensure manually since you're not using the reference counting.
This means more responsibility for you, the programmer as you always have to have total control over the lifetime of your objects. It is quite easy to make mistake with your pattern since you can't know from the code having the weak reference whether the original object still exist or it is deleted. You have to ensure this with your design pattern.
For this reason I don't suggest to "mix" the two approaches, i.e. having weak references to an object that might be freed out-of-control by a retain type property (when the value of the property changes from your object), an autorelease or by ARC.
Reference counting was introduced to take away this responsibility from the programmer and make it easier to write safe code. Your pattern is fine, it is used by millions of C++ programs but you have to be conscious of your responsibility.
As a rule, you should only use a singleton class for objects where it wouldn't make sense for more than one of them to exist. Otherwise, it's best to avoid them because they introduce coupling: every class that uses the singleton ends up tightly coupled to the singleton.
Passing references to stuff is fine. They don't have to be weak references of course, except where necessary in order to avoid retain cycles.
If you do find that you're passing the same objects around between a large number of classes, you might want to consider thinking about division of responsibility and restructuring your app.
Retain/Assign and using Singletons aren't mutually exclusive patterns. I wouldn't be so hesitant against the singleton, I was at first when I started iOS.
Coming from being a java web apps developer, Singletons were bad due to the tight coupling, and especially because if you wanted to distribute your application across a load balancer / (cloud now)... then your singleton would become a bottleneck - and wouldn't be easily scalable.
There were also problems with Unit testing with Singletons, and having to reset it's state during tests, or even trying to mock it out.
However, in Objective-C and iOS development - I'm not really seeing that many disadvantages to singletons. Your app isn't going to be scaled, and the sdk is already littered with singletons to hinder your unit tests.
I just read a couple of tutorials regarding KVO, but I have not yet discovered the reason of its existence. Isn't NSNotificationCenter an easier way to observe objects?
I am new to Stackoverflow, so just tell me if there is something wrong in the way I am asking this question!
Notifications and KVO serve similar functions, but with different trade-offs.
Notifications are easy to understand. KVO is... challenging... to understand (at least to understand how to use it well).
Notifications require modification to the observed code. The observed must explicitly generate every notification it offers. KVO is transparent to the observed code as long as the observed code conforms to KVC (which it should anyway).
Notifications have overhead even if you don't use them. Every time the observed code posts a notification, it must be checked against every observation in the system, even if no one is observing that object (even if no one is observing anything). This can be very non-trivial if there are more than a few hundred observations in the system. It can be a serious problem if there are a few thousand. KVO has zero overhead for any object that is not actually observed.
In general, I discourage KVO because of some specific implementation problems that I believe make it hard to use correctly. It's difficult to observe an object that your superclass also observes without special knowledge of your superclass. Its heavy reliance of string literals make small typos hard to catch at compile time. In general, I find code that relies heavily on it becomes complex and hard to read, and begins to pick up spooky-action-at-a-distance bugs. NSNotification code tends to be more straightforward and you can see what's happening. Random code doesn't just run when you didn't expect it.
All that said, KVO is an important feature and developers need to understand it. More and more low-level objects rely on it because of it's zero-overhead advantages. But for new developers, I typically recommend that they rely more on notification rather than KVO.
There is a third way. You can keep a list of listeners, and send them messages when things change, just like delegate methods. Some people call these "multicast delegates" but "listeners" is more correct here because they don't modify the object's behavior as a delegate does. Doing it this way can be dramatically faster than NSNotification if you need a lot of observation in a system, without adding the complexity of KVO.
Most iPhone code examples use the nonatmoc attribute in their properties. Even those that involve [NSThread detachNewThreadSelector:....]. However, is this really an issue if you are not accessing those properties on the separate thread?
If that is the case, how can you be sure nonatomic properties won't be accessed on this different in the future, at which point you may forget those properties are set as nonatomic. This can create difficult bugs.
Besides setting all properties to atomic, which can be impractical in a large app and may introduce new bugs, what is the best approach in this case?
Please note these these questions are specifically for iOS and not Mac in general.
First,know that atomicity by itself does not insure thread safety for your class, it simply generates accessors that will set and get your properties in a thread safe way. This is a subtle distinction. To create thread safe code, you will very likely need to do much more than simply use atomic accessors.
Second, another key point to know is that your accessors can be called from background or foreground threads safely regardless of atomicity. The key here is that they must never be called from two threads simultaneously. Nor can you call the setter from one thread while simultaneously calling the getter from another, etc. How you prevent that simultaneous access depends on what tools you use.
