Re: viewDidLoad and KVO
Re: viewDidLoad and KVO
- Subject: Re: viewDidLoad and KVO
- From: Graham Cox <email@hidden>
- Date: Thu, 05 Jun 2014 23:29:53 +1000
On 5 Jun 2014, at 3:03 pm, Rick Mann <email@hidden> wrote:
> I can solve this by doing an explicit UI update step in -viewDidLoad, but that ends up effectively duplicating the UI update code.
>
> I can load the view in -setFoo: by referencing self.view, but this seems like a hack.
>
> What are other people doing to address this? Any "best practice" you guys like?
I guess this is really a question about the timing of the -setFoo: call relative to the -loadView call, and that would probably imply that calling -setFoo: is actually the responsibility of a higher-level controller than this one (probably the object that alloc/inits the view controller). So maybe you could set up a simple informal delegate method such that the VC has the higher-level controller as its delegate, and when -viewDidLoad: is called, it asks the delegate to provide 'foo' at that time.
As far as observing 'foo' via KVO, the way I generally like to do this is to have my VC implement methods that have the exact same form (as setters) as the properties they're interested in on the model object, then my observer method boils down to a very simple trampoline:
- (void) observeValueForKeyPath: ....
{
[self setValue:[changeDict objectForKey:NSKeyValueChangeNewKey] forKey:keyPath];
}
This leverages the automatic unwrapping of property values that KVC already does, so I don't have to do it again myself. This is so general I have it wrapped up into a simple base class that all of these similar view controllers subclass - each subclass then just adds the set<Key> : methods it needs.
For setting up and tearing down the KVO, I do as you do, but something that can simplify this is to get the observed objects to vend a list of "observable properties" that the observer can simply iterate over and add itself as an observer for each one. This is typically a class method. The downside would be that there may well be properties of no interest, but this approach implies you still need a cover method for those properties, but of course you can also override -setValue:forUndefinedKey: not to throw an exception but to ignore instead. Otherwise, just observing the properties of interest one by one is also fine. I have found though that by generalising the approach, if you have a large number of similar but variant interfaces on a large number of similar but variant model objects, it can generalise away a lot of the tedium. I've actually found this approach a lot easier to live with than bindings, which also claim to generalise the problem (as well as moving it into IB), but can be opaque as a result.
In terms of setting up the initial state of the UI, provided -setFoo: is called at the right time (after the view is loaded), then passing the NSKeyValueObservingOptionInitial as a flag to the -addObserver: call ensures that the same trampolining method is used both for initial setup and ongoing changes to the model property.
As for tear-down at dealloc time, -setFoo:nil is fine, as it then tears down the observation on the existing (retained) foo object prior to releasing it. I've not run into issues with the unregistering observations error, at least not since this was improved a lot a few OS versions back.
--Graham
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