Re: KVO query
Re: KVO query
- Subject: Re: KVO query
- From: Jonathan Mitchell <email@hidden>
- Date: Thu, 14 Aug 2014 10:26:35 +0100
On 13 Aug 2014, at 23:41, Quincey Morris <email@hidden> wrote:
> On Aug 13, 2014, at 14:53 , Jonathan Mitchell <email@hidden> wrote:
>
>> At one point i need to invoke a manual KVO notification like so:
>>
>> [submission willChangeValueForKey:@“status”];
>> [submission didChangeValueForKey:@“status”];
>>
>> This raises like so:
>>
>> Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot update for observer <NSKeyValueObservance 0x608000ac21b0> for the key path "status.name" from <Submission 0x6080003aa800>, most likely because the value for the key "status" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the Submission class.’
>
> This is one of those infuriating errors that is hard to make sense of. Try thinking about it literally.
>
> Because you’re observing the “status.name” path from ‘submission’, the observer is *also* observing the “name” path from ‘submission.status’**. In your scenario, the originally observed ‘status’ instance (call it X) is no longer the current ‘status’ instance at willChange time (it’s now Y, say), and in general it might not be the same as that at didChange time (Z, say). Even if Y == Z, there’s been a non-KVO-compliant change in ‘status’ prior to the notification being sent.
>
> Now, we do this quite often, without problems, but I’d suggest that’s only in cases where the “temporarily non KVO compliant” change is for the *last* key of the path — in which case the last object isn’t being observed, just because it’s the last key.
This is where I am getting caught out. As you say it works fine for the last key of the path. It does for me too.
I am effectively propagating a C# PropertyChanged event. This approach by itself will never be KVO compliant and will fail when mutating non last components in an observed key path.
>
> So, in the case of a non-KVO-compliant change to the value of a non-terminal key path object, the non-KVO-compliance may not be tolerated by the frameworks. Hence this error message.
>
> What I suggest you try is to make two properties: “currentStatus” and “observableStatus”. The second of these would be updated KVO compliantly (i.e. via its setter) when you know that the first has changed and the change should be propagated (i.e. in the places where you now trigger the willChange/didChange notification manually). Code in the ‘submission’ class can use ‘self.currentStatus’ to get or set the most recent status object; observers would observe “observableStatus”, of course.
Using a proxy property sounds like it might be doable.
Thanks
Jonathan
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| References: | |
| | >KVO query (From: Jonathan Mitchell <email@hidden>) |
| | >Re: KVO query (From: Quincey Morris <email@hidden>) |