That said, to answer your question, you can't know for sure that your accessors won't be accessed on another thread in the future. This is why thread safety is hard, and a lot of code isn't thread safe. In general, if youre making a framework or library, yeah, you can try to make your code thread safe for the purposes of "defensive programming", or you can leave it non-thread safe. The atomicity of your properties is only a small part of that. Whichever you choose, though, be sure to document it so users of your library don't have to wonder.
SO is telling me this question is subjective and likely to be closed. It is indeed subjective, because I'm asking for the opinion of experienced Objective-C developers. Should I post this somewhere else? Please advise.
Fairly new to Objective-C, though fairly confident in the concept of writing OOP code, I've been struggling with the NSNotification vs Delegate dilemma from the start. I've posted a few questions about that subject alone. I do get the gist, I think. Notifications are broadcasted globally, so shouldn't be used for notifying closely related objects. Delegates exist to hand over tasks to other object, that act on behalf of the delegated object. While this can be used for closely related objects, I find the workflow to be verbose (new class, new protocol, etc), and the word "delegation" alone makes me think of armies and bosses and in general makes me feel uneasy.
Where I come from (AS3) there are things called Events. They're halfway between delegates and NSNotifications and pretty much ruled the world of flash notifying, until fairly recently, a Mr. Robert Penner came along and expressed his dissatisfaction with events. He therefore wrote a library that is now widely used in the AS3 community, called Signals. Inspired by C# events and Signals/Slots in Qt, these signals are actually properties of objects, that you access from the outside and add listeners to. There's much more you can do with a signal, but at it's core, that's it.
Because the concept is so humble, I gave it a go and wrote my own signal class in Objective-C. I've gisted Signal.h/.m here.
A way to use this for notifying class A of an event in class B could look like this:
// In class b, assign a Signal instance to a retained property:
self.awesomeThingHappened = [[[Signal alloc] init] autorelease];
// In class a, owner of class b, listen to the signal:
[b.awesomeThingHappened add:self withSelector:#selector(reactToAwesomeThing)];
// And when something actually happens, you dispatch the signal in class b:
[self.awesomeThingHappened dispatch];
// You might even pass along a userInfo dictionary, your selector should match:
[self.awesomeThingHappened dispatchWithUserInfo:userInfo];
I hope it adheres to the right memory management rules, but when the signal deallocs, it should automatically remove all listeners and pass away silently. A signal like this isn't supposed to be a generic replacement of notification and delegation, but there are lot's of close counter situations where I feel a Signal is cleaner than the other two.
My question for stackoverflow is what do you think of a solution like this? Would you instantly erase this from your project if one of your interns puts it in? Would you fire your employee if he already finished his internship? Or is there maybe already something similar yet much grander out there that you'd use instead?
Thanks for your time, EP.
EDIT: Let me give a concrete example of how I used this in an iOS project.
Consider this scenario of four object types with nested ownership. There's a view controller owning a window manager, owning several windows, each owning a view with controls, among which a close button. There's probably a design flaw in here, but that's not the point of the example :P
Now when the close button is tapped, a gesture recognizer fires the first selector in the window object. This needs to notify the window manager that it's closing. The window manager may then decide whether another window appears, or whether the windows stay hidden alltogether, at which point the view controller needs to get a bump to enable scrolling on the main view.
The notifications from window to window manager, and from window manager to view controller are the ones I've now implemented with Signals. This might have been a case of delegation, but for just a 'close' action, it seemed so verbose to create two delegate protocols. On the other hand, because the coupling of these objects is very well defined, it also didn't seem like a case for NSNotifications. There's also not really a value change that I could observe with KVO, because it's just a button tap. Listening to some kind of 'hidden' state would only make me have to reset that flag when reopening a window, which makes it harder to understand and a little error prone.
Alright, after marinating the answers and comments for a bit, I think I have come to a conclusion that the Signal class I borrowed from AS3, has very little reason for existence in Objective-C/Cocoa. There are several patterns in Cocoa that cover the ranges of use that I was thinking of covering with the Signal class. This might seem very trivial to more experienced Cocoa developers, but it for me it was hard to get the spectrum complete.
I've tried to put it down fairly concisely, but please correct me if I have them wrong.
Target-Action
Used only for notifying your application of user interaction (touches, mostly). From what I've seen and read, there's no way to 'borrow' the target-action system for your own use
KVO (key value observing)
Very useful for receiving notifications when values change in accessible objects. Not so useful for notifying specific events that have no value attached to them, like timer events or interface followup events.
