THObserversAndBinders

© 2012 James Montgomerie
jamie@montgomerie.net, http://www.blog.montgomerie.net/
jamie@th.ingsmadeoutofotherthin.gs, http://th.ingsmadeoutofotherthin.gs/

What it is

• Easy, lightweight, object-based key-value observing (KVO).
• Very lightweight object-based key-value binding (KVB).
• For iOS and Mac OS X, with ARC.
• Feels comfortable.
• Here are some examples.

Why it is

To me, Cocoa KVO has three problems (well, besides the conceptual arguments about whether KVO's a good idea in the first place):

• It makes your code messy. -[(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context] methods with huge flowing if statements in them. Need I say more?
• Lifetime management is hard to think about and therefore fragile.
• Encapsulating the above, it just doesn't 'feel comfortable'

Now, it seems like every Cocoa programmer out there has their own KVO and KVB solution, and I've tried a few of them. There are too many to enumerate here. Many of them are quite nice, but I couldn't find any that passed my personal 'feels comfortable' test with flying colours (and, though I know it's irrational, KVO just seems like it should be cleaner than a messily prefixed category on NSObject, objc_setAssociatedObject() or method swizzling).

How it works

• Observers are represented by simple, lightweight THObserver objects that are constructed with an object to observe, a keypath to observe on the object, and a block or target-action pair to call when the observed value changes.
• Optionally, you can also pass arbitrary Cocoa KVO options.
• The block or action can, again optionally, be passed the old and new value, or a whole Cocoa KVO change dictionary.
• To keep code clean, there's also an option to use "value action" target-action callbacks that don't get passed the observed object and keypath like regular actions, but instead just get passed the new, or old and new, values.
• The observation's lifetime is entirely managed by the THObserver object. Keep it around, the observation is alive. Release it, and the observations stop. You can also stop them manually by calling -stopObserving.
• The observed object and the target are weakly referenced by the THObserver.
  • Nothing is going to blow up if the target object is released before the observer
  • You must, however, still ensure that the THObserver object is deallocated, or that -stopObserving has been called on it, before the observed object is deallocated (i.e. before its dealloc method, if you have implemented one, is called). If you don't ensure this, the weak reference held by the observer gets zeroed out and it has no chance to stop KVO-observing it. This makes Cocoa KVO very upset.
    • If your THObservers are strongly held in instance variables of a parent object that also strongly holds the observed object in an ivar, and you're expecting them both be implicitly released when the parent object deallocates, remember to explicitly call -stopObserving on the observers (or perhaps just set them to nil) in the parent object's -dealloc. This will ensure that they're guaranteed to stop observing before the observed object is released.
    • If you're observing self with a THObserver strongly held in an instance variable you can also call -stopObserving inside dealloc, as when observing ivar objects. Just relying on ARC to release the observer won't work here either because by the time an object's ARC-managed ivars are released the object that contained them is already 'gone', so it's too late to remove the observation.

I like this API. It's one simple call to set up an block that fires when a property changes. Want to observe a whole bunch of things? Just set up a bunch ofTHObservers, store them in an array, then when it comes time to stop observing, release the array (maybe calling -stopObserving on the observers in the array first if there might be a reference to them lying around elsewhere, like in an autorelease pool).

Results

• Code is no longer messy. Observer functionality is easy to set up and tear down, and the observation itself is neatly encapsulated in clean blocks or action methods.
• Observation lifetime management is really easy. The observation is just an object, and managing the lifetime of objects is intuitive.
• It feels nice and looks clean in use. No messy prefixed methods etc.
  • Okay, I'll admit there is a little bit of monkeying with analysis of selectors and casting of blocks in THObserver's implementation, but it's nicely encapsulated, and the code is reasonably straightforward.

It seemed like it would be pretty easy to write a straightforward binding mechanism with a similar API on top of THObserver, so I did. The THBinder object represents a binding, and is easy to construct and manage (see the binding examples). You can optionally supply an NSValueTransformer or a block to run the value through. Lifetime is managed similarly to THObserver - it'll stop binding when it's released, and there's also a -stopBinding method.

How to use it

I've packaged this as a static library, you should be able to use it as detailed in this blog post. It's only a couple of files though, so I won't tell anyone if you just copy them into your project instead.