NSNotification
Very useful for receiving notifications when values change or other events happen in less-accessible objects. Due to the broadcast nature of the notification center, this is less suitable for cases where objects have a direct reference to another.
Delegation
Takes the most lines of code compared to the other three, but is also most suitable when the other three are not. Use this one when one object should be notified of specific events in the other. Delegates should not be abused for just accessing methods of the owner object. Stick to methods like 'should', 'will' and 'did'.
Signal
It was a fun experiment, but I mostly used this for classic delegation situations. I also used it to circumvent linked delegates (c delegate of b, b delegate of a, where a starts the event that should make it to c) without wanting to resort to NSNotification.
I still think there should be a more elegant solution for this edge case, but for now I'll
just stick to the existing frameworks. If anyone has a correction or another notification concept, please let me know. Thanks for your help!
It's an interesting idea, but I guess I don't see what makes it dramatically different from Cocoa's notification center. Compare and contrast:
self.awesomeThingHappened = [[[Signal alloc] init] autorelease]; // Your signals library
// Cocoa notifications (no equivalent code)
[b.awesomeThingHappened add:self withSelector:#selector(reactToAwesomeThing)]; // Your signals library
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(reactToAwesomeThing:)
name:#"AwesomeThingHappened"
object:n]; // Cocoa notifications
[self.awesomeThingHappened dispatch]; // Your signals library
[[NSNotificationCenter defaultCenter] postNotificationName:#"AwesomeThingHappened"
object:self]; // Cocoa notifications
[self.awesomeThingHappened dispatchWithUserInfo:userInfo]; // Your signals library
[[NSNotificationCenter defaultCenter] postNotificationName:#"AwesomeThingHappened"
object:self
userInfo:userInfo]; // Cocoa notifications
So, okay. I don't think you're trying to say that, line-for-line, a Signals library for Cocoa is different; rather, the argument goes that it terms of coupling, it isn't as tight as delegates, but not as loose as notifications. To that end, I guess I wonder how necessary it is? I guess I can see somewhat of a need to say "this object 'A' relies heavily on 'B', but doesn't need to be coupled all that closely", but to be honest, that seems like somewhat rare situation.
At any rate, NSNotificationCenter and its ilk, as well as delegates, are pretty standard in Cocoa apps. I always use the rule of thumb that if you deviate from a standard, even a de facto standard, you should have a good reason. If you have a good reason for using neither NSNotificationCenter nor delegates, then you might have a good reason to use this Signals setup. (And as an aside, I'm hesitant to associate notifications and delegates -- they each have a role and exist for different reasons.)
It's hard to say more without a specific use case. I'm inclined to say, "Hey, it looks cool in a geeky way, but it looks like it fills a role already served by notifications." Do you have any specific use cases you could cite?
What do you think of a solution like this?
I don't really see what the benefit is. To me, it seems like a combination of target/action+notifications (you can have multiple target/actions for a single notification event, but the t/a pair is registered with the object itself as opposed to a global notification center). In fact, it's more like key-value-observing that way, except that KVO is limited to observable properties.
Would you instantly erase this from your project if one of your interns puts it in?
No. It's not bad code. In fact, it seems kinda neat. But I just don't see an obvious benefit to it.
Would you fire your employee if he already finished his internship?
Of course not. You don't fire people for writing good code.
Is there maybe already something similar yet much grander out there that you'd use instead?
If you really wanted to make this neat, change the API to use blocks instead. Then you could do:
[object dispatchOnAwesomeThingHappened:^{
NSLog(#"holy cow, something awesome just happened!");
}];
Again, however, you'd be limited to reacting to stuff that the objects explicitly "dispatch". It would be much neater if you could attach stuff to immediately before and/or after any arbitrary method call. If you're interested in that, then I'd check out Aspect Objective-C on github.
I think that there is a gap between NSNotifications and Delegates. And KVO has the worst API in all of Cocoa.
As for NSNotificationCenter here are some of its problems:
it's prone to typos
it's hard to track observers of a given object, therefore hard to debug
it's very verbose
you can only pass notification data in a dictionary, which means you can't use weak references, or structs unless you wrap them. (see: very verbose)
doesn't play nice with GCD: limited support for queues (only blocks)
So there is definitely a need for something better.
I created my own observable class which I use on every project. Another alternative is to use ReactiveCocoa's RACSignal.