Examples

Block-based observation:

Simple observation block:

THObserver *observer = [THObserver observerForObject:object keyPath:@"propertyToObserve" block:^{
    NSLog(@"propertyToObserve changed, is now %@", object.propertyToObserve);
}];

Observation block with the old and new value passed in:

THObserver *observer = [THObserver observerForObject:object keyPath:@"propertyToObserve" oldAndNewBlock:^(id oldValue, id newValue) {
    NSLog(@"propertyToObserve changed, was %@, is now %@", oldValue, newValue);
}];

Observation block with custom observation options and a Cocoa change dictionary:

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                             options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew
                                         changeBlock:^(NSDictionary *change) {
                                             NSLog(@"propertyToObserve is %@", change[NSKeyValueChangeNewKey]);
                                         }];

Target-action Based Observation

Any of the calls below could be made with or without an 'options' argument.

Simple target-action:

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                              target:self
                                              action:@selector(targetActionCallback)];

Target-action, gets passed observed object

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                              target:self
                                              action:@selector(targetActionCallbackForObject:)];

Target-action, gets passed observed object and keypath

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                              target:self
                                              action:@selector(targetActionCallbackForObject:keyPath:)];

Target-action, gets passed observed object, keypath, old and new values

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                              target:self
                                              action:@selector(targetActionCallbackForObject:keyPath:oldValue:newValue:)];

Target-action with options and change dictionary:

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                             options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew
                                              target:self
                                              action:@selector(targetActionCallbackForObject:keyPath:oldValue:change:)];

"Value action" target-action callback, gets passed the new value only:

This supplies only the new value - useful in keeping code clean if you don't need the object and keypath passed in.

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                             options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew
                                              target:self
                                         valueAction:@selector(targetActionCallbackForNewValue:)];

"Value action" target-action callback, gets passed the old and new values only:

This supplies only the old and new values. Again, useful in keeping code clean (see above).

THObserver *observer = [THObserver observerForObject:object
                                             keyPath:@"propertyToObserve"
                                             options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew
                                              target:self
                                         valueAction:@selector(targetActionCallbackForOldValue:newValue:)];

Binding

Simple binding:

THBinder *binder = [THBinder binderFromObject:fromObject keyPath:@"fromKey"
                                     toObject:toObject keyPath:@"toKey"];

Binding with a Transformer Block:

THBinder *binder = [THBinder binderFromObject:fromObject keyPath:@"fromKey"
                                     toObject:toObject keyPath:@"toKey"
                          transformationBlock:^id(id value) {
                              return @([value integerValue] + 5);
                          }];

Binding with NSValueTransformer:

THBinder *binder = [THBinder binderFromObject:fromObject keyPath:@"fromKey"
                                     toObject:toObject keyPath:@"toKey"
                             valueTransformer:[[MyAddFiveTransformer alloc] init]];

This stuff seems to be making a lot of retain cycles and leaks...

I suspect you're doing something like this:

_observerIvar = [THObserver observerForObject:_objectIvar keyPath:@"propertyToObserve" block:^{
    NSLog(@"propertyToObserve changed, is now %@", _objectIvar.propertyToObserve);
}];

This will create a retain cycle. The reference of _objectIvar inside the block will cause the block - and hence the observer - to strongly retain self. The observer is in turn retained by self when you assign it to _observerIvar, creating the cycle (self retains _observerIvar, which retains the block, which retains self).

You can instead do something like this:

MyObject *blockObject = _objectIvar;
_observerIvar = [THObserver observerForObject:blockObject keyPath:@"propertyToObserve" block:^{
    NSLog(@"propertyToObserve changed, is now %@", blockObject.propertyToObserve);
}];

or:

__weak MySelf *weakSelf = self;
_observerIvar = [THObserver observerForObject:self.objectProperty keyPath:@"propertyToObserve" block:^{
    NSLog(@"propertyToObserve changed, is now %@", weakSelf.objectProperty.propertyToObserve);
}];

And remember to ensure that the observer is not observing by the time that the object in _objectIvar is released (e.g. by calling [_observerIvar stopObserving] in your dealloc).

(Thanks to Peter Steinberger for pointing out that this could use elucidation.)